Laboratory Preparation of Sulphur Dioxide

Aim of this experiment to prepare Sulphur dioxide gas and study its physical and chemical properties.

Sulphur dioxide can be prepared by the action of hot concentrated sulphuric acid on copper turnings. 

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

Reducing property 

Sulphur dioxide decolourises acidified potassium permanganate (KMnO₄) solution. Acidified potassium dichromate solution (K₂Cr₂O₇) is also turned green by SO₂. The reactions with KMnO₄ and K₂Cr₂O₇ are due to the reducing property of SO₂ and the oxidising nature of acidified KMnO₄ and K₂Cr₂O₇.

SO₂ (g) + 2H₂O (l) → K₂SO₄ (aq) + 2MnSO₄ (aq) + 2H₂SO₄ (aq)
K₂Cr₂O₇ (aq) + H₂

Apparatus and materials required are:

Apparatus

• Round bottom flask
• Thistle funnel
• Delivery tube
• Vent tube
• Double bored cork
• Lid
• Laboratory stand
• Burner
• Two test tubes
• Trough
• Gas jar
• Red and blue litmus papers

Chemicals

• Copper turnings
• Conc. sulphuric acid 
• Dil. sulphuric acid
• Potassium permanganate solution
• Potassium dichromate solution

Procedure:

• Take a round bottom flask and add copper turnings to it. Fit double bored cork into the round bottom flask. 
• Fit a thistle funnel into the double bored cork, connect the gas jar to the round bottom flask by a delivery tube and connect the vent tube to the gas jar. 
• Add concentrated sulphuric acid to the thistle funnel. 
• Start the burner to provide the heat required for the reaction.
• Sulphur dioxide is collected in the gas jar.
• To check the solubility of SO₂ in the water, take a gas jar containing Sulphur dioxide gas with its mouth closed by a lid. Invert the gas jar onto a water-filled trough and carefully remove the lid. 
• To study some chemical properties of Sulphur dioxide gas, introduce moist blue litmus paper and moist red litmus paper into a stream of SO₂ gas.
• Take an acidified solution of potassium permanganate in a test tube. 
• Now, pour the acidified solution of potassium permanganate into the gas jar and observe the colour change.
• Take an acidified solution of potassium dichromate in a test tube.
• Now, pour the acidified solution of potassium dichromate into the gas jar and observe the colour change.

Here we conclude that:

• Sulphur dioxide gas can be prepared by heating the copper turnings with concentrated sulphuric acid.
• The Sulphur dioxide gas was identified as colourless and is soluble in water.
• The nature of the gas was identified as acidic as it turns the blue litmus red. 
• The colour of the acidified potassium permanganate solution, when it reacts with Sulphur dioxide, changes from purple to colourless; this confirms the reducing nature of SO₂ gas.
• The colour of  the acidified potassium dichromate solution, when it reacts with Sulphur dioxide, changes from orange to green; this confirms the reducing nature of SO₂ gas.

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