OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), November 2020: Question 13

1 mark · Medium difficulty · Multiple Choice

Identify the correct statements about the effect of adding a catalyst to a system in dynamic equilibrium.

Practise this question

Question

Multiple-choice question 13 asking which statements are correct when a catalyst is added to a system in dynamic equilibrium. Three numbered statements are listed: 1. The rates of the forward and reverse reactions increase by the same amount. 2. The concentrations of the reactants and products do not change. 3. The value of K_c increases. Four options A, B, C, D are provided.
Question text

13 Which statement(s) is/are correct when a catalyst is added to a system in dynamic equilibrium?

1 The rates of the forward and reverse reactions increase by the same amount.

2 The concentrations of the reactants and products do not change.

3 The value of Kc increases.

A 1, 2 and 3

B Only 1 and 2

C Only 2 and 3

D Only 1

Your answer [1]

Mark scheme

Show the mark scheme The mark scheme indicates that the correct answer for question 13 is option B, worth 1 mark.

13 B 1 1.1

How to answer it

Catalysts in Dynamic Equilibrium

What this question tests

This question assesses your fundamental understanding of how catalysts affect dynamic equilibrium systems. It tests your knowledge of reaction rates, position of equilibrium, equilibrium concentrations, and the equilibrium constant ( K_c ).

Question 13

Multiple Choice Analysis

✅ Correct Answer: B (Only 1 and 2)

Adding a catalyst speeds up both the forward and reverse rates equally (Statement 1) and does not disturb the equilibrium concentrations or position (Statement 2). Therefore, statements 1 and 2 are correct, making option B the right choice.

💡 Key Knowledge

  • A catalyst provides an alternative reaction pathway with a lower activation energy.
  • It increases the rate of the forward and reverse reactions by the exact same amount.
  • It has no effect on the position of equilibrium or equilibrium yields.
  • The equilibrium constant ( K_c ) is only affected by changes in temperature.

🧠 Exam Technique

Evaluate each numbered statement individually using a true/false checklist before looking at the letter options. Once you determine a statement's validity, instantly eliminate options that do not match to save valuable time in multiple-choice sections.

❌ Common Errors

  • The K_c Trap: Many students mistakenly believe a catalyst alters K_c (Statement 3). Remember that only temperature changes the value of K_c .
  • Shifting Equilibrium: Thinking a catalyst shifts equilibrium to the left or right. It only helps the system reach equilibrium faster.
Examiner Insight: This style of question quickly exposes gaps between kinetics (rates) and thermodynamics/equilibria. Top-level candidates immediately spot that catalysts do not alter equilibrium constants or yields, swiftly arriving at option B.

Topics

Module 5: Physical chemistry and transition elements · 5.1 Rates, equilibrium and pH

Question and mark scheme from the OCR A-Level Chemistry examination, Periodic table, elements and physical chemistry (01), November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.