Catalysts in Equilibrium
Cambridge IGCSE Chemistry 0620 — Stopping the Trap
⚠️ The #1 Cambridge Trap in Topic 6.3
Students know a catalyst "speeds things up" and instinctively write that it "shifts equilibrium to the right." This is WRONG and costs marks every year. A catalyst increases the rate of both forward and reverse reactions by the same proportion — so the equilibrium position, yield, and Kc are all unchanged.
Blue = no catalyst · Green dashed = with catalyst · Note: ΔH (product–reactant gap) is IDENTICAL
The Effect
5 Questions
The Reason
5 Questions
Industrial Value
5 Questions
Test your understanding of what a catalyst does — and does NOT — do to equilibrium.
State the effect of a catalyst on the position of equilibrium.
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State the effect of a catalyst on the final yield of products at equilibrium.
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A student adds a catalyst to a reaction mixture at equilibrium.
Predict the effect on the equilibrium constant, Kc, when a catalyst is added.
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Consider the reversible reaction: N₂(g) + 3H₂(g) ⇌ 2NH₃(g)
An iron catalyst is used in this process. State whether the percentage yield of ammonia increases, decreases, or remains the same when the catalyst is introduced.
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State the effect of a catalyst on the time taken to reach equilibrium.
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Explain the mechanism behind why a catalyst cannot shift equilibrium. These are the 2-mark reasoning questions Cambridge loves.
Explain why a catalyst does not change the position of equilibrium. [2 marks]
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A catalyst works by providing an alternative reaction pathway. Using this idea, explain why the forward and reverse rates increase by the same proportion. [2 marks]
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The diagram shows an energy profile for a reaction with and without a catalyst.
Using the energy profile concept, explain why introducing a catalyst does not shift the equilibrium position. [2 marks]
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A student claims: 'A catalyst must shift the equilibrium to the right because it makes reactions go faster.' Identify and correct the error in this statement. [2 marks]
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Compare and contrast the effect of increasing temperature with the effect of adding a catalyst on a reversible exothermic reaction. [2 marks]
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Apply your understanding to real industrial processes. Why bother with a catalyst if it doesn't improve yield?
The Haber Process: N₂(g) + 3H₂(g) ⇌ 2NH₃(g) ΔH = −92 kJ/mol
If a catalyst does not increase the yield of ammonia, state TWO reasons why an iron catalyst is still used in the Haber Process. [2 marks]
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The Contact Process for sulfuric acid manufacture uses a vanadium(V) oxide catalyst: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g) ΔH = −196 kJ/mol
Explain the economic importance of the vanadium(V) oxide catalyst in this industrial process. [2 marks]
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A chemical engineer is designing a new industrial process for a reversible exothermic reaction. She wants the highest possible yield but also needs the process to be commercially viable. Explain the dilemma she faces with temperature, and state how a catalyst helps resolve it. [3 marks]
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In the Haber Process, the operating conditions are: Temperature: 450°C, Pressure: 200 atm, Catalyst: iron with promoters
The equilibrium yield of ammonia at 450°C and 200 atm is only about 15%. Suggest why the process is still economical despite this low yield. [2 marks]
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A new catalyst is discovered that is three times more active than iron for the Haber Process. Predict the effect of this new catalyst on: (a) the rate of reaching equilibrium, (b) the equilibrium yield of ammonia, (c) the equilibrium constant Kc. [3 marks]
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🎯 The Three Rules to Memorise
- 1. Catalyst → NO change in equilibrium position
- 2. Catalyst → NO change in yield or Kc
- 3. Catalyst → FASTER rate of reaching equilibrium (both directions equally)
🏭 Industrial Magic Sentence
"A catalyst allows a lower operating temperature to be used whilst still achieving a sufficient rate of reaction, reducing energy costs and making the process economically viable."
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