secondary chemistry – sulphur (IV) oxide

Sulphur (IV) oxide

Laboratory preparation
Reaction of sodium sulphite with a dilute acid
The mixture has to be heated
Na2SO3 (s) + 2HCl (aq)         →  2NaCl (aq) + SO2 (g) + 2H2O (l)

Alternatively sulphur (IV) oxide can be prepared by action of concentrated sulphuric (VI) acid on copper turnings

Cu (s) + 2H2SO4 (l)       →     CuSO4 (aq) + SO2 (g) + 2H2O (l)

The reaction should be carried out in a fume chamber since the gas is poisonous
Dried by passing it through concentrated sulphuric (VI) acid
Collected by downward delivery since it’s denser than air
The paper soaked in potassium dichromate (VI) turns from orange to green when the gas reaches its level hence used to detect when the gas jar is full of the gas

Physical properties of sulphur(IV) oxide

• Colourless
• Irritating and chocking smell
• Denser than air
• Boiling point of -10OC and easily liquefied under pressure
• Very soluble in water (can demonstrate fountain experiment)
• Turns wet blue litmus to red (it’s acidic) then to white (bleaches)
Dry gas has no effect on dry litmus paper

Chemical properties

The gas dissolves in water forming sulphuric (IV) acid
SO2 (g) + H2O (l)       →    H2SO3 (aq)

Sulphuric (IV) acid an acid hence turns blue litmus to red

When bubbled through sodium hydroxide a neutralization reaction occurs

H2SO3 (aq) + 2NaOH (aq)         →    Na2SO3 (aq) + H2O (l)

Sulphuric (IV) acid formed neutralises sodium hydroxide

Bleaching action

When coloured flowers are dropped in gas jar containing the gas the flowers turn to white (are bleached
Sulphur (IV) oxide combines with moisture to form sulphuric (IV) acid

SO2 (g) + H2O (l)      →     H2SO3 (aq)

The dye contains active oxygen which gives it colour
This oxygen reacts with sulphuric (IV) acid forming sulphuric (VI) acid and the dye is bleached

H2SO3 (aq) + dye           →     H2SO4 (aq) + (dye –O)

This bleaching is a reduction process
Oxygen in the air may re oxidise the material back to the original colour
Hence bleaching by sulphur (IV) oxide is temporary

This explains why old newspapers turn brown

Reducing action of sulphur (IV) oxide

1. Reaction with acidified potassium chromate (VI)
Reduces acidified orange potassium chromate (VI) to green chromium (III) ions
3SO2 (g) + Cr2O72-   + 2H+ (aq)    →   3SO42- (aq) + 2Cr 3+ (aq) + H2O (l)

This is the test for sulphur (IV) oxide

2. Reaction with acidified potassium manganate (VII)
Reduces purple potassium manganate (VII) to colourless manganese (II) ions

5SO2 (g) + 2MnO4(aq)   + 2H2O (l)  →  5SO42- (aq) + 2Mn 2+ (aq) +4 H+ (aq)

3. Reaction with bromine water
Acidified brown bromine water is decolourised forming hydro bromic acid

SO2 (g) + Br2 (aq)   +  2H+ (aq)  + H2O(l)  →  SO42- (aq) + 2HBr(aq) +4 H+ (aq)

When a solution of barium chloride is added to the mixture a white precipitate is formed indicating presence of sulphate ion
4. Reaction with concentrated nitric (V) acid
When bubbled through concentrated nitric (V) acid brown fumes of nitrogen (IV) oxide are given off.
Nitric (V) acid is reduced to nitrogen (IV) oxide while sulphur (IV) oxide is oxidised to sulphuric (VI) acid

SO2 (g) + 2HNO3 (aq)          →     H2SO4 (aq) + 2NO2 (g) )

5. Reaction with acidified iron (III) chloride
It reduces yellow iron (III) ions to green iron (II) ions

SO2 (g) + 2Fe3+ (aq)   + 2H2O (l)   →  2Fe2+ (aq) + 2SO42- (aq) ) + 4H+ (aq)

6. Reaction with hydrogen peroxide
Reduces hydrogen peroxide to water

SO2 (g) + 2H2O2 (aq)  + H2O (l)   →  2H2O (aq) + SO42- (aq) ) + 2H+ (aq)


Sulphur (IV) oxide does not support combustion or burn hence a burning splint inserted in a gas jar containing sulphur (IV) oxide goes off.
However under special conditions
400°C – 500°C and vanadium (V) oxide or platinum catalyst it reacts with oxygen forming sulphur (VI) oxide

Oxidising action of sulphur (IV) oxide

1. Reaction with burning magnesium
When burning magnesium is lowered in a gas jar of sulphur (IV) oxide, yellow specks of sulphur are formed
Magnesium burns at high temperature decomposing sulphur (IV) oxide to sulphur and oxygen hence continues to burn in oxygen formed.

SO2 (g) + 2Mg (s)     → 2MgO (aq) + S (s)

Magnesium is oxidised to magnesium oxide

2. Reaction with hydrogen sulphide
When a dry hydrogen sulphide is inverted over a gas jar containing sulphur (IV) oxide there is no reaction. When a few drops of water are added yellow deposit of sulphur is formed.

SO2 (g) + 2H2S (g)       →      2H2O (l) + 3S (s)

Sulphur (IV) oxide is reduced to sulphur
Hydrogen sulphide is oxidised to sulphur
The reaction only occurs in presence of moisture (water) which acts as a catalyst.

Reaction with alkalis
It forms salt and water

SO2 (g) + 2NaOH (aq)     →   Na2SO3 (aq) + H2O (l)

Uses of sulphur (IV) oxide

1. In the manufacturing of sulphuric acid
2. As a fumigant.
3. Manufacture of bleaching agent calcium hydrogen sulphite (Ca (HSO3)2 used to bleach pulp in manufacture of paper.
4. As a preservative in jam and fruit juices etc.