OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2025: Question 9

1 mark · Easy difficulty · Multiple Choice

Deduce the rate equation for the reaction of hydrogen with iodine monochloride given a two-step mechanism with a slow first step.

Practise this question

Question

Question 9 presents the overall reaction: H2 + 2ICl -> 2HCl + I2. The reaction mechanism is shown in two steps: a slow step consisting of H2 + ICl -> HCl + HI, followed by a fast step consisting of HI + ICl -> HCl + I2. Four multiple choice options for the rate equation are given: A: rate = k[HI][ICl], B: rate = k[H2][ICl], C: rate = k[H2][ICl]^2, and D: rate = k[H2][HI][ICl]^2, with an answer box for 1 mark.

Mark scheme

Show the mark scheme Mark scheme for question 9 showing the correct option is B, worth 1 mark.

How to answer it

Deducing Rate Equations from Multi-Step Reaction Mechanisms

What this question tests

This question assesses your ability to relate a multi-step reaction mechanism to the experimentally observed rate equation. Specifically, you must identify the rate-determining step (RDS) and use the stoichiometry of the reactants in and before this step to deduce the overall orders of reaction with respect to each reactant.

Multiple Choice Question 9 • 1 Mark

Deducing the Rate Equation from the Slow Step

Overall reaction: H₂ + 2ICl → 2HCl + I₂

Step 1 (slow): H₂ + ICl → HCl + HI
Step 2 (fast): HI + ICl → HCl + I₂

✅ Correct Answer

Correct Option: B

rate = k[H₂][ICl]

Because Step 1 is the slow step (the rate-determining step), the rate of the entire reaction depends solely on the reactants taking part in this step:

  • 1 molecule of H₂ → First order with respect to H₂
  • 1 molecule of ICl → First order with respect to ICl

💡 Key Knowledge

  • Rate-Determining Step (RDS): The slowest step in a multi-step reaction mechanism acts as a bottleneck, limiting the overall reaction rate.
  • Reactants in the RDS: Species that appear as reactants in the slow step (or prior fast equilibrium steps) appear in the rate equation.
  • Stoichiometric coefficients: The stoichiometric numbers of reactant particles in the RDS determine their respective orders of reaction.
  • Intermediates: Species formed in one step and consumed in another (e.g. HI ) do not appear in the overall rate equation.

📐 Step-by-Step Deduction

  1. Identify the rate-determining step: Step 1 is clearly labeled slow step .
  2. Identify the reactants in this step: Step 1 involves 1 × H₂ and 1 × ICl .
  3. Determine orders:
    • Order with respect to H₂ = 1
    • Order with respect to ICl = 1
  4. Check for preceding steps: Step 1 is the very first step, so there are no pre-equilibrium steps to consider.
  5. Construct the rate equation: rate = k[H₂]¹[ICl]¹ = k[H₂][ICl]

❌ Common Errors & Traps

  • Confusing overall stoichiometry with rate orders: Choosing C ( rate = k[H₂][ICl]² ) by wrongly assuming the stoichiometric coefficient 2 in the overall equation implies second order for ICl. The second ICl molecule reacts in the fast step, after the bottleneck!
  • Including intermediates: Choosing A or D. HI is a reaction intermediate. Standard rate equations express rate strictly in terms of reactants (and occasionally catalysts), not transient intermediates.
  • Adding all steps together: Assuming every reactant in all steps must be multiplied together.

🧠 Examiner Tips & Technique

  • Speed strategy for Section A: When Step 1 is the slow step, read the rate equation directly from the reactants of Step 1. You can solve this question in under 10 seconds!
  • "Rate equations are experimentally determined": Mechanisms are proposals to match the rate equation, not the other way around. Never write a rate equation from the overall balanced equation alone.
  • Sanity Check: Always verify that intermediate species (like HI ) cancel out completely to give the overall equation provided.
Mark Scheme Reference: Question 9 • Answer: B • Total: 1 mark

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), June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.