The Contact process: sulfur trioxide and condition choices
Use the correct sulfur dioxide sources, vanadium(V) oxide catalyst and lower typical pressure.
Extended only (Supplement) · Chemistry 0620, 2026–2028. The conditions are the typical syllabus values, not operating instructions for a plant.
Before you begin
- Use equilibrium gas coefficients and exothermic temperature responses.
- Explain why a catalyst changes speed without changing fixed-temperature equilibrium position.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which formula is sulfur trioxide?
What you will learn
- State the sulfur dioxide oxidation equation, feed sources and typical syllabus conditions.
- Explain the Contact conditions and compare their pressure and catalyst with the Haber process.
Identify the conversion step exactly
The syllabus Contact conversion is 2SO₂(g)+O₂(g)⇌2SO₃(g), an exothermic forward reaction. Sulfur dioxide is supplied by burning sulfur or roasting sulfide ores, while oxygen comes from air. The product of this equilibrium step is sulfur trioxide; do not write sulfuric acid directly as the product of the stated SO₂-to-SO₃ equation.
Typical syllabus conditions are about 450 °C and 200 kPa (about 2 atm), with vanadium(V) oxide, V₂O₅, as catalyst. This is a much lower pressure than the typical Haber value. Match each industrial reaction with its own catalyst and pressure rather than sharing one memorised list of conditions between them.
Explain why maximum pressure is not the only objective
Lower temperature favours exothermic sulfur trioxide production but slows the conversion; the typical temperature gives a useful rate while retaining favourable conversion. Increasing pressure favours the product side, which has two gaseous coefficient parts instead of three, but a high conversion can be obtained under the stated suitable conditions without the large pressure increase used in Haber. The added compression and equipment demands must be weighed against any extra conversion benefit.
The vanadium(V) oxide catalyst lowers the kinetic barrier and increases reaction speed, not the equilibrium position at fixed temperature. The typical low pressure reduces the need for expensive high-pressure compression and containment relative to a much higher-pressure alternative. Explain safety and economics in terms of plant demands and the stated conversion; do not assume lower pressure removes every hazard of handling industrial reactants or makes the process cost-free.
Pause and explain
Which catalyst belongs to the stated Contact process?
Worked example
A student proposes using the Haber process's iron catalyst and 200 atm for SO₂ oxidation because both processes are exothermic. Correct the recalled Contact conditions and evaluate the pressure argument.
Show the worked solution
- For 2SO₂(g)+O₂(g)⇌2SO₃(g), the typical Contact conditions are 450 °C, 200 kPa/about 2 atm, and vanadium(V) oxide catalyst.
- Increasing pressure would favour fewer gas particles on the SO₃ side, but the stated conversion can be favourable at the much lower typical pressure.
- A large pressure increase adds compression, equipment and safety demands, so the equilibrium direction alone does not justify copying Haber's pressure or catalyst.
Answer Contact: 450 °C, about 2 atm, V₂O₅; use a rate, conversion and plant-demand comparison.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Give the Contact conversion equation, both feed sources and typical temperature, pressure and catalyst.
Give me a hint
State where SO₂ and oxygen come from, then name the separate conversion conditions.
Compare my reasoning
- 2SO₂(g)+O₂(g)⇌2SO₃(g); SO₂ comes from burning sulfur or roasting sulfide ores, and oxygen from air.
- Use about 450 °C and 200 kPa/about 2 atm.
- The catalyst is vanadium(V) oxide, V₂O₅.
2SO₂+O₂⇌2SO₃; sulfur burning/sulfide roasting and air; 450 °C, about 2 atm, V₂O₅.
Look for these in your work
- The conversion product is SO₃.
- Sources, pressure and catalyst are assigned correctly.
Compare the typical Haber and Contact pressures and catalysts, and explain why a catalyst does not make either reaction's equilibrium yield larger at the same temperature.
Give me a hint
Match the facts to the two different equations before making the shared catalyst explanation.
Compare my reasoning
- Haber uses about 200 atm and iron; Contact uses about 2 atm and vanadium(V) oxide.
- The syllabus pressure ratio is about 200/2=100, so Haber uses roughly one hundred times the stated Contact pressure.
- Both suitable catalysts speed forward and reverse routes without changing the equilibrium position at fixed temperature.
Haber: about 200 atm/iron. Contact: about 2 atm/V₂O₅. Catalysts change speed, not fixed-temperature equilibrium yield.
Look for these in your work
- Pressure and catalyst pairs stay with their processes.
- The shared equilibrium limitation is stated.
For a hypothetical Contact pass with 1.0 mol SO₂ and 1.0 mol O₂, 0.60 mol SO₃ forms. Find unreacted amounts and explain why a catalyst could speed reaching this equilibrium but cannot guarantee all SO₂ converts. Assume no side reactions or losses.
Give me a hint
Two SO₂ and one O₂ form two SO₃ in the balanced equation.
Compare my reasoning
- 0.60 mol SO₃ uses 0.60 mol SO₂ and 0.30 mol O₂.
- Remaining amounts are 0.40 mol SO₂ and 0.70 mol O₂.
- A catalyst speeds approach to the reversible balance; it does not remove the reverse reaction or shift fixed-temperature equilibrium to complete conversion.
0.40 mol SO₂ and 0.70 mol O₂ remain; catalysis does not guarantee complete equilibrium conversion.
Look for these in your work
- The 2:1:2 coefficient ratio is applied.
- Faster approach is separated from complete conversion.
Common mistakes
- Writing sulfuric acid as the direct product of the SO₂ oxidation step.
- Using iron instead of vanadium(V) oxide for the stated Contact catalyst.
- Copying the Haber pressure of 200 atm.
- Claiming pressure has no equilibrium effect merely because the typical pressure is relatively low.
What can you explain now?
State the typical Contact pressure and catalyst for conversion of SO₂ to SO₃.
Compare with the explanation
200 kPa (about 2 atm); vanadium(V) oxide, V₂O₅.
These differ from Haber's approximately 200 atm and iron catalyst; the Contact pressure choice reflects the conversion benefit relative to plant demands.
After trying it yourself, choose your next review. This is your self-assessment.
Your review choice appears on Today. Sign in to sync it across devices.
Make it stick
Write the Contact equation and typical conditions, then compare them with Haber. Explain why a pressure that favours products still needs an economic and safety argument before an industrial increase is chosen.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.