OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), November 2020: Question 10

1 mark · Medium difficulty · Multiple Choice

Deduce the overall order of the reaction between ICl and H2 given the units of the rate constant.

Practise this question

Question

Multiple choice question 10 showing the equation 2ICl(g) + H2(g) -> 2HCl(g) + I2(g) and stating the rate constant k is 1.63 x 10^-6 dm3 mol^-1 s^-1. Four options A, B, C, D give the values 0, 1, 2, 3 respectively.
Question text

10 The equation for the reaction of ICl and H2 is shown below.

2ICl(g) + H2(g) 2HCl(g) + I2(g)

The rate constant k for this reaction is 1.63 × 10−6 dm3 mol−1 s−1.

What is the overall order of the reaction?

A 0

B 1

C 2

D 3

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme for question 10 indicating the correct answer is C, with 1 mark awarded, and allowing '2' in the answer box.

10 C 1 1.2 ALLOW 2 in the answer box

How to answer it

Determining Overall Order from Rate Constant Units

📋 What this question tests

This question assesses your ability to deduce the overall order of a chemical reaction directly from the units of its rate constant ($k$). It tests your manipulation of algebraic units and understanding of rate equations without needing experimental concentration-time data.

Question 10: Multiple Choice Solution

Full Mark Breakdown & Answer

✅ Correct Answer

Option C (2)

Mark Scheme: C (Accept writing '2' in the answer box). Worth 1 mark.

💡 Key Knowledge

  • Rate has units of mol dm⁻³ s⁻¹ .
  • The rate equation is generally written as Rate = k[A]^x[B]^y .
  • Overall order = sum of powers ( x + y ).

🧠 Exam Technique

Do not be distracted by the stoichiometry of the balanced equation ( 2ICl + H₂ → 2HCl + I₂ ). Stoichiometric coefficients never equal reaction orders unless explicitly stated as an elementary step.

❌ Common Errors

Students often incorrectly assume the overall order is 3 by adding the stoichiometric coefficients ( 2 + 1 = 3 ), or by miscalculating the indices when rearranging the units of $k$.

📐 Step-by-Step Derivation of Overall Order

  1. Write out the general rate equation:
    Rate = k[A]^n (where n is the overall order).
  2. Rearrange to make $k$ the subject:
    k = Rate / [A]^n
  3. Substitute the units into the expression:
    Units of $k$ = (mol dm⁻³ s⁻¹) / (mol dm⁻³)^n
  4. Match with the given units ( dm³ mol⁻¹ s⁻¹ ):
    Given units: mol⁻¹ dm³ s⁻¹
    Setting up the index equation for concentration units ( mol dm⁻³ ):
    (mol dm⁻³)¹ / (mol dm⁻³)^n = (mol dm⁻³)⁻¹
    Therefore, 1 - n = -1 , which means n = 2 .

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 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.