OCR A-Level Chemistry AS Breadth in chemistry (01), June 2019: Question 10

1 mark · Medium difficulty · Multiple Choice

Identify the correct statement about a reversible reaction from its enthalpy profile diagram showing activation energy and enthalpy change values.

Practise this question

Question

An enthalpy profile diagram for a reversible reaction, plotting energy in kJ mol^-1 against progress of reaction. The reactants start at an energy level, rise to a peak with an activation energy gap labeled 150, and fall to a lower products energy level with a difference of 120 between reactants and products. Below the diagram are four multiple-choice statements labeled A, B, C, and D regarding the activation energies and enthalpy changes.
Question text

10 A reversible reaction has the enthalpy profile diagram shown below.

energy

–1 reactants

/kJmol

products

progress of reaction

Which statement about this reaction is correct?

A The activation energy of the forward reaction is 120 kJ mol−1.

B The activation energy of the reverse reaction is 270 kJ mol−1.

C The enthalpy change of the forward reaction is −30 kJ mol−1.

D The reverse reaction is exothermic.

Your answer [1]

Mark scheme

Show the mark scheme The mark scheme table shows question number 10 corresponding to the correct answer B, worth 1 mark under assessment objective AO1.2.

10 B 1 AO1.2

How to answer it

Interpreting Enthalpy Profile Diagrams

What this question tests

This question assesses your ability to interpret enthalpy profile diagrams for reversible reactions. Specifically, it tests your understanding of activation energy (Eₐ) for both forward and reverse pathways, enthalpy change of reaction (ΔH), and the relationship between energy values displayed on an energy profile.

Question 10 (Multiple Choice)

Analyzing the Enthalpy Profile

✅ Correct Answer: B

Statement B ("The activation energy of the reverse reaction is 270 kJ mol⁻¹") is correct.

Mark Awarded: 1 / 1 (AO1.2)

💡 Key Knowledge

  • Forward Activation Energy (Eₐ): Energy difference from reactants to the peak (given as 150 kJ mol⁻¹ ).
  • Enthalpy Change (ΔH): Energy difference between reactants and products (given as 120 kJ mol⁻¹ , going downwards so −120 kJ mol⁻¹ ).
  • Reverse Activation Energy: Energy difference from products up to the peak. Calculated as 150 + 120 = 270 kJ mol⁻¹ .

🧠 Exam Technique

Test each option methodically rather than guessing:

  • A: Forward Eₐ is 150 , not 120 . (Incorrect)
  • B: Reverse Eₐ starts at products and goes to the peak ( 120 + 150 = 270 ). (Correct)
  • C: ΔH is −120 kJ mol⁻¹ , not −30 . (Incorrect)
  • D: The forward reaction is exothermic (products are lower than reactants). Therefore, the reverse reaction must be endothermic. (Incorrect)

❌ Common Errors

  • Confusing forward activation energy with the enthalpy change interval.
  • Subtracting values instead of adding them when calculating the activation energy of the reverse reaction from the products.
  • Assuming the reverse of an exothermic reaction is also exothermic.

Topics

Module 3: Periodic table and energy · 3.2 Physical chemistry

Question and mark scheme from the OCR A-Level Chemistry examination, AS Breadth in chemistry (01), June 2019. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.