Sulphur dioxide

Sulphur (IV) oxide

Laboratory preparation of sulphur dioxide

Sulphur (IV) oxide (sulphur dioxide) can be prepared by heating a mixture of concentrated sulphuric acid and copper or sulphur using a test shown in the diagram below.

$\text{Cu}_{(s)} + 2\text{H}_2\text{SO}_{4(l)} \rightarrow \text{CuSO}_{4(aq)} + \text{SO}_{2(g)} + 2\text{H}_2\text{O}_{(l)}$

$\text{S}_{(s)} + 2\text{H}_2\text{SO}_{4(aq)} \rightarrow 2\text{H}_2\text{O}_{(l)} + 3\text{SO}_{2(g)}$

The gas is collected by passing it through concentrated sulphuric acid.

The sulphur burns in the gas jar with a light blue flame producing a misty gas, sulphur dioxide. The solution of the gas in water turns blue litmus paper red, decolourises purple potassium permanganate solution and turns orange potassium dichromate solution green.

The flowers lose their colours in a gas jar of moist sulphur dioxide. The moist gas acts as a bleaching agent. This is why the gas is used industrially for bleaching sponges and straw for making straw hats.

Test for sulphur dioxide

1. Pure sulphur dioxide turns potassium permanganate solution from purple to colourless.
2. The gas turns acidified potassium dichromate solution from orange to green.

Physical properties of sulphur dioxide

1. Pure sulphur dioxide is a colourless gas with an irritating and choking smell.
2. It is very soluble in water.
3. It turns moist blue litmus red and bleaches it.
4. It is denser than air.

Chemical properties of sulphur dioxide

1. It is an acidic oxide.
2. It is readily dissolvable in water.
3. Sulphur dioxide dissolves in water and gives out sulphurous acid. $\text{SO}_2 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{SO}_3$
4. It reacts vigorously with sodium hydroxide solution and forms sodium sulphite. $\text{SO}_2 + 2\text{NaOH} \rightarrow \text{Na}_2\text{SO}_3 + \text{H}_2\text{O}$
5. In the cases that we pass more sulphur dioxide into this arrangement, we get sodium hydrogen sulphite. $\text{SO}_2 + \text{Na}_2\text{SO}_3 + \text{H}_2\text{O} \rightarrow 2\text{NaHSO}_3$
6. The sulphur atom in sulphur dioxide is tetravalent. Subsequently, it can increase its covalency to six by reacting with elements like $\text{O}_2$ and $\text{Cl}_2$ to form the corresponding addition compounds. For example, it reacts with chlorine under the influence of charcoal as a catalyst to give sulphuryl chloride ($\text{SO}_2\text{Cl}_2$). $\text{SO}_2 + \text{Cl}_2 \rightarrow \text{SO}_2\text{Cl}_2$
7. In the presence of vanadium pentoxide ($\text{V}_2\text{O}_5$) as a catalyst, it gives sulphur trioxide. $2\text{SO}_2 + \text{O}_2 \rightarrow 2\text{SO}_3$
8. In the presence of moisture, it can give nascent oxygen, and, thus, acts as a reducing agent. For example, it reduces ferric salts to ferrous salt, and halogens to halogen acids. $2\text{Fe}^{3+} + \text{SO}_2 + 2\text{H}_2\text{O} \rightarrow 2\text{Fe}^{2+} + \text{SO}_4^{2-} + 4\text{H}^+$

Uses of sulphur dioxide

1. Most of sulphur dioxide is used in the manufacture of sulphuric acid and sulphate compounds.
2. It is used for bleaching silk, straw, sponges and textiles.
3. It is used for preservation of fruits, fruit juices, grains and other food stuffs by preserving the food colour and preventing growth of bacteria.
4. It is used for fumigation to kill pest and fungal disease in plants.

Hazards of sulphur dioxide

Sulphur dioxide is released into the atmosphere from volcanic eruption, rotting vegetative matter and combustion of fossil fuel which contain small amount of sulphur. The following are the effects on mankind and to the environment:

1. Inhalation of Sulphur dioxide from polluted air can cause respiratory and cardiac problems to human and other animals.
2. When Sulphur dioxide dissolves in rainwater it forms sulphuric and sulphurous acid that form acid rain. The acid rain has the following effects.
3. It increases soil acidity which in turn affect plant growth and soil organism.
4. It attacks buildings especially those made from limestone. It reacts with the stone to produce a powdery substance which is easily eroded.
5. It flows into streams, rivers and lakes changing the lives of aquatic organisms.

Pollution by sulphur dioxide can be reduced by various measures such as

1. Use of scrubbers; these are devices that are used to remove acid gases from industrial process.
2. Use of low sulphur diesel in vehicles. Several countries currently have low sulphur diesel.
3. Proper management of soil waste in order to control its decomposition.