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A2 · Practice questions · Orders of reaction and rate equations

Find the orders.

Six original Cambridge-style questions on rate equations and orders of reaction.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Orders come from experiment.

01
Calculation
[4 marks]

For the reaction 2NO + 2H2 → N2 + 2H2O at constant temperature:

Expt[NO] / mol dm−3[H2] / mol dm−3initial rate / mol dm−3 s−1
10.01000.01001.20 × 10−5
20.02000.01004.80 × 10−5
30.01000.03003.60 × 10−5

Deduce the rate equation and calculate k with units.

  • Expts 1 and 2: [NO] doubles, rate ×4, so second order in NO. ✓
  • Expts 1 and 3: [H2] ×3, rate ×3, so first order in H2. ✓
  • Rate = k[NO]2[H2]. ✓
  • k = 1.20 × 10−5 / (0.01002 × 0.0100) = 12.0 dm6 mol−2 s−1. ✓
02
Calculation
[2 marks]

Using your answer to question 1, calculate the initial rate when [NO] = 0.0300 mol dm−3 and [H2] = 0.0200 mol dm−3.

  • Rate = 12.0 × 0.03002 × 0.0200. ✓
  • = 2.16 × 10−4 mol dm−3 s−1. ✓
03
Explain
[2 marks]

Explain why the order of a reaction cannot be deduced from the balanced equation.

  • Most reactions happen in several steps; the rate depends only on the species in or before the rate-determining step. ✓
  • So orders must be found by experiment, and they need not match the stoichiometric coefficients (2 for H2 in the equation above, but first order). ✓
04
Calculation
[3 marks]

The concentration of a reactant falls from 0.800 to 0.400 mol dm−3 in 35.0 s and from 0.400 to 0.200 mol dm−3 in a further 35.0 s. State the order and calculate k.

  • Successive half-lives are equal, so the reaction is first order. ✓
  • k = 0.693 / t½ = 0.693 / 35.0. ✓
  • k = 0.0198 s−1. ✓
05
Recall
[3 marks]

Sketch and describe the rate against concentration graphs for a reactant that is (a) zero order, (b) first order, (c) second order.

  • (a) horizontal line: rate is independent of [A]. ✓
  • (b) straight line through the origin: rate ∝ [A]. ✓
  • (c) curve upwards through the origin, getting steeper: rate ∝ [A]2. ✓
06
Calculation
[2 marks]

A reaction is second order overall with rate = k[A]2. When [A] = 0.150 mol dm−3 the rate is 4.50 × 10−3 mol dm−3 s−1. Calculate k with units.

  • k = 4.50 × 10−3 / 0.1502. ✓
  • k = 0.200 dm3 mol−1 s−1. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.