By the end of this note, you should be able to prepare sulphuric acid from sulphur dioxide correctly in the laboratory.

Sulphur dioxide is produced in the laboratory by heating a mixture of concentrated sulphuric acid with copper turnings or sulphur powder.

Apparatus and Materials

• Round-bottom flask with side arm
• Delivery tube
• Concentrated sulphuric acid
• Copper turnings or sulphur powder
• Bunsen burner
• Test tubes
• Safety goggles and gloves

Procedure

1. Place copper turnings or a small amount of sulphur powder in a round-bottom flask.
2. Add about 10 cm³ of concentrated sulphuric acid to the flask.
3. Heat the mixture gently using a Bunsen burner.
4. Collect the gas evolved through delivery tubes.

Chemical Equation

When copper reacts with concentrated sulphuric acid:

$\text{Cu}_{(s)} + 2\text{H}_2\text{SO}_{4(l)} \rightarrow \text{CuSO}_{4(aq)} + \text{SO}_{2(g)} + 2\text{H}_2\text{O}_{(l)}$

When sulphur reacts with concentrated sulphuric acid:

$\text{S}_{(s)} + 2\text{H}_2\text{SO}_{4(aq)} \rightarrow 2\text{H}_2\text{O}_{(l)} + 3\text{SO}_{2(g)}$

Sulphur dioxide is dissolved in water to form sulphurous acid (H₂SO₃), but this is not sulphuric acid. To make actual sulphuric acid, we must oxidize SO₂ to SO₃, then dissolve SO₃ in water.

Method A: Oxidation with Hydrogen Peroxide

Hydrogen peroxide (H₂O₂) acts as an oxidizing agent and converts sulphur dioxide to sulphuric acid.

Procedure

1. Bubble sulphur dioxide gas into a small amount of distilled water in a test tube or conical flask.
2. Add a few drops of hydrogen peroxide solution (10 volume) to the solution.
3. Stir gently and allow the reaction to occur.

Chemical Equation

First, sulphur dioxide dissolves in water:

$\text{SO}_{2(g)} + \text{H}_2\text{O}_{(l)} \rightarrow \text{H}_2\text{SO}_{3(aq)}$

Then, hydrogen peroxide oxidizes sulphurous acid to sulphuric acid:

$\text{H}_2\text{SO}_{3(aq)} + \text{H}_2\text{O}_2{(aq)} \rightarrow \text{H}_2\text{SO}_{4(aq)} + \text{H}_2\text{O}_{(l)}$

Method B: Oxidation with Potassium Permanganate

Acidified potassium permanganate solution can also oxidize sulphur dioxide to sulphuric acid.

Procedure

1. Bubble sulphur dioxide into distilled water.
2. Add dilute sulphuric acid to the solution.
3. Add potassium permanganate solution drop by drop until the purple color just disappears.

Chemical Equation

$2\text{KMnO}_4 + 5\text{SO}_2 + 2\text{H}_2\text{O} \rightarrow \text{K}_2\text{SO}_4 + 2\text{MnSO}_4 + 2\text{H}_2\text{SO}_4$

Confirm the product is sulphuric acid by testing:

1. Litmus test: The solution turns blue litmus red (acidic).
2. Barium chloride test: Add barium chloride solution to the product. A white precipitate of barium sulphate (BaSO₄) forms, confirming sulphate ions are present:

$\text{Ba}^{2+} + \text{SO}_4^{2-} \rightarrow \text{BaSO}_{4(s)}$

• Always wear safety goggles and gloves when handling concentrated acids.
• Perform the experiment in a well-ventilated area or fume cupboard.
• Sulphur dioxide is an irritating gas – do not inhale it directly.
• Add hydrogen peroxide slowly and carefully.

1. Generate SO₂ by heating copper/sulphur with concentrated H₂SO₄
2. Dissolve SO₂ in water to form H₂SO₃
3. Oxidize H₂SO₃ to H₂SO₄ using H₂O₂ or KMnO₄
4. Confirm the product with barium chloride test

In Tanzania, sulphuric acid is used in many industries, including fertilizer manufacturing, mining (for processing copper and gold ores), and battery production. Understanding how to prepare it from sulphur dioxide helps students appreciate the chemistry behind these industrial processes. For example, small-scale laboratories in schools like St. Francis Girls' Secondary School or during science fairs in Dodoma can demonstrate this preparation to help students connect classroom chemistry to real-world industrial applications.