It is prepared in the laboratory by reacting dilute HCL and iron II sulphide.

FeS(s)+HCL(aq)→FeCl2 (aq)+H2S

Physical Properties

• It is a colorless gas.
• It is denser than air.
• It is very poisonous.
• It has the smell of rotten egg.
• It is a reducing agent.

2FeCl3 (aq)+H2S→FeCl2 (s)+HCL(aq)+S(s)

SULPHUR DIOXIDE

The most convenient method of preparing SO₂ in the laboratory.

Na2SO3 (aq)+2HCL(aq)→2NaCl(aq)+H2O(l)+SO2 (g)

The gas is dried using concentrated Sulphuric acid and is collected by downward delivery.

Physical Properties

• It is poisonous.
• It is very soluble in water.
• It is denser than air.

Chemical Properties

1. It dissolves in water to form an acidic solution.

SO2 (g)+H2O(l)→H2SO3 (aq)

2. SO₂ reduces iron III to iron II

2Fe2Cl3 (aq)+SO2 (g)+2H2O(l)→FeCl2 (aq)+H2SO3 (aq)+HCL(aq)

3. SO₂oxidizes H₂S (hydrogen sulphite) to sulphur.

SO2 (g) +H2S→H2O(l)+3S(s)

NB:SO₂can also be prepared in the laboratory by the action of concentrated sulphuric acid on carbon 4 turning. It is collected by downward delivery.

Test for SO₂

• SO₂turns potassium dichromate paper to green with no precipitate.

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

• SO₂ also turns purple potassium permanganate to colorless.