Chemistry Notes Form 3 – Chemistry Form Three Pdf

Example 2: N₂(g) + 3H₂(g) 2NH₃(g)

Example 3: C₃H₈(g) + 5O₂(g) CO₂(g) + 4H₃O(l)

(a) What volume of oxygen is required to burn 25cm³ of propane, C₃H₈

(b) What volume of carbon dioxide is formed if 5dm³ of propane is burned?

(c) What volume of air (1/5th oxygen) is required to burn propane at the rate of 2dm³ per minute in a gas fire?

Stoichiometric and Ionic Equations

Chemical word equations

For any reaction, what you start with are called the reactants, and what you form are called the products

So any chemical equation shows in some way the overall chemical change of

REACTANTS -> PRODUCTS

This can be written in words or symbols/formulae

The arrow means the direction of change from reactants =to=> products
No symbols or numbers are used in word equations

Always try to fit all the words neatly lined up from left to right, especially if it is a long word equation

The word equation is presented to summarise the change of reactants to products

Stoichiometric Equations

Consider the reaction between zinc sulphide and oxygen

The equation can be written as;
ZnS(s) + O₂(g) ZnO(s) + SO₂(g)

This is an example of a stoichiometric, or normal chemical(or symbol) equation

From this equation, we can deduce;

Rules on Balancing Symbol equations

1. You should know what the reactants and products are,

2. write a word equation with appropriate reactants on the left and products on the right

3. Writing the correct symbol or formula for each equation component

Numbers in a formula are written as subscripts after the element concerned

NOTE: If the number is 1 itself, by convention, no number is shown in a formula or before a formula

4 Using numbers if necessary to balance the equation

5 If all is correct, then the sum of atoms for each element should be the same on both side of the equation arrow

a) in other words: atoms of products = atoms of reactants

This is a chemical conservation law of atoms and later it may be described as the ‘law of conservation of mass

b) the equations are first presented in ‘picture’ style and then written out fully with state symbols

c) The individual formulas involved and the word equations have already been presented above

Ionic Equations

1. Acid-base reactions:

Acids can be defined as proton donors

A base can be defined as a proton acceptor

Any acid-alkali neutralisation involves the hydroxide ion is (base) and this accepts a proton from an acid

2. Insoluble salt formation:

An insoluble salt is made by mixing two solutions of soluble compounds to form the insoluble compound in a process called ‘precipitation’

A precipitation reaction is generally defined as ‘the formation of an insoluble solid on mixing two solutions or a gas bubbled into a solution’

(a) Silver chloride is made by mixing solutions of solutions of silver nitrate and sodium chloride

(3) Redox reaction analysis:

(a) Magnesium + iron (II) sulphate magnesium sulphate + iron

No electrons show up in the full equations because electrons lost by x = electrons gained by y!!

11. MOLARITY

It is very useful to be known exactly how much of a dissolved substance is present in a solution of particular concentration or volume of a solution

So we need a standard way of comparing the concentrations of solutions

The concentration of an aqueous solution is usually expressed in terms of moles of dissolved substance per cubic decimetre, mol dm^-3 , this is called molarity, sometimes denoted in shorthand as M

Note: 1dm³ = 1 litre = 1000ml = 1000 cm³ , so dividing cm³ /1000 gives dm³ , which is handy to know since most volumetric laboratory apparatus is calibrated in cm³ (or ml), but solution concentrations are usually quoted in molarity, that is mol/dm³ (mol/litre)

Equal volumes of solution of the same molar concentration contain the same number of moles of solute i.e the same number of particles as given by the chemical formula

You need to be able to calculate:

You should recall and be able to use each of the following relationships

(1) molarity of Z = moles of Z / volume in dm³
(2) molarity x formula mass of solute = solute concentration in g/dm³ ,
dividing this by 1000 gives the concentration in g/cm³
(3) (concentration in g/dm³ ) / formula mass = molarity in mol/dm³ ,
(4) moles Z = mass Z / formula mass of Z

(5) 1 mole = formula mass in grams

Example 1

What mass of sodium hydroxide (NaOH) is needed to make up 500 cm³ (0.5 dm³ ) of a 0.5M solution?

[Ar’s: Na = 23, O = 16, H = 1]

1 mole of NaOH = 23 + 16 + 1 = 40g

for 1000 cm³ (1 dm³ ) of 0.5M you would need 0.5 moles NaOH

which is 0.5 x 40 = 20g
however only 500 cm³ of solution is needed compared to 1000 cm³
so scaling down: mass NaOH required = 20 x 500/1000 = 10g

Example 2

How many moles of H₂SO₄ are there in 250cm³ of a 0.8M sulphuric acid solution? What mass of acid is in this solution?

[Ar’s: H = 1, S = 32, O = 16]

formula mass of sulphuric acid = 2 + 32 + (4×16) = 98, so 1 mole = 98g
if there was 1000 cm³ of the solution, there would be 0.8 moles H2SO4
but there is only 250cm³ of solution, so scaling down

moles H₂SO₄ = 0,8 x (250/1000) = 0.2 mol
mass = moles x formula mass, which is 0

2 x 98 = 19.6g of H₂SO₄

Example 3

5.95g of potassium bromide was dissolved in 400cm³ of water [Ar’s: K = 39, Br = 80]

moles = mass / formula mass, (KBr = 39 + 80 = 119)

mol KBr = 5.95/119 = 0.05 mol

400 cm3 = 400/1000 = 0.4 dm³

molarity = moles of solute / volume of solution
molarity of KBr solution = 0.05/0.4 = 0.125M

Example 4

What is the concentration of sodium chloride (NaCl) in g/dm³ and g/cm³ in a 1.50 molar solution?

At Masses: Na = 23, Cl = 35.5, formula mass NaCl = 23 + 35.5 = 58.5
Therefore concentration = 1.5 x 58.5 = 87.8 g/dm³ , and
concentration = 87.75 / 1000 = 0.0878 g/cm³

Example 5

A solution of calcium sulphate (CaSO₄) contained 0.5g dissolved in 2dm³ of water

Calculate the concentration in (a) g/dm³ ,

(b) g/cm³ and

(c) mol/dm³

(a) concentration = 0.5/2 = 0.25 g/dm³ , then since 1dm³ = 1000 cm³

(b) concentration = 0.25/1000 = 0

00025 g/cm³
(c) At.masses: Ca = 40, S = 32, O = 64, f

mass CaSO₄ = 40 + 32 + (4 x 16) = 136
moles CaSO₄ = 0.5 / 136 = 0.00368 mol

concentration CaSO₄ = 0.00368 / 2 = 0.00184 mol/dm³

12. Titration: Acid and Alkali

Titrations can be used to find the concentration of an acid or alkali from the relative volumes used and the concentration of one of the two reactants

You should be able to carry out calculations involving neutralisation reactions in aqueous solution given the balanced equation or from your own practical results

1. Note again: 1dm³ = 1 litre = 1000ml = 1000 cm³ , so dividing cm³ /1000 gives dm³

2. and other useful formulae or relationships are:

In most volumetric calculations of this type, you first calculate the known moles of one reactant from a volume and molarity

Then, from the equation, you relate this to the number of moles of the other reactant, and then with the volume of the unknown concentration, you work out its molarity

Example 1:

25 cm³ of a sodium hydroxide solution was pipetted into a conical flask and titrated with 0

2M hydrochloric acid

Using a suitable indicator it was found that 15 cm³ of acid was required to neutralise the alkali

Calculate the molarity of the sodium hydroxide and concentration in g/dm³

equation NaOH(aq) + HCl(aq) NaCl(aq) + H₂O(l)

moles HCl = (15/1000) x 0.2 = 0.003 mol

moles HCl = moles NaOH (1 : 1 in equation)

so there is 0.003 mol NaOH in 25 cm³
scaling up to 1000 cm³ (1 dm³ ), there are

0.003 x (1000/25) = 0.12 mol NaOH in 1 dm³
molarity of NaOH is 0.12M or mol dm^-3
since mass = moles x formula mass, and Mr(NaOH) = 23 + 16 + 1 = 40

concentration in g/dm³ is 0

12 x 40 = 4.41g/dm³

Example 2:

20 cm³ of a sulphuric acid solution was titrated with 0.05M potassium hydroxide

If the acid required 36 cm³ of the alkali KOH for neutralisation what was the concentration of the acid?

equation 2KOH(aq) + H₂SO₄(aq) K₂SO4 + 2H₂O(l)

mol KOH = 0.05 x (36/1000) = 0

0018 mol
mol H₂SO4 = mol KOH / 2 (because of 1 : 2 ratio in equation above)
mol H₂SO4 = 0.0018/2 = 0.0009 (in 20 cm³ )
scaling up to 1000 cm³ of solution = 0

0009 x (1000/20) = 0.045 mol
mol H₂SO4 in 1 dm³ = 0

045, so molarity of H2SO4 = 0.045M or mol dm^-3
since mass = moles x formula mass, and Mr(H₂SO₄) = 2 + 32 + (4×16) = 98
Concentration in g/dm³ is 0

045 x 98 = 4.41g/dm³
How to carry out a titration

The diagrams show the typical apparatus (1)-(6) used in manipulating liquids and on the left a brief three stage description of titrating an acid with an alkali:

Volumetric Analysis (Titrations)

A titration is a laboratory procedure where a measured volume of one solution is added to a known volume of another reagent until the reaction is complete

The operation is an example of volumetric (titrimetric) analysis

The equivalence point is usually shown by the colour change of an indicator and is known as the end-point

Volumetric analysis is a powerful technique, which is used in a variety of ways by chemists in many different fields

Practical Aspects

The practical aspects of titrations are required in the assessment of practical skills

Knowledge of the techniques of titrations is expected but it would be normal to assume that all apparatus would have been washed with distilled/deionised water

The description should include which reagent is placed in the burette, name of indicator (but no reason for choice of indicator), detection of endpoint and what should be observed, and repetition for accuracy

When you have finished this section you should be able to:

Use of a Volumetric Flask

To prepare a solution of precisely known concentration (a standard solution), a definite amount of solute must be dissolved in a solvent to give a definite volume of solution

The definite amount of material is measured by weighing, and the definite volume of solution prepared in a volumetric flask

A volumetric flask contains a definite volume when correctly filled to the calibration mark at the temperature stated on the flask

Tip the solid from a weighing bottle into a large (250 cm₃) beaker and add about 50 cm₃ of distilled water from a wash bottle

Stir well with a glass rod to dissolve

Take great care not to lose any of the solution and remember to wash the solution off the stirring rod back into the beaker

Rinse out the volumetric flask with distilled water and pour the cold solution into the flask through a clean filter funnel

Wash out the beaker several times and add all the washings to the flask

Now fill the flask to within about 1 cm of the calibration mark on the neck

Finally add water dropwise until the meniscus just rests on the calibration mark

Stopper the flask and invert a number of times to thoroughly mix the contents

Use of a Pipette
The pipette is designed to deliver a definite fixed volume of liquid when correctly filled to its calibration mark

Before use, a pipette must be washed out with the solution it is to measure

To fill the pipette, use a safety filler to suck solution up a few centimetres above the calibration mark

Let the solution down until the bottom of the meniscus just touches the calibration mark

For a titration the contents of the pipette are run into a conical flask, which has been well washed with distilled water

Allow the pipette to drain for about 20 seconds, then touch the tip to the surface of the liquid in the conical flask

The volume of solution delivered by the pipette is known as the aliquot

Use of a Burette

The burette is designed to deliver definite but variable volumes of liquid

First rinse out the burette with the solution it is to contain

Clamp the burette vertically in a stand

Fill the burette carefully using a beaker and a filter funnel

Open the tap briefly to fill the burette below the tap making sure there are no trapped air bubbles

Read the burette scale by observing the position of the bottom of the liquid meniscus, making sure your eyes are level with the graduation mark

To observe the meniscus more clearly, hold a white card behind the burette

Record the volume reading to the nearest 0.05 cm₃

Some common indicators

Titration Technique

When performing a titration, place the conical flask containing the aliquot on a white tile under the burette so that the tip of the burette is inside the mouth of the conical flask to avoid splashing

Add a few drops of a suitable indicator to the solution in the conical flask

First perform a ‘rough’ titration by taking the burette reading and running in the solution in approximately 1 cm₃ portions, while swirling the flask vigorously

When the end-point is reached, as shown by the indicator changing colour, quickly close the tap

The new burette reading will give you a rough idea (to within about 1 cm₃) of the volume to be added

Now repeat the titration with a fresh aliquot

As the rough end-point volume is approached, add solution from the burette one drop at a time until the indicator changes colour

Record the volume

The volume run out from the burette to reach the end point is known as the titre

Recording Titration Results

The results of a titration should be recorded;

Record the titration results in the form of a table

Accuracy

Record burette readings to the nearest 0.05 cm₃ (approximately 1 drop)

Consecutive titrations should agree to within 0.10 cm₃ and, strictly, you should repeat the titrations until this is achieved

However you may not have either the time or materials available to do this

With practice, your technique should improve so that you should not need to do more than 4 titrations (1 trial + 3 accurate)

Calculate and use the mean (average) of the two (or preferably three) closest consecutive readings and quote this to the nearest 0.05 cm3

What do examiners look for in your answer sheet?

Calculating the Concentration of a Solution from Titration Data

When you have finished this section you should be able to:

14.0.0 Organic Chemistry 1

Products from Oil

Coal, Oil and Natural Gas Formation – Fossil Fuels

Just as coal has formed by the action of heat and pressure on the remains of trees and plants on land over millions of years, so oil and natural gas have formed by the action of heat and pressure on the remains of sea plants and animals over millions of years

The remains were buried in sediments which excluded the air (kept out oxygen) and stopped them decaying

More sediment buried the remains deeper and deeper until pressure and heat eventually turned them into coal, oil and natural gas

They are called fossil fuels because they are buried underground (from Latin fossilis – dug up)

Fossil fuels are a finite resource and non-renewable

The oil deposits are formed in porous rock sediments

Porous rock has pores in it

Pores are small holes (see for example sandstone)

The small holes allow the oil and natural gas to pass through the rock and rise until they are stopped by a layer of non-porous rock

Non-porous rock (for example shale) has no holes, and acts as a barrier to prevent the oil and natural gas rising

The oil and natural gas become trapped underground

The oil is called crude oil (or petroleum, from Latin – rock oil), and has natural gas in it or in a pocket above it trapped by non-porous rock

Drilling through the rock allows the oil and gas to escape to the surface

Natural gas is mostly methane (CH₄)

Crude oil is a mixture of substances (mostly hydrocarbons)

So what are Hydrocarbons?

Crude oil is a mixture of substances which are mostly hydrocarbons

A hydrocarbon is a compound containing hydrogen and carbon only

Since crude oil is a mixture of different hydrocarbon compounds, the different hydrocarbons will have different boiling points

A sample of crude oil will therefore have a range of boiling points, and the mixture can be separated by fractional distillation

Fractional Distillation of Crude Oil

Naming the fractions

The hydrocarbon fractions are mainly alkanes

Crude oil is heated until it boils and then the hydrocarbon gases
are entered into the bottom of the fractionating column

As the gases go up the column the temperature decreases

The hydrocarbon gases condense back into liquids
and the fractions are removed from the sides of the column

The smaller the hydrocarbon molecule,
the further it rises up the column before condensing

The fractionating column operates continuously

The temperatures shown are approximate

A sample of crude oil may be separated in the laboratory
by fractional distillation

The collection vessel is changed
as the temperature rises to collect the different fractions

Naming hydrocarbons

Hydrocarbons are named
according to the number of carbon atoms in the molecule

Meth is pronounced meeth (like teeth),
Eth is pronounced eeth (like teeth),
Prop is pronounced prope (like rope),
But is pronounced bute (like beauty)

Pent is pronounced pent (like pentagon)

Hex is pronounced hex (like hexagon)

Hept is pronounced hept (like heptagon)

Oct is pronounced oct (like octagon)

The hydrocarbon fractions are mainly alkanes

Properties of Different Fractions

The different hydrocarbon fractions obtained from crude oil
condense at different temperatures

The larger the hydrocarbon molecule (the more carbon atoms it has)

1) The higher the condensing temperature (the higher the boiling point)

2) The more viscous it is (it takes longer to flow – like syrup)

3) The less volatile it is (it evaporates less quickly)

4) The less flammable it is (it does not set fire so easily)

Gases from volatile hydrocarbons are denser than air
and pose a fire hazard at ground level

This is why ignition sources
(such as smoking) are not allowed at petrol stations

Families of organic compounds

Homologous series

Organic compounds belong to different families, though all are based on carbon C, hydrogen H, and other elements such as oxygen O and nitrogen N etc The compounds in each family have a similar chemical structure and a similar chemical formula

Each family of organic compounds forms what is called a homologous series

Different families arise because carbon atoms readily join together in chains (catenation) and strongly bond with other atoms such as hydrogen, oxygen and nitrogen

The result is a huge variety of ‘organic compounds’

The name comes from the fact that most of the original organic compounds studied by chemists came from plants or animals

A homologous series is a family of compounds which have a general formula and have similar chemical properties because they have the same functional group of atoms (e.g C=C alkene, C-OH alcohol or -COOH carboxylic acid)

Members of a homologous series have similar physical properties such as appearance, melting/boiling points, solubility etc but show trends in them e.g steady increase in melting/boiling point with increase in carbon number or molecular mass

The molecular formula represents a summary of all the atoms in the molecule

The structural or displayed formula shows the full structure of the molecule with all the individual bonds and atoms shown (though there are different ‘sub-styles’ of varying detail, see below)

What are alkanes?

These are obtained directly from crude oil by fractional distillation

They are saturated hydrocarbons and they form an homologous series called alkanes with a general formula CnH₂n+2 Saturated hydrocarbons have no C=C double bonds, only carbon-carbon single bonds, and so has combined with the maximum number of hydrogen atoms

i.e no more atoms can add to it

Alkanes are the first homologous series

Examples of alkanes are:

The gases Methane CH₄, ethane C₂H₆, propane C₃H₈, butane C₄H₁₀

Liquids Pentane C₅H₁₂, hexane C₆H₁₄ C7H₁₆ etc

The Names of all alkanes end in …ane

Names

Alkanes are the simplest homologous series of compounds and their names follow this pattern,

CH4 – methane

C2H6 – ethane

C3H8 – propane

C4H10 – butane

C5H12 – pentane

I.e they have a prefix (meth-, eth-, prop-, but-, etc), which depends on the number of carbon atoms in the molecule and a common suffix (-ane)

The general chemical form la for an alkane is CnH₂n+2

Structural formulae

As well as using a normal type of molecular formula to describe an organic molecule, they can be represented by drawing out their structure i.e by showing how the atoms are connected, or bonded to each other

In order to do this a few rules have to be followed;

(i) Carbon atoms must be bonded four times

(iii) Hydrogen atoms must bond only once

Name Molecular formula Structural formula

Isomerism

Isomerism occurs when two or more compounds have the same chemical formula but have different structures, e.g for the molecular formula C₄H₁₀ there are two possibilities – one ‘linear’ and one with carbon chain ‘branching’

Butane is linear while its branched isomer is methyl propane

As the number of carbon atoms increases, the number of possible isomers increases rapidly

All families or homologous series exhibit isomerism

Physical properties of alkanes

Physical state Lower molecular weight alkanes are gases

Methane, ethane, propane and butane are gases at ordinary room temperature

Higher alkanes up to those having 17 carbon atoms are liquids; higher alkanes are solids at room temperature

Melting and boiling points Homologous alkanes show increase in melting and boiling points

Similar to the behavior of elements in the same group in a periodic table

Solubility Alkanes, like all other organic chemicals are insoluble in water

They are however soluble in organic liquids

Alkanes are non-polar and are hence soluble in other non-polar liquids and not in water, as water is a polar molecule

Chemical Reactions of Alkanes

1.Substitutional reactions of alkanes

Alkanes are most inert of all homologous series

They are not very reactive unless burned

But they will react with strong oxidising chemicals like chlorine when heated or subjected to u.v light

A substitution reaction occurs and a chloro-alkane is formed e.g a hydrogen atom is swapped for a chlorine atom and the hydrogen combines with a chlorine atom forming hydrogen chloride

This process is called halogenation

The UV light causes the formation of free radical halogen atoms by providing enough energy for the bond between the two halogen atoms to break

A halogen atom attacks the alkane, substituting itself for a hydrogen atom

This substitution may occur many times in an alkane before the reaction is finished

2. Combustion

Alkanes, along with all other types of hydrocarbon, will burn in an excess of oxygen to give carbon dioxide and water only as the products,

e.g CH₄ (g) + 2O₂(g) CO₂(g) + 2H₂O(g)
in general,

CnH₂n+2(g) + (1.5n+0.5)O₂(g) nCO₂(g) + (n+1)H₂O(g)

If there is not enough oxygen present then instead of carbon dioxide, carbon monoxide, CO, is produced

Carbon monoxide is particularly toxic and absorbed into blood, through respiration, very easily

For domestic heating systems it is particularly important that enough air can get to the flame to avoid carbon monoxide being generated in the home

Car engines also require a lot of air and there is a lot of research going on to make the internal combustion engine more efficient, and so put out less carbon monoxide

3. Reactivity

Alkanes are saturated hydrocarbons

Molecules of saturated hydrocarbons contain only single bonds between all carbon atoms in the series

Hence their reactivity with other chemicals is relatively low

What are alkenes?

Hydrocarbons, which contain two hydrogen atoms less than the corresponding alkanes, are called alkenes

They have one double bond and are unsaturated carbon compounds

Alkenes cannot be obtained directly from crude oil

They can only be obtained by cracking of alkanes

Cracking

In industry the fractions obtained from the fractional distillation of crude oil are heated at high pressure in the presence of a catalyst to produce shorter chain alkanes and alkenes

E.g C₁₀H₂₂ C₅H₁₂ + C₅H₁₀

They are unsaturated hydrocarbons with a general formula CnH₂n

Unsaturated means the molecule has a C=C double bond to which atoms or groups can add after breaking the double bond

Alkenes all have a C=C double bond in their structure and their names follow this pattern

Their names end i.e ….ene

C2H4 – ethene

C3H6 – propene

C4H8 – butene

C5H10 – pentene

The general chemical formula for an alkene is CnH₂n

The general chemical formula for an alkene is CnH₂n

(2) Addition reactions of alkenes :

(i) Bromination

The double bond of an alkene will undergo an addition reaction with aqueous bromine to give a dibromo compound

The orange bromine water is decolourised in the process

E.g ethene reacts with bromine water to give 1,2-dibromoethane,

(ii) Hydrogenation

Alkenes may be turned into alkanes by reacting the alkene with hydrogen gas at a high temperature and high pressure

A nickel catalyst is also needed to accomplish this addition reaction

E.g ethene reacts with hydrogen to give ethane,

This reaction is also called saturation of the double bond

In ethene the carbon atoms are said to be unsaturated

In ethane the carbon atoms have the maximum number of hydrogen atoms bonded to them, and are said to be saturated

(iii) Oxidation

The carbon-carbon double bond may also be oxidised i.e have oxygen added to it

This is accomplished by using acidified potassium manganate (VII) solution at room temperature and pressure. The purple manganate (VII) solution is decolourised during the reaction

E.g ethene reacts with acidified potassium manganate (VII)(aq) to give ethan-1,2-diol,

(3) Addition polymerisation :

All alkenes will react with free radical initiators to form polymers by a free radical addition reaction

Some definitions

monomer – a single unit e.g an alkene

The alkene monomer has the general formula:

Where R is any group of atoms, e.g R=CH₃ for propene

The reaction progresses by the separate units joining up to form giant, long chains

Polymer– a material produced from many separate single monomer units joined up together

An addition polymer is simply named after the monomer alkene that it is prepared from

The structure above shows just 4 separate monomer units joined together

In a real polymer, however, there could be 1000’s of units joined up to form the chains

This would be extremely difficult to draw out and so the structure is often shortened to a repeat unit

There are 3 stages to think about when drawing a repeat unit for a polymer

1) Draw the structure of the desired monomer

2) Change the double bond into a single bond and draw bonds going left and right from the carbon atoms

3) Place large brackets around the structure and a subscript n and there is the repeat unit

Laboratory preparation of ethene gas

In the lab ethene is prepared by cracking kerosene or candle wax

Kerosene is poured over sand and this is kept at the bottom of a hard glass test tube

A few pieces of pumice stone or porcelain is kept a little distance away

The sand is slowly heated

After a while the porcelain portion of the test tube is heated

This is done alternately

The heated kerosene first vaporizes and then cracks

When the vapours pass over the hot porcelain, they crack again into smaller and smaller molecules

The gases are then passed over water

Ethene is collected by downward displacement of water

It can be understood that this method for collecting ethene gas does not give pure ethene gas

This is because from cracking, we get many types of molecules

All those, which are lighter than water and insoluble in water, will be collected

Ethene by dehydration of alcohols

To obtain pure ethene gas, another method is followed

This is from a chemical reaction with ethanol and concentrated sulphuric acid

The temperature of the mixture of ethanol or ethyl alcohol and concentrated sulphuric acid is increased to 160°C

The acid acts as a dehydrating agent and picks up a water molecule from the ethanol molecule, leaving the reaction product as ethene gas

The laboratory equipment to produce ethene gas is shown below

About 20 to 25 ml of ethanol is taken in a round bottomed flask

Concentrated sulphuric acid is added to it from a thistle funnel slowly

Heat is supplied from a Bunsen burner and the temperature of the flask is raised to 160°C

Ethene gas starts evolving and it can be collected over water by downward displacement of water

Uses of ethene

Ethylene glycol is used for making artificial fibbers like polyesters

These plastics are made from polymerization of ethene into polythene

Polythenes are used for making bags, electrical insulation, etc

Ethene is used artificial ripening of fruits such as mangoes, bananas, etc

What are Alkynes?

Hydrocarbons that have two carbon atoms in a triple bond are called alkynes

They are unsaturated bonds

Their general formula is CnH2n-2 and their names are derived from the alkanes by changing the ending “ane’ of the alkane by “yne”, for example, ethyne, propyne, butyne, etc

The simplest of alkynes has two carbon atoms in triple bond and is called ethyne

The table below gives names of the first three alkynes

Chemical properties of ethyne

1. Combustion: Ethyne burns in air with a sooty flame

It forms carbon dioxide and water and gives out heat

The sooty flame is due to higher amount of carbon in ethyne than in methane

All the carbon atoms cannot get oxidized while burning this makes the flame sooty

But if ethyne is burnt with a proper control, for example, if the gas is made to pass through a small nozzle, then it gets ample air mixture to burn completely

This type of complete combustion is used for acetylene lamps in industries

Acetylene lamps produce very luminous non-sooty flame

Ethyne combined well with oxygen can burn to give a flame whose temperature is 3000°C

This oxy-acetylene flame is used for welding metals, where very high temperatures are required

2. Reactivity: Alkynes are more reactive than the alkanes or alkenes due to the presence of unsaturated bonds

Such a reaction is called addition reaction

In an addition reaction, the alkynes will become an alkane

For example if ethyne is reacted with chlorine, it becomes 1,1,2,2 tetra-chloro-ethane

Similarly, addition reaction with bromine will give rise to 1,1,2,2, tetra-bromo-ethane

Bromine water decolorizes on reaction with ethyne

This is a prominent test for testing unsaturated nature of hydrocarbons

When hydrogen is added to ethyne, and heated in the presence of nickel, it becomes ethene and then proceeds to become ethane

The bonds become saturated

This is known as the process of hydrogenation

The addition of hydrogen to a double or triple bonded hydrocarbon leads to saturation of the bonds

When hydrochloric acid is added to ethyne, it becomes first chloro-ethene and then
1,1- dichloro-ethane

The reaction is shown below

Uses of ethyne

Hawkers use this as lamps

15.0.0 Nitrogen and Its Compounds
Nitrogen

Nitrogen is a colourless and odourless gas, N₂, which is insoluble in water

Although it does not support life, it is not poisonous

It reacts only with difficulty with other elements, requiring either high temperatures, a catalyst, or both, in order to form compounds

The most important of these are ammonia and ammonium salts, certain nitrogen oxides and nitric acid and its salts

Composition of air

The atmosphere is the gaseous envelope which surrounds the earth

This gas, air, is a mixture consisting of about 78% nitrogen and 21 % oxygen

Water vapour is present in variable amounts (up to 5%), and so the composition of unpolluted air is normally based on the dry gas mixture

The figures below are percentages of the normal constituents by volume

Nitrogen comprises about 78.1% of the earth’s atmosphere and it is the source of the commercial and industrial gas

Traces of other gases, notably He, Ne, Kr and Xe are also found, while near cities and industrial areas, all sorts of pollutants are also found

Air is liquefied, and the oxygen (about 20.9%) boiled off at -183 ºC, leaving liquid nitrogen (which boils at -196 ºC) behind

This process is known as fractional distillation

Preparation of Nitrogen from Air

Industrial preparation

The chief source of free nitrogen is atmospheric air and nitrogen is usually prepared from it

Air free from dust, water vapour and carbon dioxide is compressed in a compression chamber for liquefaction

Firstly, the pressure on the air is increased to about 200 atmospheres

It is then released through a spiral into a low-pressure area, where intense cooling of the air takes place

The cold air goes upwards and further cools the spiral that brings in a fresh batch of purified air

In this way the cold air in the spiral gets progressively cooled when released

This procedure continues and the cooling becomes gradually more and more intense

Ultimately, the cooling becomes so great that the temperature drops to nearly -200^oC

At this temperature the air condenses to form liquid air (Nitrogen becomes liquid at -196^oC)

Liquid air is then led into a chamber, and allowed to warm up, by absorbing heat from the atmosphere

The boiling point of nitrogen is -196^oC; when this temperature is reached, nitrogen starts boiling and the vapours (gas) is collected and packed

The liquid left behind is mainly oxygen, which has a higher boiling point of 183^oC

Prepararion from Ammonia and Ammonium Compounds

(i) By treating excess ammonia with chlorine, ammonium chloride and nitrogen are formed

The products obtained are bubbled through water

The vapours of ammonium chloride dissolve in the water while nitrogen is collected separately

(ii) By passing ammonia over heated metallic oxides like copper oxide and lead oxide

(Fig.12.3)

(iii) By burning ammonia in oxygen

Ammonia burns in oxygen to yield water vapour and nitrogen (Fig.12.4)

(v) By the action of heat on ammonium compounds: (ammonium dichromate)

Ammonium dichromate is an orange coloured crystalline substance

When heated it starts decomposing, with the evolution of heat

Sparks can be seen inside the test tube and therefore further heating is not necessary

The products of decomposition are, a green coloured solid of chromic oxide, water vapour and nitrogen gas (Fig.12.5)

However, collecting nitrogen by this method is difficult

As the reaction is accompanied by heat and light, it is quite violent

Also the green coloured fluffy chromic oxide gets sprayed all over and thrown out of the test tube

It is therefore difficult to control this reaction

Laboratory Preparation of Nitrogen

Method A

Nitrogen can be prepared from the air as shown

Air flows into the respirator and onto caustic soda which dissolves carbon dioxide gas

It is then passed through a heated combustion tube containing heated copper turnings which remove oxygen

Nitrogen is then collected over water

Traces of noble gases present in air still remain in the final product

Method B

Nitrogen can also be obtained by heating a mixture of sodium nitrite and ammonium chloride as shown

The gas collected by this method is purer than one in method A, even though it contains water vapour which could have been removed if the gas is passed through concentrated sulphuric acid before collection

In the laboratory, nitrogen is prepared by heating a mixture of ammonium chloride and sodium nitrite and a small quantity of water

If ammonium nitrite is heated by itself it decomposes to produce nitrogen gas

However, this reaction is very fast and may prove to be explosive

For safety, a mixture of ammonium chloride and sodium nitrite approximately in the ratio of 4:5 by mass, is heated mildly with a small quantity of water

The presence of water prevents ammonium chloride form subliming when heated

Initially, the two substances undergo double decomposition to form sodium chloride and ammonium nitrite

The ammonium nitrite so formed then decomposes to form nitrogen gas and water vapor

Nitrogen gas is collected by downward displacement of water

Uses of Nitrogen

(i) Nitrogen is used in high temperature thermometers where mercury cannot be used

This is because mercury boils at 356.7^oC and hence cannot be used in such thermometers

A volume of nitrogen is enclosed in a vessel and introduced into the region of high temperature

Depending upon the temperature, expansion of the nitrogen volume takes place

Then applying the gas equation, the temperature is calculated

(ii) Nitrogen mixed with argon is used in electric bulbs to provide an inert atmosphere

It helps in prevention of oxidation and evaporation of the filament of the bulb, giving it a longer life

(iii) It is used to produce a blanketing atmosphere during processing of food stuff, to avoid oxidation of the food

It is also used when food is being canned, so that microorganisms do not grow

(iv) It is used in metal working operations to control furnace atmosphere and in metallurgy to prevent oxidation of red-hot metals

(v) Nitrogen in the air helps as a diluting agent and makes combustion and respiration less rapid

(vi) It is used by the chemical, petroleum, and paint industries to provide inactive atmosphere to prevent fires or explosions

(vii) It is used in the industrial preparation of ammonia, which is converted into ammonium salts, nitric acid, urea, calcium cyanamide fertilizers etc

(viii) Liquid nitrogen is used as a refrigerant for food, for storage of blood, cornea etc in hospitals

Meat, fish etc

, can be frozen in seconds by a blast of liquid nitrogen, which can provide temperatures below -196oC

(ix) Liquid nitrogen is used in scientific research especially in the field of superconductors

(x) Nitrogen is essential for synthesis of proteins in plants

Proteins are essential for synthesis of protoplasm, without which life would not exist

(xi) Liquid nitrogen is used in oil fields, to extinguish oil fires

Summary

Physical Properties

Nitrous oxide is a linear molecule

It has a boiling point of -88 ºC, and a melting point of -102 ºC

It is colourless and has a faintly sweet smell

It is used as an anaesthetic, popularly called laughing gas

Nitric oxide:

Nitric oxide, NO, may be prepared by the action of dilute nitric acid on copper:

3Cu + 8HNO,₃ 3Cu (NO₃ )2 +2NO + 4H₂ O

It is a colourless gas, insoluble in water, which reacts with oxygen to form the brown gas nitrogen dioxide, NO₃:
O₃ NO + O₃ 2NO₃

Nitrogen dioxide:

Nitrogen dioxide, NO₂ is a planar molecule

3

Dissolving Ammonia in Water

Due to its high solubility, ammonia cannot be passed through water like many other gases

Ammonia is dissolved in water, as shown below

This arrangement is called funnel arrangement and its principle is the same as that discussed for HCl gas

The funnel arrangement prevents back suction of water, which can cause damage to the apparatus used

It provides larger surface area for dissolution of ammonia

A very strong solution of ammonia in water is called liquor ammonia

Ammonia can be obtained from it by boiling

Action with Acids

Ammonia reacts with the acids to form their respective ammonium salts

The ammonium salts appear as white fumes

Ammonia gas + acid -> ammonium salt

Uses of Ammonia

Following are the chief uses of ammonia:

1) Ammonia is used in the industrial preparation of nitric acid by Ostwald’s process

2) Fertilisers, such as ammonium sulphate, ammonium nitrate, ammonium phosphate, urea etc

are manufactured with the help of ammonia

3) It is used in the manufacture of other ammonium salts, such as ammonium chloride, ammonium carbonate, ammonium nitrite etc

4) It finds use in the manufacture of nitrogen compounds such as sodium cynamide, plastics, rayon, nylon, dyes etc

5) It is used in the manufacture of sodium carbonate by Solvay’s process

(Ammonia and carbon dioxide are treated with aqueous sodium chloride, crystals of sodium hydrogen carbonate are formed

They are heated to yield sodium carbonate)

6) Ammonia acts as refrigerant in ice plants

Evaporation of a liquid needs heat energy

About 17g of liquid ammonia absorb 5700 calories of heat from the surrounding water

This cools the water and ultimately freezes it to ice

7) Ammonia is used to transport hydrogen

Hydrogen is dangerous to transport, as it is highly combustible

So it is converted to ammonia, liquefied, transported and then catalytically treated to obtain hydrogen

8) Many ammonium salts are used in medicines

Inhaling the fumes produced by rubbing ammonium carbonate in the hands can revive people who have fainted

9) It is used as a cleansing agent

Ammonia solution emulsifies fats, grease etc

so it can be used to clean oils, fats, body grease etc from clothes

It is also used to clean glassware, porcelain, floors etc

10) It is used as laboratory reagent

Nitric acid:

Nitric acid is produced industrially from ammonia by the Oswald process

It is a strong acid, converting bases to salts called nitrates:
CuO + 2HNO₃ Cu(NOO₃)2 + HO₂O

Copper(11) nitrate

NaOH + HNOO₃ NaNOO₃ + HO₂O

Sodium nitrate

Nitric acid is also a strong oxidizing agent and may be reduced to nitric oxide or nitrogen dioxide:

Cu + 4HNOO₃ Cu (NOO₃)O₂ + 2H2O + 2NO2
3Cu + 8HNOO₃ 3Cu (NOO₃)O₂ + 2NO +4HO₂O

Pure nitric acid slowly decomposes to form water, nitrogen dioxide and oxygen

This causes the nitric acid to become yellow

The process is accelerated on heating:
4HNOO₃ 2H₂O + 4NOO₂ + OO₃

Oswald Process

Nitric acid is prepared in large scale from ammonia and air (Fig.6.14)

Reactants

Pure dry ammonia (1 volume) and air (10 volumes)

Reactions

1) 1st step – Catalytic oxidation of ammonia to form nitric oxide

2) 2nd step – Oxidation of nitric oxide to nitrogen dioxide

3) 3rd step – Absorption of nitrogen dioxide in water to give nitric acid

Catalyst

Platinum (for oxidation of NH₃)

Temperature

700o – 800oC

Reactions of Nitric Acid

Cuprous Oxide, Cu₂O reacts with dilute Nitric Acid, HNO₃, in the cold to form a solution of Cupric Nitrate, Cu (NO

)2, and Copper, Cu

Cu2O + 2 HNO₃ Cu (NO₃)₂ + Cu + H₂O

Cuprous Oxide, Cu₂O reacts with concentrated Nitric Acid, HNO₃, or with dilute Nitric Acid, HNO₃, on heating, when the Cuprous Oxide, Cu₂O dissolves with evolution of Nitric Oxide, NO

3Cu₂O + 14HNO₃ 6Cu (NO₃)₂ + 2NO + 7H₂O

Dinitrogen Pent oxide, N₂O₅, is best prepared by dehydrating concentrated Nitric Acid, HNO₃, by Phosphorus Pent oxide, P₂O₅

2 HNO₃ + P₂O₅ N₂O₅ + 2 HPO₃

Nitric Oxide, NO is prepared by the action of Copper, Cu, or Mercury, Hg, on dilute Nitric Acid, HNO₃, and was called Nitrous Air

3 Cu + 8 HNO₃ 3 Cu (NO₃)2 + 2 NO + 4 H₂O

Nitrogen dioxide, NO₂, is a mixed acid anhydride and reacts with water to give a mixture of nitrous and nitric acids

2 NO₂ + H₂ HNO₂ + HNO₃

If the solution is heated the nitrous acid decomposes to give nitric acid and nitric oxide

3 HNO₂ HNO₃ + 2 NO + H₂O

Sulphur Dioxide, SO₂, and Nitrogen Oxides, NOx, are toxic acidic gases, which readily react with the Water, H2O in the atmosphere to form a mixture of Sulphuric Acid, H₂SO₄, Nitric Acid, HNO₃, and Nitrous Acid, HNO₂,

The dilute solutions of these acids which result give rain water a far greater acidity than normal, and is known as Acid Rain

Nitrates are the salts of nitric acid, and are strong oxidising agents

The Oswald Process is the tree stage process by which Nitric Acid, HNO₃, is manufactured

Firstly, Ammonia, NH₃, is oxidised, at high temperature (900 0C

) over a platinum-rhodium catalyst, to form Nitrogen Monoxide, NO

4 NH₃ (g) + 5O₂ (g) 4 NO (g) + 6H₂O

The Nitrogen Monoxide, NO, cools and reacts with oxygen, O₂, to produce Nitrogen Dioxide, NO₂

2 NO (g) + O₂ 2 NO₂ (g)

Finally, the Nitrogen Dioxide, NO₂ reacts with Water and Oxygen, O₂, oxygen to produce Nitric Acid,

4 NO₂ (g) + 2 H₂O (l) + O₂ 4 HNO₃ (l)

Cu₂O + 2 HNO₃ Cu (NO₃)₂ + Cu + H₂O

Cuprous Oxide, Cu₂O reacts with concentrated Nitric Acid, HNO3, or with dilute Nitric Acid, HNO3, on heating, when the Cuprous Oxide, Cu₂O dissolves with evolution of Nitric Oxide, NO

3 Cu2O + 14 HNO₃ 6 Cu (NO₃)₂ + 2 NO + 7 H₂O

Nitrates:

Salts of metals with nitric acid are called nitrates

Most nitrates are soluble in water

The nitrates of alkali metals form nitrites when strongly heated:

2NaNO₃ 2NaNO₂ + O₂

The nitrate of other metals decompose on heating to form nitrogen dioxide and the metal oxide, or, in the case of some metals such as silver and gold, the pure metal, nitrogen dioxide, and oxygen:

2Pb(NO₃) 2PbO + 4NO₂ + O₂
2AgNO₃ 2Ag + 2NO₂ + O₂

Summary of Action of Heat on Nitrates

Generally compounds of very reactive metals such as sodium and potassium are more stable to heat than the metals lower down in the reactivity series of metals

Nitrogen pollution ??

16.0.0 Sulphur and Its Compounds

Sulphur

It takes the form of a yellow solid naturally and can be found in this state near volcanoes

Sulphur is also present in a number of metal ores, for example zinc blende (zinc sulphide, ZnS )

Sulphur has chemical symbol S

It has 16 protons and 16 neutrons

An atom of S is represented as 3216S

Sulphur is a non-metal and exists in the earth’s crust either as pure sulphur or as a metal-sulphide

Since S has 16 protons, it also has 16 electrons; the electronic configuration of S is 2, 8, 6

S is placed in Group VI A of the periodic table, just after phosphorus, and below oxygen

The reaction of S is similar to oxygen

Sulphur is found as a free element or in combined state in nature

Free sulphur is found in at a large depth below the earth’s surface

Metal sulphides such as Zn, Fe, Ag, Ca, Pb, Cu are found in abundant quantities

Mineral ores containing S are:

Sulphur is found as H₂ S gas in petroleum gas, coal gas

H2S is the familiar pungent smell of onions

It is present in hair, eggs, many proteins and wool

Extraction of pure sulphur : Frasch process

Since sulphur in free state is found at depths of more than 150 to 300 meters below the earth’s surface, the method of extraction of sulphur differs from other metal or non-metal extractions

Sulphur’s relatively low melting point (115°C) is utilized in this process

This is known as the Frasch process

Here compressed super heated water (at 170°C) is pressed into a pipe which reaches up to the sulphur deposits

The sulphur here melts

Introducing hot compressed air through another pipe brings it up

The molten sulphur and water mixture is forced up and is collected in a settling tank

The sulphur is cooled and water is evaporated

The sulphur extracted in this way is more than 99% pure

The sulphur obtained by Frasch process is a yellow and brittle solid or powder

Physical properties of sulphur :
Since S has 6 electrons in its outermost shell, it needs 2 more electrons to complete its shell

But S combines with 7 other atoms to make a sulphur molecule that has a total of 8 sulphur atoms

Thus each S atom shares 2 electrons with its neighboring atom

The bonds are covalent in nature

A molecule of sulphur is represented as S8

It is a ringed molecule

The structure is shown below

These large molecules each have many electrons , so the Van der Waals forces are quite strong and the melting point is quite high (119oC)

There are two ways of packing the sulphur rings, so solid sulphur exists in two crystalline forms, called rhombic and monoclinic

Allotropes of sulphur

Sulphur is a yellow crystalline solid

It is tasteless and odourless

The melting point of S is 115°C

Sulphur is an insulator and is a poor conductor of heat and electricity

S is insoluble in water but is soluble in CS₂

Sulphur forms covalent bonds and shows allotropic forms

The allotropes have different crystalline shapes such as rhombic and monoclinic

There is another allotrope which has no shape and is called plastic sulphur

Vapours of sulphur are pungent and although not poisonous, they can cause health problems to humans

Chemical properties of sulphur :

1. Valence : Since S has 6 electrons in its outer shell

Hence S does not give off its electrons easily

It readily forms covalent bonds to complete its outer shell

It shows variable valence of 2 or 6 S is quite a reactive element and forms oxides, chlorides and sulphides readily

2. Action of oxygen: Sulphur reacts with oxygen and burns with a blue flame

It forms sulphur dioxide which is a colourless gas having a pungent smell

Sulphur dioxide forms an acidic solution, sulphurous acid, when dissolved in water i.e it turns damp blue litmus paper red

It will also react with oxidising agents to produce sulphate ions e.g Orange acidified dichromate (VI) ions are turned green and purple acidified manganese (VII) ions are turned colourless

3. S reacts with other non-metals also

In all cases sulphur has to be heated or boiled for the reaction to take place

4. Reaction with metals : Heated S reacts with metals like Fe, Cu, Zn, Sb directly to give metal-sulphide

A mixture of powdered zinc and sulphur, when heated up to a high temperature, will react together to produce an extremely exothermic change

A few reactions are shown below

5. Reaction with acids : S is oxidized by strong concentrated oxidizing acids such as sulphuric acid and nitric acid

In both the reactions S acts as a reducing agent

Effect of heat on sulphur:

A sulphur molecule consists of 8 atoms in a ring form

When heated, S melts at 115°C and a pale yellow liquid is formed

The S8 ringed molecules are connected to other molecules in a long chain

On heating, the long chain breaks up

The individual molecules can slip over each other when melted

On further heating, the liquid becomes dark brown and viscous

When the temperature rises beyond 160°C, the intra-molecular bonds break

Sulphur boils at 444°C

At this temperature the large molecule breaks up into pieces of S2 molecule

This molecule is pale yellowish-brown in colour

The vapours of S contains S2 molecules

Vulcanization of rubber :

Natural rubber is a soft and sticky solid

Rubber is a long chain polymer made out of isoprene (2 – methyl butadiene) monomer

The long molecule forms a coil like structure

The unique property of rubber is that it is elastic

When rubber is stretched, the molecular bonds can be extended out

When released, the molecules coil back to their original shape

Natural rubber looses its rubber-like properties at temperatures above 60°C

Also its wear resistance and tensile strengths are low

The process of vulcanization can improve the quality of rubber

Raw rubber is heated with sulphur during vulcanization

This makes the rubber hard, more elastic and strong

During the process of vulcanization, the sulphur atoms attach themselves to extra loose bonds in the rubber molecule and also cross-link the molecules

The cross-linking locks the molecules in place and prevents slipping

Thus making the vulcanized rubber more strong

Vulcanized rubber is non-sticky and has higher elasticity

It does not loose its properties easily and can be used in a temperature range of – 40°C to 100°C

Uses of sulphur :

The Properties Of Sulphur:

Sulphur Dioxide

Moist sulphur dioxide (or sulphurous acid) is a reducing agent

This fact is used as a test for the detection of sulphur dioxide

1. There is a colour change from purple (pink in dilute solution) to colourless on the addition of the gas to a solution of potassium manganate (VII) (permanganate)
2MnO4- + 5SO

There is a colour change from orange to blue on adding the gas to a solution of potassium dichromate (VI)

Cr2O72- + 3SO₂ +2H+ 2Cr₃+ + 3SO42- + H₂O

Sulphurous acid and Sulphites
Sulphur dioxide dissolves in water forming sulphuric (IV) acid (sulphurous acid)

SO₂ (aq) + H₂O (l) H₂SO₃ (aq)
This is a weak dibasic acid and ionises producing hydrogen ions and sulphite SO32- ions

H₂SO₃ 2H+ + SO₃^2-

1. Sulphites give sulphur dioxide on heating with dilute acids

Na₂SO₃+ 2HCl NaCl + SO₂ + H₂O

2. With barium chloride they give a white precipitate of barium sulphite which is soluble in dilute hydrochloric acid

Ba₂+ (aq) + SO₃^2- (aq) BaSO₃ (s)
(White precipitate)
BaSO₃ (s) + 2HCl BaCl₂ + SO₂ (g) + H₂O
This reaction is used as a test for sulphite ions in solution

Sulphur Dioxide

Sulphur dioxide is a colourless gas, about 2.5 times as heavy as air, with a suffocating smell, faint sweetish odour

Occurrence
Sulphur dioxide occurs in volcanic gases and thus traces of sulphur dioxide are present in the atmosphere

Other sources of sulphur dioxide are the combustion of the iron pyrites which are contained in coal

Sulphur dioxide also results from various metallurgical and chemical processes

Preparation of Sulphur Dioxide

Sulphur dioxide is prepared by burning sulphur in oxygen or air

S + O₂ SO₂

Sulphur dioxide is usually made in the laboratory by heating concentrated sulphuric acid with copper turnings

Cu + 2 H₂SO4 CuSO4 + SO₂ + 2 H₂O

Sulphur dioxide is released by the action of acids on sulphites or acid sulphites (e.g by dropping concentrated sulphuric acid into a concentrated solution of sodium hydrogen sulphite)

Sulphur dioxide is a colourless liquid or pungent gas, which is the product of the combustion of sulphur on air

Its melting point is -72.7 0C, its boiling point is -100C and its relative density is 1.43

Sulphur Dioxide is an acidic oxide which reacts with water to give sulphurous acid

SO₂ + H₂O H₂SO₃
Sulphur dioxide is a good reducing and oxidising agent

Summary

Uses of Sulphur Dioxide

a). Sulphur dioxide is a reducing agent and is used for bleaching and as a fumigant and food preservative

b). Large quantities of sulphur dioxide are used in the contact process for the manufacture of sulphuric acid

c). Sulphur dioxide is used in bleaching wool or straw, and as a disinfectant

d). Liquid sulphur dioxide has been used in purifying petroleum products

e). It is used as a bleaching agent

The Contact Process

This page describes the Contact Process for the manufacture of sulphuric acid, and then goes on to explain the reasons for the conditions used in the process

It looks at the effect of proportions, temperature, pressure and catalyst on the composition of the equilibrium mixture, the rate of the reaction and the economics of the process

A brief summary of the Contact Process

The Contact Process:

Making the sulphur dioxide

This can either be made by burning sulphur in an excess of air:

or by heating sulphide ores like pyrite in an excess of air:

In either case, an excess of air is used so that the sulphur dioxide produced is already mixed with oxygen for the next stage

Converting the sulphur dioxide into sulphur trioxide

This is a reversible reaction, and the formation of the sulphur trioxide is exothermic

A flow scheme for this part of the process looks like this:

Converting the sulphur trioxide into sulphuric acid
This can’t be done by simply adding water to the sulphur trioxide – the reaction is so uncontrollable that it creates a fog of sulphuric acid

Instead, the sulphur trioxide is first dissolved in concentrated sulphuric acid:

The product is known as fuming sulphuric acid or oleum

This can then be reacted safely with water to produce concentrated sulphuric acid – twice as much as you originally used to make the fuming sulphuric acid

Summary

Explaining the conditions

The proportions of sulphur dioxide and oxygen

The mixture of sulphur dioxide and oxygen going into the reactor is in equal proportions by volume

Avogadro’s Law says that equal volumes of gases at the same temperature and pressure contain equal numbers of molecules

That means that the gases are going into the reactor in the ratio of 1 molecule of sulphur dioxide to 1 of oxygen

That is an excess of oxygen relative to the proportions demanded by the equation

According to Le Chatelier’s Principle, Increasing the concentration of oxygen in the mixture causes the position of equilibrium to shift towards the right

Since the oxygen comes from the air, this is a very cheap way of increasing the conversion of sulphur dioxide into sulphur trioxide

Why not use an even higher proportion of oxygen? This is easy to see if you take an extreme case

Suppose you have a million molecules of oxygen to every molecule of sulphur dioxide

The equilibrium is going to be tipped very strongly towards sulphur trioxide – virtually every molecule of sulphur dioxide will be converted into sulphur trioxide

Great! But you aren’t going to produce much sulphur trioxide every day

The vast majority of what you are passing over the catalyst is oxygen which has nothing to react with

By increasing the proportion of oxygen you can increase the percentage of the sulphur dioxide converted, but at the same time decrease the total amount of sulphur trioxide made each day

The 1: 1 mixture turns out to give you the best possible overall yield of sulphur trioxide

The temperature

Equilibrium considerations

You need to shift the position of the equilibrium as far as possible to the right in order to produce the maximum possible amount of sulphur trioxide in the equilibrium mixture

The forward reaction (the production of sulphur trioxide) is exothermic

According to Le Chatelier’s Principle, this will be favoured if you lower the temperature

The system will respond by moving the position of equilibrium to counteract this – in other words by producing more heat

In order to get as much sulphur trioxide as possible in the equilibrium mixture, you need as low a temperature as possible

However, 400 – 450°C isn’t a low temperature!

Rate considerations

The lower the temperature you use, the slower the reaction becomes

A manufacturer is trying to produce as much sulphur trioxide as possible per day

It makes no sense to try to achieve an equilibrium mixture which contains a very high proportion of sulphur trioxide if it takes several years for the reaction to reach that equilibrium

You need the gases to reach equilibrium within the very short time that they will be in contact with the catalyst in the reactor

The compromise

400 – 450°C is a compromise temperature producing a fairly high proportion of sulphur trioxide in the equilibrium mixture, but in a very short time

The pressure

Equilibrium considerations

Notice that there are 3 molecules on the left-hand side of the equation, but only 2 on the right

According to Le Chatelier’s Principle, if you increase the pressure the system will respond by favouring the reaction which produces fewer molecules

That will cause the pressure to fall again

In order to get as much sulphur trioxide as possible in the equilibrium mixture, you need as high a pressure as possible

High pressures also increase the rate of the reaction

However, the reaction is done at pressures close to atmospheric pressure!

Economic considerations

Even at these relatively low pressures, there is a 99

5% conversion of sulphur dioxide into sulphur trioxide

The very small improvement that you could achieve by increasing the pressure isn’t worth the expense of producing those high pressures

The catalyst

Equilibrium considerations

The catalyst has no effect whatsoever on the position of the equilibrium

Adding a catalyst doesn’t produce any greater percentage of sulphur trioxide in the equilibrium mixture

Its only function is to speed up the reaction

Rate considerations

In the absence of a catalyst the reaction is so slow that virtually no reaction happens in any sensible time

The catalyst ensures that the reaction is fast enough for a dynamic equilibrium to be set up within the very short time that the gases are actually in the reactor

Properties of Sulphuric Acid

Sulphuric acid is a dense, oily liquid once known as oil of vitriol

Pure sulphuric acid is almost twice as dense as water (1.98 g cm-2)

As water is added the density drops

Car batteries contain concentrated sulfuric acid

As the battery is discharged, the concentration of the acid falls

By measuring the density of the acid the driver can check whether the battery is flat or not

Action as an oxidising agent

It behaves as an oxidising agent only when hot and concentrated:

Cu + 2H₂SO₄ CuSO₄ + H₂O + SO₂
The sulphuric acid is reduced to sulphur dioxide

Action as a dehydrating agent

Concentrated sulphuric acid has a great affinity for water

(It is important when diluting the concentrated acid to add the acid to water and NEVER water to acid

) The reaction is highly exothermic

So great is its affinity for water that it can dehydrate compounds containing hydrogen and oxygen:

acid
It is used for drying gases, especially SO₂ and HCl, but cannot be used to dry a reducing gas such as H₂S or an alkaline gas such as NH₃

Action as a dehydrating agent

The properties of acids are due to the hydrogen ions in solution

Concentrated sulphuric acid contains molecules, rather than ions

Since it contains very few hydrogen ions it does not react significantly with metals and can safely be stored in steel containers

A piece of magnesium ribbon does not dissolve in concentrated sulphuric acid

Diluted with water, sulphuric acid behaves as a typical acid:

Industrial uses

17.0.0 Chlorine and Its Compounds
Chlorine

Halogen is elements in group (vii) of the periodic table

Chlorine is a halogen as well as fluorine, bromine, iodine and astatine

Chlorine has a symbol 35

5 Cl because it is made up of two isotopes 37 Cl and 35 Cl

It has an electronic arrangement of 2:8:7, hence justifying its position in group (vii)

Laboratory preparation

In order to convert hydrogen chloride to chlorine, it is necessary to remove hydrogen

Removal of hydrogen is oxidation

A powerful oxidizing agent such as manganese (IV) oxide converts hydrogen chloride (HCl) to chloride (Cl₂)

The most common laboratory method for preparation of Chlorine is to heat of Manganese Dioxide with concentrated Hydrochloric Acid

The gas is bubbled through water to remove any traces of hydrochloric gas that may be present and then it is dried by bubbling it through concentrated sulphuric acid

Chlorine may also be prepared by dropping cold concentrated Hydrochloric Acid on crystals of Potassium Permanganate

2 KMnO₄ + 16 HCl 2 MnCl₂ + 2 KCl + 8 H₂O + 5 Cl₂

The gas is also bubbled through water to remove any traces of Hydrochloric Acid gas that may be present and then it is dried by bubbling it through concentrated Sulphuric Acid

Manufacture of Chlorine

Membrane cell

Chlorine is manufactured industrially as a by-product in the manufacture of Caustic Soda by the electrolysis of brine

2 NaCl + 2 H₂O Cl₂ + H₂ + 2 NaOH

The membrane cell has titanium anode and a nickel cathode

Titanium is chosen because it is not attacked by chlorine

The anode and the cathode compartments are separated by an ion exchange membrane

The membrane is selective; it allows Na+ ions, H+ and OH- ions to flow but not Cl- ions

These ions cannot flow backwards, so products are kept separate and cannot react with each other

At anode, the Cl- ions are discharged more readily than OH- ions because they are in higher concentration and are hence preferred

2 Cl- Cl₂ + 2e-
A pale green gas is seen coming off at the anode
At cathode, it is the H+ ions that accept electrons, as sodium is more reactive than hydrogen

2 H+ + 2e- H₂

Bubbles of hydrogen are seen at the cathode

The remaining ions of Na+ and OH- join up and come off as sodium hydroxide, NaOH

Products and uses

Properties of chlorine

Test for Chlorine Gas, Cl₂(g)

1) Will turn moist litmus or universal indicator paper red,
and then bleach it white

(2) Is green-yellow in colour

(3) Has a pungent choking smell and is poisonous

It is twice as dense as air

(4) Will put out a lit splint

Collecting Chlorine

Chlorine is denser than air and can be collected by downward delivery or using a gas syringe

Reactions

1.Chlorine is a highly reactive element, and undergoes reaction with a wide variety of other elements and compounds

2.Chlorine is a good bleaching agent, due to its oxidising properties

3.Chlorine is soluble in water (which solution is called Chlorine Water) and this loses its yellow colour on standing in sunlight, due to the formation of a mixture of Hypochlorous Acid and Hydrochloric Acid

Cl₂ + H₂O HOCl + HCl

4.Chlorine gas supports the vigorous combustion of many elements to form their chlorides

For example, Sulphur and Phosphorus burn in the gas

Cl₂ + S SCl₂
Cl₂ + P PCl₃ + PCl₅

Bleaching Action

If chlorine is passed through water, it forms two acids, hydrochloric acid

Cl₂ (g) + H₂O (l) HCl (aq) + HOCl (aq)

Hypochlorous acid (the second acid) is the source of oxygen and is responsible for the bleaching of chlorine

HClO (aq) + Dye HCl (aq) + oxidized Dye
(Coloured) (Colourless)

It is important to wash bleached clothes thoroughly to remove hydrochloric acid formed after the process

Reaction of Chlorine with Hydrogen

A mixture of Chlorine and Hydrogen explodes when exposed to sunlight to give Hydrogen Chloride

In the dark, no reaction occurs, so activation of the reaction by light energy is required

Hydrogen and chlorine gas also combine directly in presence of sunlight

A jar of hydrogen is inverted and placed on a jar containing chlorine in the sun

Soon hydrogen chloride is formed

In diffused sunlight, the reaction slows down, and in the dark it is very slow

Cl₂ + H₂ 2 HCl
Hydrogen chloride is highly soluble in water

It dissolves to form hydrochloric acid

This reaction can be used to produce hydrochloric when the hydrogen chloride gas produced is dissolved in water as shown

2. Reaction of Chlorine with Non-Metals

Chlorine combines directly with most non-metals (except with Nitrogen, Oxygen and Carbon, C)

3. Reaction of Chlorine with Metals

Thin foils of metals like sodium, copper, etc. when plunged into a jar of chlorine gas catch fire spontaneously to form their respective chlorides

With yellow phosphorous

Yellow phosphorus first melts and then catches fire spontaneously when introduced into a jar of chlorine gas

It forms thick white fumes of phosphorus (III) chloride and phosphorus (V) chloride

Reaction with hydrogen sulphide

On passing chlorine and hydrogen sulphide through separate vents in a upright combustion tube hydrogen sulphide gets oxidised to hydrogen chloride and sulphur

Hydrogen chloride comes out through the middle tube

If chlorine is passed through a solution of hydrogen sulphide in water the solution turns turbid due to the formation of free sulphur

Reaction with aqueous sodium sulphite

When chlorine is passed through an aqueous solution of sodium sulphite it gets converted to sodium sulphate

Chlorine first reacts with the water to form nascent oxygen

Displacement of the Halogens by Chlorine

Halogens are the most electronegative elements

Fluorine is the most electronegative, followed by chlorine then bromine and then iodine

The relative reactivity of the halogens, as described in group trends, can be shown by displacement reactions

These are similar to the metal displacement reactions

For example, Bromine gas bubbled through a solution of potassium iodide in water will displace (take the place of) the less reactive iodine, forming iodine and potassium bromide

Bromine + potassium iodide potassium bromide + iodine

Br2(g) + 2KI(aq) 2KBr(aq) + I₂(s)

Similarly, chlorine will displace less reactive halogens

Chlorine will displace both bromine and iodine from the appropriate salt

This can be used in the extraction of bromine from sea water

Chlorine + potassium iodide potassium chloride + iodine

Cl2(g) + 2KI(aq) 2KCl(aq) + I2(s)

The equations can be written in terms of ions (called ionic equations)

For example, the last equation can be written as

Cl2(g) + 2I-(aq) 2Cl-(aq) + I2(s)

Potassium iodide, on the left, exists as potassium ions (K+) and iodide ions (I-) and potassium chloride, on the right, exists as potassium ions (K+) and chloride ions (Cl-)

Potassium ions (or other metal ions) can be left out of the ionic equation because they do not take part in the reaction

They are called ‘spectator ions’, as though they just sit back and watch!

Fluorine being the most electronegative halogen can displace all the other elements of this group (chlorine, bromine and iodine) from the aqueous solution of their salts

Chlorine can displace bromine and iodine from bromides and iodides respectively

However, it cannot displace fluorine from fluorides

Bromine can displace iodine from iodides

But it cannot displace chlorine or fluorine from chlorides or fluorides respectively

Iodine being the least electronegative of the halogens cannot displace any other halogen from their respective halides

Summary of Displacement Reactions

Bleaching Action of Chlorine

Chlorine bleaches (removes the color) organic colors by the process of oxidation in presence of moisture

The bleaching action takes place in few steps:

a) Chlorine first dissolves in water to give a mixture of hydrochloric acid and hypochlorous acid

b) As mentioned earlier hypochlorous acid is very unstable and decomposes to give hydrochloric acid and nascent oxygen

c) The nascent oxygen oxidises the coloring matter to colorless matter thereby bleaching them

Bleaching by chlorine is permanent

Chlorine bleaches cotton fabrics, wood pulp litmus etc

However, it is not used to bleach delicate articles such as silk, wool etc

, as it is a strong bleaching and oxidizing agent

This dual action will damage the base material

You must have noticed that in the very first stage, presence of water is essential to produce hypochlorous acid

The dry coloured fabric does not bleach

The coloured fabric soaked in water is bleached

This shows that chlorine only bleaches in the presence of water

If water is absent no bleaching can take place

Dry chlorine therefore does not bleach

Tests for Chlorine

1. Chlorine is a greenish yellow gas with a pungent and irritating odor

2. It turns wet blue litmus paper red and then bleaches it

3. Colored petals and the leaves of plants can be bleached by it

4. Chlorine turns wet starch potassium iodide paper blue by displacing the iodine from the potassium iodide, and causing iodine to turn the starch blue

Oxidizing Reaction of Chlorine

Chlorine is a strong oxidising agent

Chlorine oxidises Iron (II) Chloride, FeCl₂, to the salt containing Iron in the higher oxidation state Iron (III) Chloride, FeCl₃

This is possible because Iron has a variable valency

2 FeCl2 + Cl2 2 FeCl₃
Chlorine displaces the less electronegative Bromine and Iodine from their respective salts

Cl₂ + 2 KBr 2 KCl + Br₂ Chlorine removes Hydrogen from the hydrides of non-metals, forming Hydrogen Chloride, and leaving the non-metal element

Cl₂ + H₂S 2 HCl + S

Affinity for Hydrogen

Chlorine combines with free hydrogen to form hydrogen chloride

It can also react with the hydrogen present in other compounds such as water, ammonia, hydrocarbon, hydrogen sulphide etc

With water

Chlorine dissolves in water to form chlorine water

It slowly reacts with the water to form a mixture of hydrochloric acid and hypochlorous acid

Hypochlorous acid is very unstable and in presence of sunlight it decomposes to give hydrochloric acid and a nascent oxygen atom

Two such nascent oxygen atoms combine to form a molecule of oxygen

So if a solution of chlorine in water is exposed to sunlight as shown in figure 14

14, oxygen is formed

With ammonia

Depending on which of the two gases is in excess, chlorine reacts with ammonia in two ways

(a) When ammonia is in excess the final products are ammonium chloride and nitrogen

Hydrogen chloride thus produced reacts with excess of ammonia to produce ammonium chloride

The overall reaction can be written as:

(b) When chlorine is in excess the final product is an oily explosive liquid called nitrogen trichloride

With alkalis

Alkalis, at different temperatures and at different levels of concentration, behave differently with chlorine

(i) With cold dilute alkalis

Chlorine reacts with cold dilute alkalis to form their respective chlorides, hypochlorites and water

(ii) With hot concentrated alkalis

Chlorine reacts with hot concentrated alkalis to form their respective chlorides, chlorates and water

Uses

Chlorine is used

Hydrogen Chloride
Hydrogen chloride is an hydrogen compound of Chlorine

Chlorine is a highly reactive element and is mainly found in combined state

Its most important source is sodium chloride which is mainly found in large underground deposits, sea and lake such as lake Magadi

Sodium chloride is the main source of chlorine which is used to make hydrogen chloride

Preparation of hydrogen chloride

Hydrogen Chloride may be prepared in the laboratory by heating Concentrated Sulphuric Acid, with Sodium Chloride

Preparing and making a solution of hydrogen chloride

It is prepared industrially by the combustion of Hydrogen, H₂, in Chlorine, Cl₂

H₂ + Cl₂ -> 2 HCl

Other way of producing chlorine

Chlorine, removes Hydrogen, from the hydrides of non-metals, forming Hydrogen Chloride, and leaving the non-metal element

Cl₂ + H₂S 2 HCl + S

When Chlorine Water, (i.e a solution of Chlorine gas, in Water) in a flask inverted in a basin of the same liquid is exposed to bright sunlight, the Chlorine is decomposed and a solution of Hypochlorous Acid remains

H₂O + Cl₂ HCl + HClO

The Hypochlorous Acid, is not very stable and the solution readily decomposes, especially when exposed to sunlight, yielding Oxygen,

2 HClO 2 HCl + O₂
Chlorine is soluble in water (which solution is called Chlorine Water) and this loses its yellow colour on standing in sunlight, due to the formation of a mixture of Hypochlorous Acid, and Hydrochloric Acid

Cl2 + H₂O HOCl + HCl

Properties of Hydrogen Chloride

Hydrogen chloride in solution

Hydrogen chloride is a colourless fuming gas

The polar covalent gas Hydrogen Chloride is very soluble in Water

In aqueous solution, the molecule exists in ionic form, as the positively charged Hydrogen Ion, H+, and the negatively charged Chloride Ion, Cl-

HCl + aq H (+) + Cl (-)
Hydrogen Hydrogen Chloride
Chloride Ion Ion

Its solution in water turns blue litmus paper red

Hydrogen chloride has no effect on dry litmus paper as no ions are present in dry gas

The importance of water

The gas hydrogen chloride is made up of covalently bonded molecules

If the gas is dissolved in an organic solvent, such as methylbenzene, it does not show any of the properties of an acid

Dissolving hydrogen chloride in water and methylbenzene

For example, it does not conduct electricity or turn a piece of blue litmus paper red

However, when the gas is dissolved in water, a strongly acidic solution is produced

The acidic oxides of sulphur, phosphorus and carbon are the similar

They are covalent molecules when pure, but show acidic properties only when dissolved in water

Reaction with Group 1 Alkali Metals

Alkali metals burn very exothermically and vigorously when heated in chlorine to form colourless crystalline ionic salts e.g NaCl or Na+Cl-

This is a very expensive way to make salt! It’s much cheaper to produce it by evaporating sea water!

E.g sodium + chlorine sodium chloride

2Na(s) + Cl₂(g) 2NaCl(s)

The sodium chloride is soluble in water to give a neutral solution pH 7, universal indicator is green

The salt is a typical ionic compound i.e a brittle solid with a high melting point

Similarly potassium and bromine form potassium bromide KBr, or lithium and iodine form lithium iodide LiI

Again note the group formula pattern

Hydrochloric acid reacts with metals above hydrogen in the reactivity series

All of these metals react with hydrochloric acid liberating hydrogen which puts out a burning splint with a pop sound

Zn(s) + 2HCl (aq) -> ZnCl₂ (aq) + H₂ (g)

This is a common reaction for preparing hydrogen gas in the school laboratory

The hydrogen can then be collected over water

The salts produced can be obtained from the solution by filtration to obtain the filtrate which then transferred onto an evaporation dish for evaporation and crystallization

No hydrogen is liberated when the acid is reacted with lead, copper and mercury

Despite the fact that lead is above hydrogen in the reactivity series, the hydrogen chloride is not a strong enough oxidizing agent to liberate hydrogen

Reaction with bases

Hydrochloric acid is a strong acid as it is well ionized in solution

It neutralizes bases and alkalis forming salts and water

KOH (aq) + HCl (aq) -> KCl (aq) + H₂O (l)

CuO(s) + HCl (aq) -> CuCl₂ (aq) + H₂O (l)

FeO(s) + 2 HCl (aq) -> FeCl₂ (aq) + H₂O (l)

Reaction with carbonates

Hydrogen chloride reacts with carbonates and hydrogen carbonates producing carbon dioxide, salt and water

Carbon dioxide turns lime water milky

CaCO₃ (aq) + 2HCl (aq) CaCl₂ (aq) + CO₂ (g)+H₂O (l)

Ionic equation
CO_{3/sub>2- (aq) + 2H+ (aq) CO2(g) + H2O (l)}

NaHCO₃ (aq) + HCl (aq) NaCl (aq) + CO₂ (aq) + H₂O (l)

Ionic equation

HCO₃-(aq) + H+ (aq) CO₂(g) + H₂O(l)

Reaction with hydrogen H₂

Halogens readily combine with hydrogen to form the hydrogen halides which are colourless gaseous covalent molecules

E.g hydrogen + chlorine hydrogen chloride

H₂ (g) + Cl₂ (g) -> 2HCl (g)

The hydrogen halides dissolve in water to form very strong acids with solutions of pH1 e.g hydrogen chloride forms hydrochloric acid in water HCl(aq) or H+Cl-(aq) because they are fully ionised in aqueous solution even though the original hydrogen halides were covalent!

An acid is a substance that forms H+ ions in water

Bromine forms hydrogen bromide gas HBr (g), which dissolved in water forms hydrobromic acid HBr (aq)

Iodine forms hydrogen iodide gas HI (g), which dissolved in water forms hydriodic acid HI (aq)

Note the group formula pattern

Oxidation of hydrochloric acid

MnO₂ + 4 HCl MnCl₂ + 2 H₂O + Cl₂
The gas is bubbled through water to remove any traces of hydrochloric gas that may be present and then it is dried by bubbling it through concentrated sulphuric acid

Chlorine may also be prepared by dropping cold concentrated Hydrochloric Acid, on Potassium Permanganate

2KMnO₄ + 16HCl 2MnCl₂ + 2KCl + 8H₂O + 5Cl₂Summary

Defination of Common Terms in Chemistry

Word Definition:

Acid :A corrosive substance which has a pH lower than 7

Acidity is caused by a high concentration of hydrogen ions

Acid rain: Acid rain is caused when sulphur dioxide – released by the burning of coil and oil – dissolves in rainwater to form sulphuric acid

Activation energy: The minimum energy required for a collision between particles, in order for a reaction to occur

Addition polymer: Addition polymers are made when monomers (simple molecules) add together across a double bond

Addition reaction: A reaction in which a small molecule adds on across a double bond

Alkali: A base, which is soluble in water

Alkali metals The alkali metals are the elements in Group 1 of the periodic table

They have one electron in the outer shell

Alkanes: Alkanes are saturated hydrocarbons

This means that each carbon atom has four bonds to other atoms

Alkenes: Alkenes are unsaturated hydrocarbons with a double bond between the carbon atoms

Allotropes: Allotropes are structurally different forms of an element

They differ in the way the atoms bond with each other and arrange themselves into a structure

Because of their different structures, allotropes have different physical and chemical properties

Alloy: An alloy is a mof two or more elements, at least one of which is a metal

Anode: An anode is the electrode (electrical conductor) attached to the positive terminal of a battery

Atom: All elements are made of atoms

An atom consists of a nucleus containing protons and neutrons, surrounded by electrons

Atomic number: The atomic number (Z) of an element is the number of protons in the nucleus of the atom

Bases: Substances with a pH higher than 7, and which have a high concentration of hydroxyl ions

Bases react with acids to form a salt and water (called neutralisation)

Metal hydroxides, oxides and carbonates are all bases

Biocatalyst :A biocatalyst is an enzyme or microorganism that activates or speeds up a biochemical reaction

Boiling point: The temperature at which a liquid changes its state to gas

Brittle If something is brittle it is easily broken

Catalyst: A catalyst changes the rate of a chemical reaction without being changed by the reaction itself

Catalytic converter: A device in internal combustion engines which catalyses reactions converting harmful exhaust gases such as carbon monoxide into normal atmospheric gases

Cathode: A cathode is the electrode (electrical conductor) attached to the negative terminal of a battery

Chemical change: A chemical change involves new substances being formed and is very difficult to reverse

Combustion: Combustion is the process of burning by fire

Compound: A compound is a substance formed by the chemical union (involving bond formation) of two or more elements

Condensation: Condensation is a change of state in which gas becomes liquid by cooling

Conduct To allow electricity, heat or other energy forms to pass through

Conductor: An electrical conductor is a material which allows an electrical current to pass easily

It has a low resistance

A thermal conductor allows thermal energy to be transferred through it easily

Corrode: To deteriorate (get weaker) due to the action of water, air, or an acid

Covalent bond: A covalent bond between atoms forms when atoms share electrons to achieve a full outer shell of electrons

Covalent compounds: A covalent compound is a compound of neutral atoms in which the atoms are held together by covalent bonds

Covalent bonds between atoms form when atoms share electrons to achieve a full outer shell of electrons

Cracking: Cracking is the breaking down of large hydrocarbon molecules into smaller, more useful hydrocarbon molecules by vapourizing them and passing them over a hot catalyst

Crude oil : Crude oil is formed from the remains of small animals and plants that died and fell to the bottom of the sea

Their remains were covered by mud

As the sediment was buried by more sediment, it started to change into rock as the temperature and pressure increased

The plant animal remains were “cooked” by this process and changed into crude oil

Decomposition: A reaction in which substances are broken down, by heat, electrolysis or a catalyst

Denatured: If a protein is denatured, its structure and function is altered

This can be caused by heat, altered pH or by chemical agents

Displacement reactions: Displacement reactions happen when a more-reactive element replaces a less-reactive element in a compound

Distillation: Distillation is when we make a liquid evaporate and then condense the vapour back to a purer liquid

Double bond :A double bond is a covalent bond resulting from the sharing of four electrons (two pairs) between two atoms

Ductile: If a material is ductile it is capable of being drawn into thin sheets or wires without breaking

Electrode: Electrodes are conductors used to establish electrical contact with a circuit

The electrode attached to the negative terminal of a battery is called a negative electrode, or cathode

The electrode attached to the positive terminal of a battery is the positive electrode, or anode

Electrolysis: Electrolysis is the decomposition (separation or break-down) of a compound using an electric current

Electrolyte An electrolyte is a substance which in solution will conduct an electric current

Electron: An electron is a very small negatively charged particle found in an atom in the space surrounding the nucleus

Electrostatic: An electrostatic force is generated by differences in electric charge (i.e positive and negative) between two particles

It can also refer to electricity at rest

Element All atoms of an element have the same atomic number, the same number of protons and electrons and so the same chemical properties

Endothermic: In an endothermic reaction, energy is taken in from the surroundings

The surroundings then have less energy than they started with, so the temperature falls

Equilibrium: If the rate of the forward reaction and the rate of the back reaction in a reversible reaction are equal, the reaction is in equilibrium

Eutrophication: Eutrophication is the enrichment of a water body with nutrients – such as nitrates – which results in excessive growth of algae and other aquatic plants, leading to depletion of oxygen

Evaporation: Evaporation is a change in state in which a liquid becomes a gas (vapour); molecules near the surface of a liquid may leave the liquid to become a vapour

Exothermic: Heat energy is released in an exothermic reaction

We know this because the surroundings get warm

Filtrate: Filtrate is fluid that has passed through a filter

Formula: A formula is a combination of symbols that indicates the chemical composition of a substance

Fossil: Fossils are the remains of animals and plants from a past geological age, preserved in the Earth’s crust

Fossil fuels: Fossil fuels have been created over millions of years by the decay and compression of living things, particularly plants

Coal, gas and oil are fossil fuels

Fractional distillation: In fractional distillation a mixture of several substances, such as crude oil, is distilled and the evaporated components are collected as they condense at different temperatures

Groups: The groups of elements in the periodic table are the elements which have the same number of electrons in their outer shells and so have similar chemical properties

A group of elements all lie in same column in the periodic table

Halogens: The halogens are the elements in Group VII of the periodic table

They have seven electrons in the outer shell

Hydrocarbons: Hydrocarbons are a group of compounds, which contain the elements hydrogen and carbon

Ion :An ion is a charged particle formed by loss or gain of electrons

When atoms lose an electron they become a positive ion

When they gain an electron they become a negative ion

Ionic bond: An ionic bond forms between two atoms when an electron is transferred from one atom to the other, forming a positive-negative ion pair

Ionic compound: An ionic compound occurs when a negative ion (an atom that has gained an electron) joins with a positive ion (an atom that has lost an electron)

The ions swap electrons to achieve a full outer shell

Isotopes: Atoms of the same element that have different numbers of neutrons are called isotopes

Lattice: A lattice is a regular grid-like arrangement of atoms in a material

Liquefy: To liquefy means to become liquid, for example through heating a solid or cooling a gas

Lubricant: A substance used to reduce the friction between two solid surfaces

Mass: Mass is a measure of the amount of material in an object

It is measured in grams (g)

Mass number: The mass number (A) of an element is the number of protons plus the number of neutrons in the nucleus of the atom

Molecular compound: A molecular compound is made up of at least two different elements, which share electrons to form covalent bonds

Molecule: A molecule is a collection of two or more atoms held together by chemical bonds

It is the smallest part of a substance that displays the properties of the substance

Molten: Molten means reduced to liquid form by heating

It is mainly used to describe rock, glass or metal

Monomer: A monomer is a simple molecule

Natural gas: Natural gas is a fossil fuel formed from decaying plant and animal material

Neutralisation: Neutralisation is the reaction between an acid and a base to form a salt plus water

Neutron: A neutron is a particle that is found in the nucleus of an atom, has a mass approximately equal to that of a proton, and has no electric charge

Noble gases: The noble gases are the elements in Group 0/Group VIII of the periodic table

They have a full outer shell of electrons and so are unreactive

Nucleus: Found at the centre of an atom, the nucleus contains protons and neutrons

Ore: An ore is a rock containing enough quantities of a mineral that it is profitable to extract it

Oxidation: Oxidation is a reaction in which oxygen combines with a substance

Oxidation also means a loss of electrons

Periods: The periods of elements in the periodic table are the elements in which the same outer shell is being filled up

A period of elements in the periodic table all lie in the same row

Polymer: A polymer is a large molecule formed from many identical smaller molecules (monomers)

Polymerise: Polymerisation is the reaction in which many identical monomers are joined together to make a polymer

Product: A product is a substance formed in a chemical reaction

Protons: A proton is a small particle with a positive charge found in the nucleus of the atom

Radioactive: A radioactive isotope gives off (or is capable of giving off) radiant energy in the form of particles or rays by the spontaneous disintegration of the nucleus

Reactant: A reactant is a substance put together with another substance/substances to undergo a chemical reaction

Reactant: A reactant is a substance put together with another substance/substances to undergo a chemical reaction

Re-crystallisation: When rocks are heated and put under pressure new crystals can form

These are often elongated in the direction of least pressure

Redox: reaction Oxidation and reduction always take place together

The combined reaction is called a redox reaction

Reduction: Reduction is a reaction in which oxygen is removed from a substance

Reduction also means a gain in electrons

Relative atomic mass: The relative atomic mass is the number of times heavier an atom is compared to one twelfth of a carbon-12 atom

Relative mass: The relative mass is the number of times heavier a particle is, compared to another

Reversible reactions :Reversible reactions are chemical reactions, which can go both ways

The direction of the reaction depends on the condition of the reactants

Salt: A compound formed by neutralisation of an acid by a base (e.g a metal oxide) – the result of hydrogen atoms in the acid being replaced by metal atoms or positive ions

Sodium chloride: – common salt – is one such compound

Saturated: Filled to capacity

In a saturated hydrocarbon there are no more available bonds

Slag :Slag is the non-metallic by-product resulting from the extraction of metals such as iron from their ores

Solute: A solute is the material that dissolves in a solvent to form a solution

Solution: A solution is the mixture formed when a solute dissolves in a solvent

Solvent: A solvent is the liquid in which the solute dissolves to form a solution

Stable: Atoms are stable if their outer shell contains its maximum number of electrons

Sterilize: To sterilize means to make something free from microorganisms such as bacteria

Thermal decomposition: A reaction in which substances are broken down by heat

Unsaturated: An unsaturated compound contains at least one double or triple bond

Vapour: Vapour is a cloud of liquid particles

Steam is water vapour

Vapourized: Turned (generally through heating) into vapour.Vapour is a cloud of liquid particles

Steam is water vapour

KCSE Revision Notes Form 1 – Form 4 All Subjects