OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2022: Question 10

1 mark · Easy difficulty · Multiple Choice

Determine the signs of enthalpy change (ΔH) and entropy change (ΔS) for the condensation of ammonia gas.

Practise this question

Question

Multiple choice question 10 asking for the signs of enthalpy change and entropy change for the condensation of ammonia gas, with four options A, B, C, D representing different combinations of positive and negative signs for delta H and delta S.
Question text

10 For the condensation of ammonia gas, what are the signs of ∆H and ∆S?

A ∆H –ve ∆S –ve

B ∆H –ve ∆S +ve

C ∆H +ve ∆S +ve

D ∆H +ve ∆S –ve

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme indicating the correct answer is option A.

10 A 1 2.1

How to answer it

Signs of Enthalpy and Entropy for Condensation

What this question tests

This question tests your understanding of phase transitions (specifically condensation) in terms of thermodynamic property changes: enthalpy change ( ΔH ) and entropy change ( ΔS ). You need to recall how molecular interactions and disorder change when a gas turns into a liquid.

Question 10 (Multiple Choice)

Determining ΔH and ΔS for Condensation

✅ Correct Answer: A

ΔH is negative ( -ve ) and ΔS is negative ( -ve ).

💡 Key Knowledge

  • Condensation: Gas turns into liquid ( NH₃(g) → NH₃(l) ).
  • Enthalpy ( ΔH ): Making bonds or intermolecular forces releases energy to the surroundings, making ΔH exothermic ( -ve ).
  • Entropy ( ΔS ): Gases have high disorder (random movement). Liquids have less disorder. Going from gas to liquid decreases system disorder, making ΔS negative ( -ve ).

🧠 Exam Technique

Always associate state changes with physical intuition first:
• Boiling/Melting/Sublimation = Endothermic ( +ve ΔH ), Increased disorder ( +ve ΔS )
• Condensation/Freezing = Exothermic ( -ve ΔH ), Decreased disorder ( -ve ΔS )

❌ Common Errors

Students often confuse the entropy change of the system with the entropy change of the surroundings. Remember that the question asks for ΔS of the process (system), where molecules become constrained in a liquid volume compared to a gas volume.

Mark Allocation: [1 mark] awarded for selecting option A.

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), June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.