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IGCSE · Practice questions · The Haber and Contact processes

Compromise for profit.

Six original Cambridge-style questions on the Haber and Contact processes: raw materials, equations, conditions and the reasons behind them.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Yield, rate, safety, cost.

01
Recall
[4 marks]

For the Haber process, write the equation with state symbols and the reversible sign, and state the sources of the two raw materials and the three conditions used.

  • N2(g) + 3H2(g) ⇌ 2NH3(g). ✓
  • Nitrogen from the air; hydrogen from methane. ✓
  • 450 °C and 20 000 kPa (200 atm). ✓
  • Iron catalyst. ✓
02
Recall
[3 marks]

For the Contact process, state the equation for the reversible step, the catalyst, and two sources of sulfur dioxide.

  • 2SO2 + O2 ⇌ 2SO3. ✓
  • Vanadium(V) oxide. ✓
  • Burning sulfur; roasting sulfide ores. ✓
03
Explain
[3 marks]

(Extended) The forward reaction in the Contact process is exothermic. Explain why a temperature of 450 °C is used.

  • A lower temperature would move the equilibrium to the right and give a higher yield of sulfur trioxide. ✓
  • But at a lower temperature the rate would be too slow. ✓
  • 450 °C is a compromise giving a reasonable yield at an acceptable rate. ✓
04
Explain
[4 marks]

(Extended) Explain why a pressure of 20 000 kPa is used in the Haber process, rather than a lower or a much higher pressure.

  • There are 4 gas molecules on the left and 2 on the right, so higher pressure moves the equilibrium to the right and increases the yield. ✓
  • Higher pressure also increases the rate, because the gas particles collide more frequently. ✓
  • But very high pressures are expensive to produce and need stronger, costlier equipment, ✓
  • and they are more dangerous; 20 000 kPa is a compromise. ✓
05
Explain
[2 marks]

Describe how ammonia is separated from the unreacted gases in the Haber process, and what happens to those gases.

  • The mixture is cooled; ammonia condenses to a liquid and is removed. ✓
  • The unreacted nitrogen and hydrogen are recycled back to the reactor. ✓
06
Explain
[2 marks]

(Extended) Explain why a catalyst is used in the Haber process even though it does not increase the yield.

  • It increases the rate, so equilibrium is reached faster and more ammonia is made per hour. ✓
  • It allows a reasonable rate at a lower temperature, which gives a better yield and saves energy costs. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.