Rate equations and orders of reactionEdexcel A-Level Chemistry: Subtopic test

1. 1
   The reaction between peroxodisulfate ions and iodide ions is S₂O₈²⁻(aq) + 2I⁻(aq) → 2SO₄²⁻(aq) + I₂(aq). Initial rates were measured at constant temperature. Experiment 1: [S₂O₈²⁻] = 0.020 mol dm⁻³, [I⁻] = 0.040 mol dm⁻³, initial rate = 2.4 × 10⁻⁶ mol dm⁻³ s⁻¹. Experiment 2: [S₂O₈²⁻] = 0.040 mol dm⁻³, [I⁻] = 0.040 mol dm⁻³, initial rate = 4.8 × 10⁻⁶ mol dm⁻³ s⁻¹. Experiment 3: [S₂O₈²⁻] = 0.040 mol dm⁻³, [I⁻] = 0.080 mol dm⁻³, initial rate = 9.6 × 10⁻⁶ mol dm⁻³ s⁻¹.

   (a)

   Comparing Experiments 1 and 2, what is the order of reaction with respect to S₂O₈²⁻?

   [1 mark]

   • AZero
   • BSecond
   • CFirst
   • DThird

   (b)

   The order with respect to I⁻ is first order. What is the overall order of the reaction?

   [1 mark]

   • A2
   • B3
   • C1
   • D0

   (c)

   Calculate the value of the rate constant, k, for this reaction, including its units.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Dinitrogen pentoxide, N₂O₅, decomposes in solution at constant temperature. The initial concentration of N₂O₅ is 0.80 mol dm⁻³. It falls to 0.40 mol dm⁻³ after 120 s and to 0.20 mol dm⁻³ after a further 120 s. The rate constant for the reaction at this temperature is 5.78 × 10⁻³ s⁻¹.

   (a)

   What is the order of reaction with respect to N₂O₅?

   [1 mark]

   • AZero
   • BSecond
   • CThird
   • DFirst

   (b)

   What is the concentration of N₂O₅ after 360 s from the start?

   [1 mark]

   • A0.20 mol dm⁻³
   • B0.10 mol dm⁻³
   • C0.05 mol dm⁻³
   • D0.00 mol dm⁻³

   (c)

   Calculate the initial rate of reaction, including units.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Nitrogen monoxide reacts with hydrogen: 2NO(g) + 2H₂(g) → N₂(g) + 2H₂O(g). Initial rates were measured at constant temperature. Experiment 1: [NO] = 0.010 mol dm⁻³, [H₂] = 0.010 mol dm⁻³, initial rate = 2.0 × 10⁻⁷ mol dm⁻³ s⁻¹. Experiment 2: [NO] = 0.020 mol dm⁻³, [H₂] = 0.010 mol dm⁻³, initial rate = 8.0 × 10⁻⁷ mol dm⁻³ s⁻¹. Experiment 3: [NO] = 0.010 mol dm⁻³, [H₂] = 0.030 mol dm⁻³, initial rate = 6.0 × 10⁻⁷ mol dm⁻³ s⁻¹.

   (a)

   Use the data to deduce the order of reaction with respect to NO and with respect to H₂, and write the rate equation.

   [3 marks]

   (b)

   Calculate the value of k, with units, and hence calculate the initial rate when [NO] = 0.015 mol dm⁻³ and [H₂] = 0.040 mol dm⁻³.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student investigates the decomposition of ammonia on a hot tungsten wire, 2NH₃(g) → N₂(g) + 3H₂(g), at constant temperature. The concentration of ammonia falls from 0.100 mol dm⁻³ to 0.080 mol dm⁻³ in the first 40 s, to 0.060 mol dm⁻³ after 80 s and to 0.040 mol dm⁻³ after 120 s. The student then studies a second, homogeneous reaction in solution and records its concentration data continuously, but the order with respect to the reactant is not yet known.

   (a)

   The ammonia decomposition is zero order with respect to ammonia. Use the data to explain this, describe the shape of the concentration-time graph and of the rate-concentration graph, and explain why the rate does not depend on the concentration of ammonia.

   [6 marks]

   (b)

   Describe how the student could use the continuous concentration–time data for the second reaction to decide whether it is first order or second order with respect to the reactant.

   [6 marks]

   Total for question 4: 12 marks

End of questions