How to explain the Contact Process for sulphuric acid

The Contact Process is how sulphuric acid, one of the most important industrial chemicals in the world, is manufactured, and it is a favourite topic in manufactured substances in industry. Examiners like it because it rewards students who can lay out the stages in order, give the correct conditions, and explain the reasoning. This guide gives you a way to explain it that earns every mark.

Explain it in four stages

The trick is to present the process as four clean stages, each with its own equation. Answer in this order and you will not miss anything.

Stage 1, Making sulphur dioxide. Solid sulphur is burned in dry air, or sulphide ores are roasted, to produce sulphur dioxide gas:

S + O₂ → SO₂

This is the same combustion you meet in combustion of carbon and sulfur.

Stage 2, Making sulphur trioxide. This is the heart of the process. Sulphur dioxide reacts with more oxygen in a reversible reaction to form sulphur trioxide:

2SO₂ + O₂ ⇌ 2SO₃

This step needs specific conditions, covered below, and is the one examiners probe most.

Stage 3, Making oleum. Sulphur trioxide is absorbed into concentrated sulphuric acid to form oleum (fuming sulphuric acid):

SO₃ + H₂SO₄ → H₂S₂O₇

Stage 4, Making sulphuric acid. The oleum is then diluted with the right amount of water to produce concentrated sulphuric acid:

H₂S₂O₇ + H₂O → 2H₂SO₄

If you can write those four equations in sequence, you already have the backbone of a full-mark answer. The full reaction set is laid out in the Contact Process page.

The conditions for Stage 2, and why

The reversible step 2SO₂ + O₂ ⇌ 2SO₃ is where the conditions matter, so learn them with the reasoning attached.

• Catalyst: vanadium(V) oxide, V₂O₅. It speeds up the reaction so that equilibrium is reached quickly. It does not shift the position of equilibrium, it only makes the process fast enough to be economical.
• Temperature: about 450–500 °C. The forward reaction is exothermic, so a lower temperature would give a higher yield of SO₃. But too low a temperature makes the reaction very slow. Around 450–500 °C is the compromise between a good yield and a workable rate.
• Pressure: about 1 atmosphere (normal pressure). The forward reaction already gives a very high conversion at ordinary pressure, so there is no need for expensive high-pressure equipment. This is a good contrast to make with processes that do need high pressure.

Notice the pattern: the catalyst fixes the rate, the temperature is a compromise, and the pressure is kept low because the yield is already high. Being able to say why each condition is chosen is what separates a top answer from a memorised list.

Why sulphur trioxide is not dissolved directly in water

This is a classic follow-up question. You might expect Stage 3 to be simply SO₃ + H₂O → H₂SO₄. In principle that gives sulphuric acid, but in practice the reaction is extremely exothermic and violent. It produces a thick mist of tiny sulphuric acid droplets that is very hard to condense and would escape as pollution. To avoid this, sulphur trioxide is instead absorbed into concentrated sulphuric acid to form oleum, which is then diluted safely with water. Stating this reasoning clearly is often worth its own mark.

What sulphuric acid is used for

A short paragraph on uses rounds off a good answer and shows you understand why the process matters. Sulphuric acid is used to make fertilisers such as ammonium sulphate, in detergents, in paints and pigments, as the electrolyte in car (lead-acid) accumulators, and in the manufacture of many other chemicals. It is one of the reasons a country’s sulphuric acid output is sometimes treated as a rough measure of its industrial activity.

How to structure your exam answer

1. Name the process and the product.
2. Give the four stages in order, each with its equation.
3. State the conditions for Stage 2, with a reason for each.
4. If asked, explain why SO₃ is absorbed in acid rather than water.
5. Finish with a use or two of sulphuric acid.

Write it in that sequence and the marks look after themselves. If industrial processes like this feel like a wall of facts to memorise, that is exactly the kind of topic our online one-to-one teachers turn into a clear, ordered story, English lessons from RM50 an hour, with a paid one-hour trial. You can see how we work on the how it works page.