Non Metals And Their Compounds
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Non Metals And Their
Compounds
Chemistry Form 4
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General Chemical Properties of Non Metals
Chlorine
Hydrogen Chloride
Sulphur
Sulphur Dioxide
Preparation of sulphur dioxide
In the laboratory, sulphur dioxide is prepared by heating a mixture of sodium sulphite and
dilute hydrochloric acid. The reaction equation is:
Alternatively, sulphur dioxide can be prepared by heating a mixture of concentrated
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sulphuric acid and copper. In this case, there is no reaction until the mixture in the flask Ticha
Kidevu
becomes hot. Then rapid effervescence occurs and the gas is usually collected as shown.
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A dark brown mixture is left in the flask. It contains anhydrous copper (II) sulphate and
certain impurities.
The Properties of Sulphur Dioxide
Describe the properties of sulphur dioxide
Physical properties
These are:
1. The gas is colourless with an irritating (pungent), choking smell.
2. It is denser than air. Its density is 2½ times that of air.
3. It is readily soluble in water and forms an acidic solution of sulphurous acid.
Acidic characteristics of sulphur dioxide
As indicated above, sulphur dioxide gas dissolves in water to form an acid solution of
sulphurous acid, H2SO3. The solution turns blue litmus paper red. Sulphur dioxide is
thus an acidic gas.
Chemical properties
The reaction characteristics described below explains the chemical properties of
sulphur dioxide gas.
The reducing properties of sulphur dioxide
(i) Reduction of dyes in flower petals or colour in paper (bleaching)
Sulphur dioxide bleaches the colours in dyes such as flower pigments. When the
flower pigments or dyes contain oxygen, they are coloured. Sulphur dioxide reduces
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the dye (removes oxygen from it) and the dye, therefore, turns colourless. This Ticha
process can be summarized as follows: Kidevu
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In this process, sulphur dioxide dissolves in water to form sulphurous acid:
Then the acid takes up oxygen from the dye of the flowers or paper and forms
sulphuric acid. The removal of oxygen from the dye converts it to a colourless
compound:
However, the oxygen from the air may oxidize the reduced colourless compound
back to the original coloured compound.
(ii) Reduction of nitric acid
If sulphur dioxide gas is bubbled through concentrated nitric acid, brown fumes of
nitrogen dioxide gas are formed:
The nitric acid is reduced to nitrogen dioxide and the sulphur dioxide is oxidized to
sulphuric acid.
(iii) Reduction of acidified potassium permanganate solution
Sulphur dioxide decolourized purple potassium permanganate solution.
Sulphur dioxide is first converted to a sulphite (SO32-), after reacting with water, and
then oxidized to a sulphate (SO42-) by the permanganate. Ask
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The ionic equation is:
Non Metals And Their Compounds
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The oxidation state of manganese changes from +7 to +2. This is a reduction
reaction.
(iv) Reduction of acidified potassium dichromate solution
When sulphur dioxide is mixed with potassium dichromate (VI) solution the orange
colour of the solution changes to green. The dichromate (VI) is reduced to chromate
(III) which is green in colour when in aqueous state:
The oxidation state of chromium changes from +6 in potassium dichromate (VI) to +3
in chromic sulphate. On the other hand, sulphur dioxide is oxidized by the dichromate
(VI) to sulphate (SO42-).
Other reduction reactions involving sulphur dioxide gas
Sulphur dioxide reduces chlorine, bromine and iodine to the hydrogen halides in the
presence of water:
In all these cases, the solution changes from brown to colourless.
The oxidizing properties of sulphur dioxide
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Ticha
(i) Oxidation of hydrogen sulphide Kidevu
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Iron (II) sulphide reacts with dilute hydrochloric acid to produce hydrogen sulphide
Non Metals And Their Compounds
Chemistry Form 4
gas and iron (III) chloride:
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Hydrogen sulphide gas has a smell similar to that of a rotten egg. The hydrogen
sulphide gas is oxidized by sulphur dioxide in the gas jar to sulphur:
The sulphur produced is a yellow residue.The reaction takes place in the presence of
moisture which acts as a catalyst. That is why water is added in the jar.
(ii) Oxidation of magnesium
When burning magnesium is lowered into a jar containing sulphur dioxide gas, a
white solid, magnesium oxide, and yellow pieces of sulphur are formed:
Tests for sulphur dioxide gas
Includes
1. The gas can be identified by its characteristic pungent and choking smell.
2. It can also be detected by putting into it a filter paper that has been previously
dipped into an acidified solution of potassium dichromate (VI). The colour of the
filter paper changes from orange to green due to the reduction of dichromate
(VI) to chromate (III).
3. Sulphur dioxide also decolourized acidified potassium permanganate solution.
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The Uses and Hazards of Sulphur Dioxide
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Explain the uses and hazards of sulphur dioxide
Non Metals And Their Compounds
Sulphur
Chemistry Form 4 dioxide has got a number of uses in daily life. However, there are also some
hazards
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Complete can be caused by the gas if its production is not controlled.
Uses of sulphur dioxide
The following are some uses of sulphur dioxide gas:
1. The gas is used in the industrial manufacture of sulphuric acid in the Contact
Process.
2. It is used as a bleaching agent for wood pulp, silk, wool and straw.
3. Its poisonous nature makes it a useful fumigant. So it is used in fumigation. The
gas is poisonous to all organisms, particularly bacteria.
4. It is used as a preservative and sterilizing agent in making soft drinks and jam,
and in dried fruits. A very low concentration of the gas in food prevents
fermentation as it stops the growth of bacteria and moulds. Its reaction with
oxygen prevents oxidation of juices and other liquids to which it is added for
preservation.
Hazards of sulphur dioxide gas
The hazards of sulphur dioxide gas are due to its effect in environmental pollution
and the health problems accompanied with that pollution. Sulphur dioxide is a major
air pollutant. The major sources of sulphur dioxide in the air are power plants that
use fossil fuels such as coal, diesel and petrol; industrial boilers; and exhaust
emissions from motor vehicles. The gas is also produced during metal smelting and
other industrial processes.
Half of sulphur dioxide output comes from burning coal in coal-fired power stations.
All coal contains small amounts of sulphur. So when the coal is burnt to produce
energy, the sulphur in the coal reacts with oxygen in the air to produce sulphur
dioxide
Sulphur dioxide is a very irritating gas and is thought to be the cause of bronchitis
and other lung diseases. Exposure to higher concentrations of the gas can cause Ask
impairment of the respiratory function and heart diseases. Ticha
Kidevu
Sulphur dioxide also causes acid rain. This occurs when the gas comes in contact
with
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The sulphur dioxide dissolves in water vapour from the clouds and /
combines with oxygen from the atmosphere to form an acid – sulphuric acid:
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Acid rain damages the leaves and barks of plants making them more vulnerable to
diseases, weather and insects. When acid rain reaches the lake, river or other water
bodies it makes the whole water body acidic. Even a low concentration of acid in the
water can kill fish and other marine organisms.
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Sulphuric Acid
Nitrogen
Ammonia
Carbon
Carbon Dioxide
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