AQA A-Level Chemistry Paper 3, June 2018: Question 12

1 mark · Medium difficulty · Multiple Choice

Identify a possible rate equation for a reaction given the effect of doubling all reactant and catalyst concentrations on the reaction rate.

Practise this question

Question

Multiple choice question showing the chemical equation: 2W + X -> Z, reacting in the presence of soluble catalyst Y. It states that when the concentrations of W, X, and Y are all doubled, the rate of reaction increases by a factor of four. Four options for the rate equation are given: A: rate = k [W]^2 [X]; B: rate = k [W]^2 [Y]; C: rate = k [X] [Y]; D: rate = k [X] [Z].
Question text

12 Solutions of two compounds, W and X, react together in the presence of a soluble catalyst,

Y, as shown in the equation

2W + X → Z

When the concentrations of W, X and Y are all doubled, the rate of reaction increases by a

factor of four.

Which is a possible rate equation for this reaction?

[1 mark]

A rate = k [W]2 [X]

B rate = k [W]2 [Y]

C rate = k [X] [Y]

D rate = k [X] [Z]

Mark scheme

Show the mark scheme Mark scheme showing the answer for question 12 is option C.

12 C

How to answer it

Deducing Rate Equations from Overall Reaction Orders

📋 What this question tests

This question assesses your understanding of chemical kinetics and rate equations:

  • Relating changes in concentration to changes in overall reaction rate.
  • Recognising that homogeneous catalysts (here, Y) can appear in the rate equation.
  • Distinguishing between stoichiometric coefficients and kinetic orders of reaction.
  • Calculating overall order of a reaction from proportional changes.

Question 12 Analysis

Reaction: 2W + X → Z (Catalysed by Y)  |  1 Mark

✅ Correct Answer

C: rate = k [X] [Y]

Awarded 1 mark for selecting option C.

💡 Key Knowledge

  • Overall Order: If every species in the rate expression is doubled simultaneously, the rate increases by 2(overall order).
  • Catalysts: Catalysts participate in the rate-determining step and can appear in the rate equation, even though they do not appear in the overall balanced equation.
  • Stoichiometry ≠ Rate Law: The stoichiometric coefficients from the equation (2 for W, 1 for X) do not dictate the powers in the rate equation.

📐 Step-by-Step Deduction

  1. Determine the total overall order required:
    Let the reaction have an overall order of n with respect to the species present in the rate equation.
    When concentrations of W, X, and Y are all doubled (×2), the rate increases by a factor of 4:
    2n = 4 = 22  ⇒  n = 2 (overall order must be 2).
  2. Evaluate each given option:
    • Option A: rate = k [W]² [X]
      Order = 2 + 1 = 3. Doubling [W] and [X] gives a factor increase of 2² × 2¹ = 4 × 2 = 8 (Incorrect).
    • Option B: rate = k [W]² [Y]
      Order = 2 + 1 = 3. Doubling [W] and [Y] gives a factor increase of 2² × 2¹ = 8 (Incorrect).
    • Option C: rate = k [X] [Y]
      Order = 1 + 1 = 2 (and W is zero order, order = 0).
      Doubling [W], [X], and [Y] gives: (2⁰) × (2¹) × (2¹) = 1 × 2 × 2 = 4 (Correct).
    • Option D: rate = k [X] [Z]
      Z is a product, not one of the starting reactants/catalyst whose concentrations were doubled at the start (Incorrect).

🧠 Exam Technique

  • Fast Elimination: Immediately calculate the required sum of the powers. Since doubling all species quadrupled the rate, the sum of powers of {W, X, Y} must be 2.
  • Option A has sum = 3. Option B has sum = 3. This immediately leaves C as the only viable choice without complex math!

❌ Common Errors

  • Assuming stoichiometry equals order: Choosing A because students see 2W + X in the equation and assume the rate equation must be k [W]² [X] .
  • Believing catalysts cannot be in rate equations: Ruling out B and C because Y is a catalyst. A catalyst frequently appears in rate laws!
  • Confusing products with reactants: Overlooking that Z in option D is a reaction product.

Topics

Physical Chemistry · 3.1.9 Rate Equations

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, June 2018. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.