OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), November 2021: Question 11

1 mark · Medium difficulty · Multiple Choice

Identify the correct signs for enthalpy change and entropy change for a reaction that is feasible at high temperatures but not feasible at low temperatures.

Practise this question

Question

Multiple choice question 11 asking to identify the row with the correct signs for enthalpy change (Delta H) and entropy change (Delta S) for a reaction feasible at high temperatures and not feasible at low temperatures. A table shows four options: A (+ Delta H, + Delta S), B (+ Delta H, - Delta S), C (- Delta H, + Delta S), and D (- Delta H, - Delta S), with an answer box below.
Question text

11 The feasibility of a chemical reaction depends on the temperature and the signs of ΔH and ΔS.

Which row has signs for ΔH and ΔS for a reaction that is feasible at high temperatures but is not

feasible at low temperatures?

ΔH ΔS

A + +

B + –

C – +

D – –

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme showing that the correct answer for question 11 is option A, worth 1 mark under assessment objective AO1.2.

11 A 1 AO1.2

How to answer it

Feasibility, Enthalpy, and Entropy Changes

OCR A-Level Chemistry • Multiple Choice Question

What this question tests

This question assesses your understanding of chemical thermodynamics, specifically the Gibbs Free Energy equation ( ΔG = ΔH - TΔS ) and how temperature affects reaction feasibility based on the sign combinations of enthalpy change ( ΔH ) and entropy change ( ΔS ).

Question 11 (Part A Analysis)

Determining the Temperature Dependence of Feasibility

✅ Correct Answer: A

ΔH is positive ( + ) and ΔS is positive ( + ).

Marks awarded: 1 / 1 (AO1.2)

💡 Key Knowledge

  • A reaction is feasible when ΔG ≤ 0 .
  • Equation: ΔG = ΔH - TΔS .
  • When ΔH is + and ΔS is + , the TΔS term must outweigh ΔH for ΔG to be negative. This requires a high temperature ( T ).

🧠 Exam Technique

Break down the Gibbs equation mathematically. For ΔG to become negative when ΔH is positive:

ΔH - TΔS < 0 ⇒ ΔH < TΔS

If ΔS is also positive, increasing temperature ( T ) makes the TΔS term larger, eventually making ΔG negative at high temperatures.

❌ Common Errors

  • Confusing the sign rules for temperature-dependent feasibility combinations.
  • Forgetting that temperature T is always in Kelvin (absolute temperature, so always positive), meaning the sign of TΔS depends entirely on the sign of ΔS .

Topics

Module 5: Physical chemistry and transition elements · 5.2 Energy

Question and mark scheme from the OCR A-Level Chemistry examination, Periodic table, elements and physical chemistry (01), November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.