OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), November 2021: Question 9

1 mark · Medium difficulty · Multiple Choice

Identify which container will have the largest concentration of carbon monoxide at equilibrium given different initial amounts of reactants and products.

Practise this question

Question

The question presents an equilibrium system: CO(g) + NO2(g) <=> CO2(g) + NO(g). A table lists the initial moles of CO, NO2, CO2, and NO for four containers labeled A, B, C, and D. Container A has 1 mol CO and 1 mol NO2; Container B has 1 mol CO, 1 mol CO2, and 1 mol NO; Container C has 1 mol CO, 1 mol NO2, and 1 mol CO2; Container D has 1 mol NO2, 1 mol CO2, and 1 mol NO. Below the table, the question asks which container would have the largest concentration of CO(g) at equilibrium, followed by a box for the answer.
Question text

9 An equilibrium system is shown below.

CO(g) + NO2(g) CO2(g) + NO(g)

Different amounts of CO, NO2, CO2 and NO are added to four containers, as shown below.

Container CO / mol NO2/ mol CO2/ mol NO / mol

A 1 1 0 0

B 1 0 1 1

C 1 1 1 0

D 0 1 1 1

Which container would have the largest concentration of CO(g) at equilibrium?

Your answer [1]

Mark scheme

Show the mark scheme The mark scheme table indicates the correct answer is B, worth 1 mark, classified under assessment objective AO2.2.

9 B 1 AO2.2

How to answer it

Equilibrium Position & Le Chatelier's Principle

What this question tests

This question assesses your understanding of chemical equilibria, Le Chatelier's Principle, and how the relative initial amounts of reactants and products dictate the direction in which a system shifts to re-establish equilibrium (Applying Knowledge AO2.2).

Question 9 Analysis

Evaluating Containers for Maximum CO(g) at Equilibrium

✅ Correct Answer

B

Mark Awarded: 1 / 1 for selecting B

💡 Key Knowledge

  • Le Chatelier's Principle: If a factor is changed, the position of equilibrium shifts to counteract that change.
  • Position of Equilibrium: To maximise the amount of CO(g) at equilibrium, the system needs to shift heavily to the left (towards the reactants).
  • Equilibrium Constant Expression: For CO(g) + NO₂(g) ⇌ CO₂(g) + NO(g) , Kc has equal moles on both sides (1:1 stoichiometric ratio), meaning total volume changes don't shift equilibrium, but changes in concentration do.

🧠 Exam Technique

Look at the position of CO in the chemical equation—it is a reactant. Therefore, to maximise its concentration at equilibrium, you must choose conditions that favour the reverse reaction (shifting left).

Container B contains initial moles of CO = 1 , NO₂ = 0 , CO₂ = 1 , and NO = 1 . Having zero initial NO₂ combined with products already present forces the equilibrium to shift entirely to the left to produce reactants, resulting in the highest final concentration of CO .

❌ Common Errors

  • Assuming containers with the highest initial moles of CO straight out of the table will automatically give the highest at equilibrium without factoring in shifts.
  • Confusing the direction of the shift by forgetting that reactants are on the left side of the equation.

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 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.