Sulphur dioxide (SO2) : Preparation, Properties and Uses.

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Preparation of Sulphur dioxide:

  • By burning sulphur in air.

  • By roasting metal sulphide : When metal sulphides (sulphide ores) are heated in excess of air at high temperature, then SO2 gas is produced.

Laboratory preparation of sulphur dioxide (SO2) :

Sulphur dioxide gas is prepared in laboratory by heating copper turnings with conc. H2SO4.

lab preparation of sulphur dioxide SO2

lab preparation of sulphur dioxide SO2

Procedure: Copper turnings are taken in a round bottom flask fitted with thistle funnel and delivery tube as shown in figure. When conc. H2SO4 is poured through thistle funnel and mixture is heated sulphur dioxide gas is evolved, which is collected in the gas jar by upward displacement of air.

Drying of SO2:  If sulphur dioxide gas is to be dried, the gas obtained is passed through conc. Sulphuric acid . The moisture is absorbed by conc. H2SO4.

Chemical Properties of sulphur dioxide (SO2)

1. Weak acidic nature of SO2 :

It dissolves in water to give sulphurous acid, a weak diprotic (dibasic) acid.

Sulphurous acid ionizes in two steps:

It produces two series of salts if reacted with base.Eg.

acidic nature of SO2

2. SO2 as a reducing agent :

Aqueous sulphurdioxide shows reducing character and SO2 itself gets oxidized to H2SO4.

  • It reduces halogens (Cl2, Br2, I2) in aq. Solution to respective halides. Eg.

  • It reduces acidified potassium permanganate solution and pink colour of KMnO4 is discharged.

reducing nature of SO2

  • It reduces acidified K2Cr2O7 solution and orange color of K2Cr2O7 changes to light green.

  • It reduces ferric salt to ferrous salt and colour changes from yellow to light green.

  • It reduces potassium iodate(KIO3) solution to iodine.

3. SO2 as an oxidizing agent :

SO2 oxidizes powerful reducing agents like H2S, HI, Mg, Fe, etc and itself get reduced to sulphur or sulphide.

oxidizing nature of SO2

4. SO2 as a bleaching agent:
Formation of colourless product from coloured substance is called bleaching. Chemical substances such as SO2, H2O2, CaOCl2, Cl2, etc. show bleaching action.

SO2 can act a a bleaching agent in presence of moisture. It can bleach coloured wool, silk, flower, hair, etc. The bleaching action of sulphur dioxide is due to the formation of nascent hydrogen in presence of moisture, which reduces colouring substance to colourless reduced product.

bleaching action of sulphurdioxide(SO2)

In some cases, bleaching action of SO2 is due to formation of colourless addition product.

→ The bleaching action of SO2 is temporary. The bleached colourless compound will regain its original colour slowly on standing in air due to oxidation by air.

Comparison of bleaching action of SO2 and Cl2:
Sulphur dioxide (SO2) Chlorine (Cl2)
 1. Sulphur dioxide reacts with water to yield nascent hydrogen which bleaches coloured substance to colourless product by reduction.

SO2+2H2O →H2SO4+2[H]

2. Bleaching by SO2 may be sometimes due to formation of colourless addition products.

3. Bleaching by SO2 is reversible i.e. a temporary process.

4. SO2 is mild bleaching agent.

5. It is used to bleach delicate articles such as wool, silk, cane sugar, etc.

  1. Chlorine reacts with water to yield nascent oxygen which bleaches coloured substance to colourless product by oxidation.

Cl2+H2O →2HCl+2[O]

2. No such colourless addition product is formed with chlorine.

3. Bleaching by Cl2 is irreversible ie. a permanent process.

4. Cl2 is strong bleaching agent.

5. It is used to bleach wood, textiles, paper, etc.

Test for sulphur dioxide (SO2) :

  • Odour : Sulphur dioxide has typical suffocating smell of burning sulphur.
  • Potassium dichromate paper test : When SO2 comes in contact with acidified potassium dichromate paper, the orange coloured paper turns to green due to formation of chromium sulphate.

test of sulphur dioxide SO2

Uses of SO2

  • It is used as bleaching agent.
  • It is used in the manufacture of sulphuric acid.
  • It can be used as disinfectant, fungicide and food preservative.

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