OCR A-Level Chemistry AS Breadth in chemistry (01), June 2019: Question 13

✅ Correct Answer: Option B

Both rates increase by the same factor.

A catalyst lowers the activation energy (Eₐ) for both the forward and reverse reactions by the exact same amount. Consequently, it speeds up both directions equally, allowing the system to reach dynamic equilibrium faster without altering the equilibrium yield (position).

💡 Key Knowledge

• Definition of a Catalyst: A substance that increases the rate of a chemical reaction without being used up. It does this by providing an alternative reaction pathway with a lower activation energy.
• Equilibrium Position: A catalyst has zero effect on the position of equilibrium or the numerical value of the equilibrium constant (K_c or K_p).
• Symmetry: Because the forward and reverse pathways share the transition state / intermediate relationship in terms of energy barriers, lowering the barrier affects both directions symmetrically.

🧠 Exam Technique

• Eliminate distractors: Options C and D falsely assume a catalyst favours one direction over the other. Remember Le Chatelier's principle—catalysts do not change yields, they only change time-to-equilibrium.
• Read carefully: Ensure you look for wording that mentions *both* rates increasing equally rather than assuming a catalyst "starts" a reaction or shifts positions.

❌ Common Errors

• The Yield Myth: Students often mistakenly believe a catalyst increases the amount of product formed at equilibrium (confusing rate with yield).
• Directional Bias: Assuming a catalyst only speeds up the forward reaction to make more product faster. It speeds up the reverse reaction by the exact same multiplier!