OCR A-Level Chemistry AS Breadth in chemistry (01), June 2022: Question 13

1 mark · Medium difficulty · Multiple Choice

Identify which of the four given equilibrium reactions has a numerical Kc value of 0.01 when the concentration of each gas in the equilibrium mixtures is 0.1 mol dm-3.

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Question

Multiple choice question 13 stating that four equilibrium reactions are set up where the concentration of each gas is 0.1 mol dm-3. Option A: CH4(g) + 2H2O(g) right-left-arrow CO2(g) + 4H2(g). Option B: N2(g) + 3H2(g) right-left-arrow 2NH3(g). Option C: H2(g) + I2(g) right-left-arrow 2HI(g). Option D: 2NO2(g) right-left-arrow N2O4(g). An answer box and mark allocation of [1] is at the bottom right.
Question text

13 Four equilibrium reactions are set up.

The concentration of each gas in the equilibrium mixtures is 0.1 mol dm–3.

Which equilibrium has a numerical Kc value of 0.01?

A CH4(g) + 2H2O(g) CO2(g) + 4H2(g)

B N2(g) + 3H2(g) 2NH3(g)

C H2(g) + I2(g) 2HI(g)

D 2NO2(g) N2O4(g)

Your answer [1]

Mark scheme

Show the mark scheme The mark scheme shows the correct answer is A for question number 13, worth 1 mark.

13 A 1 AO2.6

How to answer it

Calculating Equilibrium Constants (Kc)

What this question tests

This question assesses your ability to construct equilibrium constant expressions (Kc) from balanced chemical equations, substitute constant equilibrium concentration values (0.1 mol dm⁻³), and evaluate indices to match a target numerical value.

Question 13

Exam Breakdown & Step-by-Step Solution

✅ Correct Answer

A

CH₄(g) + 2H₂O(g) ⇌ CO₂(g) + 4H₂(g)

💡 Key Knowledge

  • The general Kc expression is products over reactants, each raised to the power of their stoichiometric coefficient.
  • Since all gases have an equilibrium concentration of 0.1 mol dm⁻³ , you substitute 0.1 into every term in the expression.
  • 0.1 can be written in index form as 10⁻¹ , making index arithmetic much faster.

🧠 Exam Technique

Do not waste time calculating full values if you can use indices mentally. For option A: Kc = [0.1][0.1]⁴ / [0.1][0.1]² . Cancel out common terms before final evaluation to prevent calculator errors.

❌ Common Errors

Students often invert the Kc expression (putting reactants on top) or forget to raise concentrations to the power of the balancing numbers (stoichiometry), leading to incorrect powers of 10.

📐 Step-by-Step Calculation for Option A

  1. Write the Kc expression:
    Kc = ([CO₂][H₂]⁴) / ([CH₄][H₂O]²)
  2. Substitute the concentration ( 0.1 mol dm⁻³ or 10⁻¹ ) into all terms:
    Kc = ((10⁻¹)(10⁻¹)⁴) / ((10⁻¹)(10⁻¹)²)
  3. Simplify the powers:
    Numerator = (10⁻¹)(10⁻⁴) = 10⁻⁵
    Denominator = (10⁻¹)(10⁻²) = 10⁻³
  4. Calculate final value:
    Kc = 10⁻⁵ / 10⁻³ = 10⁻² = 0.01
Mark Scheme Note: 1 mark awarded for selecting option A. (AO2.6: Applying chemical knowledge to numerical formats).

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, AS Breadth in chemistry (01), June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.