State the sources of the sulfur dioxide (burning sulfur or roasting sulfide ores) and oxygen (air) in the Contact process

Chemical reactions – Reversible reactions and equilibrium

The Contact Process – Where do the gases come from?

The Contact Process is a key industrial method for making sulphuric acid.

The overall reaction is:

\$\mathrm{SO2 + \tfrac{1}{2}O2 \;\;\xrightarrow{\,\text{Vulcanised catalyst}\,}\;\; SO_3}\$

To run this reaction you need two gases: sulphur dioxide (SO₂) and oxygen (O₂).

Below is a quick guide to where each gas comes from in the real world.

Exam tip: Remember that SO₂ is produced by burning sulphur or roasting sulphide ores, and O₂ is supplied directly from air. This is often asked in the “sources of reactants” question.

Sources of Sulphur Dioxide (SO₂)

  • 🔥 Burning elemental sulphur – Think of a campfire where you light a lump of sulphur; it glows bright orange and releases SO₂.

  • ⛏️ Roasting sulphide ores – When ores like pyrite (FeS₂) are heated in air, the sulphur part turns into SO₂.

    Analogy: Roasting is like baking a cake – the heat breaks bonds and releases a new ingredient (SO₂) into the air.

Source of Oxygen (O₂)

Oxygen is simply taken from the air that surrounds us. In the Contact Process, air is passed over the catalyst so that the O₂ can react with SO₂.

Exam tip: If asked “Where does the O₂ come from?” answer “From air” – no need to mention the 21 % composition, just that it’s supplied by the atmosphere.

Putting It All Together – A Simple Flow Chart

StepSource / ProcessResulting Gas
1Burning sulphur or roasting pyrite\$\mathrm{SO_2}\$
2Air (21 % \$\mathrm{O_2}\$)\$\mathrm{O_2}\$
3Catalytic converter (Vulcanised catalyst)\$\mathrm{SO3}\$ (and back to \$\mathrm{SO2}\$ in equilibrium)

Quick Recap – Flashcard Style

Flashcard: Q: What are the two main sources of gases in the Contact Process?

A: SO₂ from burning sulphur or roasting sulfide ores, and O₂ from air.

Remember: the Contact Process is all about converting one gas into another using heat and a catalyst. Keep the sources in mind, and you’ll ace any question about the reaction setup.