Chemical Properties of Sulfur Dioxide

Chemical Properties of Sulfur Dioxide:

Chemically, sulfur dioxide gas is highly reactive in nature and gives a large number of chemical reactions.

(1) Combustion- Sulfur dioxide is neither combustible nor a supporter of combustion. However, Mg and K continue to burn in a jar of SO2 gas.

2Mg + SO2 ———–> 2MgO + S
4K + 3SO2 ———–> K2SO3 + K2S2O3

Similarly, Fe and Sn also behave in the same manner.

(2) Decomposition- It is a fairly stable compound. It, however, undergoes decomposition when heated to 1200°C to give sulfur trioxide and sulfur.

3SO2 ⇌ 2SO3 + S

(3) Acidic Properties- It is an acidic oxide. On dissolving in water, it forms sulfurous acid which is a weak dibasic acid. It is also called sulfurous anhydride.

H2O + SO2 ———–> H2SO3
H2SO3 ⇌ 2H+ + SO3

The acidic nature of the aqueous solution of the gas is supported as under-

(a) Action of blue litmus paper- It turns blue litmus paper red.

(b) Action of alkalis- On treatment with alkalis like NaOH, it gives rise to the formation of the corresponding acid and normal salts.

NaOH + H2SO3 ———–> NaHSO3 (Acid Salt) + H2O
2NaOH + H2SO3 ———–> Na2SO3 + 2H2O

(c) Action of carbonates- It decomposes carbonates to give carbon dioxide.

Na2CO3 + 2H2SO3 ———–> 2NaHSO3 + H2O + CO2

(d) Action of lime water- It turns lime water milky.

Ca(OH)2 + H2SO3 ———–> CaSO3 (Milkiness) + 2H2O

(e) Action of metals- It reacts with metals like Mg to give rise to the evolution of hydrogen gas.

Mg + H2SO3 ———–> MgSO3 + H2

(f) Action of basic oxides- It reacts with basic oxides like CaO, BaO etc. to form salts.

CaO + H2SO3 ———–> CaSO3 + H2O
BaO + H2SO3 ———–> BaSO3 + H2O

(4) Reducing Properties- Sulfur dioxide, in presence of moisture (water) liberates nascent hydrogen and hence, it acts as a strong reducing agent.

SO2 + 2H2O ———–> H2SO4 + 2H

Some important reducing reactions of sulfur dioxide are as under-

(a) Action of halogens- Halogens (Cl2, Br2 or I2) are reduced to the corresponding halogen acids.

Cl2 + SO2 + 2H2O ———–> 2HCl + H2SO4

(b) Action of ferric salts- Ferric salts are reduced to ferrous salts.

Fe2(SO4)3 (Ferric Sulfate) + SO2 + 2H2O ———–> 2FeSO4 (Ferrous Sulfate) + 2H2SO4

(c) Action of acidified KMnO4 It decolourizes the pink colour of the acidified solution of KMnO4.

2KMnO4 + 5SO2 + 2H2O ———–> K2SO4 + 2MnSO4 + 2H2SO4

(d) Action of acidified K2Cr2O7The orange colour of acidified K2Cr2O7 solution is changed to green colour.

K2Cr2O7 + 3SO2 + H2SO4 ———–> K2SO4 + Cr2(SO4)3 (Green Colour) + H2O

(5) Oxidizing Properties- In many instances, sulfur dioxide acts as an oxidizing agent as well. Some typical oxidizing reactions of SO2 are as under-

(a) Action of H2S- Hydrogen sulfide is oxidized to sulfur.

2H2S + SO2 ———–> 2H2O + 3S

(b) Action of lead dioxide- It oxidizes lead dioxide to lead sulfate on heating.

PbO2 + SO2 ———–> PbSO4

(c) Action of magnesium- It oxidizes magnesium to magnesium oxide.

2Mg + SO2 ———–> 2MgO + S

(d) Action of iron- Iron is oxidized to iron oxide.

3Fe + SO2 ———–> 2FeO + FeS

(6) Addition Reactions- Sulfur dioxide shows unsaturated character and, thus, is capable of forming a large number of addition compounds. Example-

(a) Action of halogens- It combines with halogens like Cl2, Br2 etc. in the presence of sunlight to give the corresponding sulfury halides.

SO2 + Cl2 ———–> SO2Cl2

(b) Action of sodium peroxide- It forms sodium sulfate.

SO2 + Na2O2 ———–> Na2SO4

(c) Action of Oxygen- It combines with oxygen on heating in presence of V2O5 at 450°C to give sulfur trioxide.

2SO2 + O2 ———–> 2SO3

(7) Bleaching Property- Sulfur dioxide acts as a powerful bleaching agent in the presence of moisture. It bleaches vegetable colouring matter in presence of moisture. The bleaching action of SO2 is due to the reduction of colouring matter to a colourless product. On long-standing, the colourless product is reoxidized by the oxygen of the air and thus, colour is restored. Hence, bleaching by SO2 is temporary in nature.

SO2 + 2H2O ———–> H2SO4 + 2H
Colouring matter + H ———–> Colourless matter

Uses of Sulfur Dioxide:

Sulfur dioxide is used-

  • As a bleaching agent for delicate articles such as straw, wool, hair and other articles.
  • As a disinfectant for household fumingation.
  • As a refrigerant.
  • In the manufacture of paper as calcium bisulfite.
  • In the refining and bleaching of sugars.
  • In the manufacture of sulfuric acid.
  • As a germicide for killing of germs.
  • As an important solvent for organic compounds.
  • As an antichor in removing excess of chlorine from the bleached articles.

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