AQA A-Level Chemistry Paper 3, 2017: Question 20

1 mark · Medium difficulty · Multiple Choice

Identify which property increases when the temperature of the exothermic equilibrium between carbon monoxide, chlorine, and phosgene is decreased.

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Question

Multiple-choice question 20 showing the reversible reaction: CO(g) + Cl2(g) ⇌ COCl2(g) with an enthalpy change of -108 kJ mol⁻¹. The question asks which quantity is greater for the new equilibrium when the temperature of the mixture in a container of fixed volume is decreased: A (The mole fraction of carbon monoxide), B (The partial pressure of chlorine), C (The total pressure of the mixture), or D (The value of the equilibrium constant, Kp).
Question text

20 An equilibrium mixture is prepared in a container of fixed volume.

CO(g) + Cl (g) ⇌ COCl (g) ΔH = −108 kJ mol−1

The temperature of this mixture is decreased and the mixture is allowed to reach a

new equilibrium.

Which is greater for the new equilibrium than for the original equilibrium?

[1 mark]

A The mole fraction of carbon monoxide

B The partial pressure of chlorine

C The total pressure of the mixture

D The value of the equilibrium constant, Kp

Mark scheme

Show the mark scheme Mark scheme table showing question number 20 corresponds to the correct answer D.

20 D

How to answer it

Equilibria: Effect of Temperature on Yield and Kp

📋 What this question tests

This question assesses your understanding of dynamic equilibria in gaseous systems, specifically applying Le Chatelier's Principle to predict shifts caused by temperature changes, understanding the conditions that alter the numerical value of an equilibrium constant (Kp), and tracking changes in mole fractions and partial/total pressures.

Question 20 • Multiple Choice [1 Mark]

Temperature Decrease on an Exothermic Equilibrium

Reaction: CO(g) + Cl₂(g) ⇌ COCl₂(g)    ΔH = -108 kJ mol⁻¹

✅ Correct Answer: D

The value of the equilibrium constant, Kp is greater for the new equilibrium.

Award [1 mark] for selecting option D.

💡 Core Chemical Principles

  • Enthalpy sign: ΔH = -108 kJ mol⁻¹ indicates the forward reaction is exothermic.
  • Le Chatelier's Principle: Lowering temperature shifts the position of equilibrium in the exothermic direction (to the right) to release thermal energy and oppose the decrease.
  • Kp dependence: Temperature is the only factor that changes the value of an equilibrium constant. Because the forward reaction is favoured, the ratio of product partial pressure to reactant partial pressures increases, so Kp increases.

📐 Step-by-Step Analysis of All Options

  1. A is incorrect: The shift to the right consumes CO(g) and produces COCl₂(g). The amount of CO decreases relative to total moles, so the mole fraction of CO decreases.
  2. B is incorrect: Equilibrium shifts to the right, reducing moles of Cl₂(g). Both lower moles of gas and lower temperature cause the partial pressure of chlorine to decrease.
  3. C is incorrect: 2 moles of gas react to form 1 mole of gas (decrease in total moles). Furthermore, reducing temperature directly decreases pressure in a fixed volume ( p = nRT/V ). Hence, total pressure must decrease.
  4. D is correct: Since Kp = p(COCl₂) / [p(CO) × p(Cl₂)] , shifting towards products as temperature falls raises the numerator and lowers the denominator, meaning Kp increases.

🧠 Exam Technique & Elimination Strategy

  • Golden Rule of K: Whenever an exam question changes only temperature, check the sign of ΔH first.
    • Exothermic (ΔH < 0) + Temp ↓ → Kp ↑
    • Endothermic (ΔH > 0) + Temp ↓ → Kp ↓
  • Spotting the Odd One Out: Options A, B, and C all describe reactant quantities or total system pressures, all of which fall when shifting forward in a rigid container. Only D reflects the increase in forward yield.

❌ Common Misconceptions & Traps

  • Believing "Kp is constant under all conditions": Many students remember that changing concentration, pressure, or adding a catalyst does not change Kp, and mistakenly apply this to temperature as well. Temperature changes DO change Kp.
  • Confusing the effect of temperature on pressure: Lowering temperature lowers gas particle kinetic energy and collision frequency with the container walls ( p ∝ T ). Even without a shift, total pressure drops; the shift from 2 moles of gas to 1 mole reduces it even further.
  • Sign errors for ΔH: Misreading a negative value ( -108 kJ mol⁻¹ ) as endothermic will reverse the entire logical chain. Always pause and confirm: negative = exothermic.

Topics

Physical Chemistry · 3.1.6 Chemical Equilibria, Le Chatelier's Principle and Kc · 3.1.10 Equilibrium Constant Kp

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, 2017. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.