Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

6.6Chemical Reactions

The Contact process — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions

5 min read 10 parts Lesson 7 of 7 Unit quiz

📚 Cambridge IGCSE Chemistry 0620 Hub / Topical Revision
🩺
100% Curriculum Specification Match Lesson Doctor Verified
The only platform with 100% curriculum specification match and verified interactive question banks.
0%read
Lesson summary

6.6 The Contact process — Notes — IGCSE Chemistry notes: Describe how sulfuric acid is made in the Contact process, including raw materials, catalyst and conditions, and explain why these conditions are chosen.Purpose — the Contact process is the industrial method for making sulf…

  • Uses of sulfuric acid
  • Sulfur dioxide
  • Low temperature
  • Compromise
  • High pressure
  • Why it is low

Section 1 of 10Purpose and key reaction

1Purpose
  • Purpose the Contact process is the industrial method for making sulfuric acid.
2Uses of sulfuric acid
  • Uses of sulfuric acid fertilisers such as ammonium sulfate, detergents, paints and car batteries.
3Key step
  • 2SO2(g) + O2(g) ⇌ 2SO3(g), a reversible, exothermic reaction.
4Gas moles
  • 3 mol of gas on the left become 2 mol on the right.
5
Sulfuric acid is also used to make superphosphate fertiliser.

Section 2 of 10Raw materials

Sulfur dioxide
  • Sulfur dioxide from burning sulfur in air (S + O2 → SO2) or roasting sulfide ores such as zinc sulfide.
Oxygen
  • Oxygen from the air.

Section 3 of 10Making sulfur trioxide

Burn sulfur (or roast a sulfide ore) in air to make sulfur dioxide.
Pass dry, purified SO2 and air over layers of V2O5 catalyst.
Keep the temperature at about 450 °C and the pressure at about 200 kPa (2 atm).
Unreacted gases may pass over further catalyst beds to improve conversion.
Catalyst
  • Catalyst vanadium(V) oxide, V2O5, speeds up the reaction without being used up.

Section 4 of 10Choice of temperature

Low temperature
  • Low temperature the forward reaction is exothermic, so a lower temperature shifts the equilibrium right and gives a higher yield of SO3.
Drawback
  • Drawback at low temperature the rate is too slow and the catalyst works poorly.
Compromise
  • 450 °C balances a good yield with a fast enough rate.

Section 5 of 10Choice of pressure

1High pressure
  • High pressure would push the equilibrium towards SO3, because 3 mol of gas become 2 mol.
2Why it is low
  • Why it is low the equilibrium already lies far to the right, with about 96% SO3, so extra pressure is not worth the cost of equipment and energy.
3Safety
  • Safety high pressures are dangerous and could liquefy the sulfur dioxide.
4Slightly above atmospheric
  • Slightly above atmospheric about 200 kPa keeps the gases moving through the plant.

Section 6 of 10Turning SO3 into sulfuric acid

Not added to water
  • Not added to water the reaction is very exothermic and forms a hazardous, corrosive acid mist.
Oleum
  • SO3 is absorbed in concentrated (98%) sulfuric acid to form oleum, SO3 + H2SO4 → H2S2O7.
Dilution
  • Dilution oleum is carefully diluted with water, H2S2O7 + H2O → 2H2SO4.

Section 7 of 10Effect of changing conditions

ChangeEffect on yield of SO3Effect on rateIndustrial choice
Raise temperatureDecreasesIncreasesModerate: 450 °C
Raise pressureIncreasesIncreasesLow: 200 kPa (yield already high)
Add catalystNo changeIncreasesV2O5 used

Section 8 of 10More from the specification

1
Contact process
  • Contact process makes sulfuric acid by burning sulfur to sulfur dioxide, converting it to sulfur trioxide, then absorbing it
2
Reversible stage
  • 2SO2 + O2 forms 2SO3 over a vanadium(V) oxide catalyst at about 450 degrees Celsius and 1 to 2 atmospheres, an exothermic equilibrium system
3
Compromise conditions
  • Compromise conditions a lower temperature gives a higher yield of products but too slow a rate,
  • so a moderate temperature is used, like in the Haber process for nitrogen and hydrogen
4
Absorption stage
  • SO3 is dissolved in concentrated sulfuric acid to form oleum,
  • then diluted with water,
  • since adding SO3 directly to water makes a hazardous mist
5
Le Chatelier's principle
  • Le Chatelier's principle an endothermic or exothermic change shifts equilibrium to oppose changes in temperature and pressure
6
Reactants
  • The substances that react together at the start of a reaction.
7
Product
  • A substance formed in a reaction.
8
Endothermic
  • A reaction that takes in thermal energy from the surroundings.
9
Molecule
  • Two or more atoms held together by covalent bonds.
10
Hydrated
  • Containing water of crystallisation.
11
Anhydrous
  • Containing no water of crystallisation.
12
Hydrogen
  • A gas that gives a squeaky pop with a lighted splint.
13
Haber process
  • Industrial manufacture of ammonia from nitrogen and hydrogen.

Section 9 of 10Key terms

TermMeaning
Contact processIndustrial manufacture of sulfuric acid.
Vanadium(V) oxideV2O5, the catalyst for converting SO2 to SO3.
OleumH2S2O7, formed when SO3 is absorbed in concentrated sulfuric acid.
Roasting sulfide oresHeating a metal sulfide in air to give a metal oxide and sulfur dioxide.
Reversible reactionA reaction that can go in both directions (⇌).
Compromise conditionsConditions balancing rate, yield, cost and safety.

Section 10 of 10Exam technique

1
Saying SO3 is added directly to water — it is absorbed in concentrated sulfuric acid to make oleum.
2
Using the Haber conditions — the Contact process uses 200 kPa and V2O5, not 20 000 kPa and iron.
3
Saying a high pressure is used — the yield is already high at low pressure.
4
Saying the catalyst increases the yield — it only increases the rate.
5
Stating that a high temperature increases the yield of SO3 — the forward reaction is exothermic, so a high temperature lowers the yield

Last updated: