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

1. 1
   A technician studies the gas-phase reaction 2NO(g) + 2H₂(g) → N₂(g) + 2H₂O(g) at constant temperature. Experiments show that the rate equation is rate = k[NO]²[H₂].

   (a)

   What is the overall order of this reaction?

   [1 mark]

   • A1
   • B2
   • C3
   • D4

   (b)

   What are the units of the rate constant k, when concentrations are in mol dm⁻³ and time is in s?

   [1 mark]

   • Adm⁶ mol⁻² s⁻¹
   • Bdm³ mol⁻¹ s⁻¹
   • Cmol dm⁻³ s⁻¹
   • Ds⁻¹

   (c)

   The concentration of NO is doubled and the concentration of H₂ is halved, with the temperature unchanged. Deduce the factor by which the rate changes.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Peroxodisulfate ions oxidise iodide ions: S₂O₈²⁻(aq) + 2I⁻(aq) → 2SO₄²⁻(aq) + I₂(aq). A student measures the initial rate of this reaction in three experiments 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⁻] = 0.020 mol dm⁻³, [I⁻] = 0.080 mol dm⁻³, initial rate = 4.8 × 10⁻⁶ mol dm⁻³ s⁻¹.

   (a)

   What is the order of reaction with respect to S₂O₈²⁻?

   [1 mark]

   • A0
   • B1
   • C2
   • D3

   (b)

   The reaction is first order with respect to each reactant. What is the value of the rate constant, with its units?

   [1 mark]

   • A1.2 × 10⁻⁴ dm³ mol⁻¹ s⁻¹
   • B6.0 × 10⁻⁵ dm³ mol⁻¹ s⁻¹
   • C1.9 × 10⁻⁹ dm³ mol⁻¹ s⁻¹
   • D3.0 × 10⁻³ dm³ mol⁻¹ s⁻¹

   (c)

   Calculate the initial rate of the reaction at the same temperature when [S₂O₈²⁻] = 0.050 mol dm⁻³ and [I⁻] = 0.060 mol dm⁻³.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A reactant R decomposes in solution at constant temperature. Its concentration is 0.800 mol dm⁻³ at the start, 0.400 mol dm⁻³ after 120 s, 0.200 mol dm⁻³ after 240 s and 0.100 mol dm⁻³ after 360 s.

   (a)

   Deduce the order of reaction with respect to R and justify your answer.

   [3 marks]

   (b)

   The initial rate, found from a tangent to the concentration–time curve at t = 0, is 4.62 × 10⁻³ mol dm⁻³ s⁻¹. Calculate the rate constant, with units, and then the rate when [R] = 0.500 mol dm⁻³.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Hydrogen peroxide decomposes: 2H₂O₂(aq) → 2H₂O(l) + O₂(g). The reaction is very slow at room temperature but is catalysed by aqueous iodide ions and, separately, by solid manganese(IV) oxide. With iodide ions present, the rate of reaction is found to depend on the concentration of the iodide ions.

   (a)

   Explain the difference between a homogeneous catalyst and a heterogeneous catalyst, using the two catalysts above as examples, and explain how a catalyst increases the rate of a reaction. Suggest why the concentration of iodide ions affects the rate even though iodide ions do not appear in the overall equation.

   [6 marks]

   (b)

   In a series of initial-rate experiments with iodide ions as catalyst, at constant temperature, the following results were obtained. Experiment 1: [H₂O₂] = 0.020 mol dm⁻³, [I⁻] = 0.010 mol dm⁻³, rate = 1.2 × 10⁻⁶ mol dm⁻³ s⁻¹. Experiment 2: [H₂O₂] = 0.040 mol dm⁻³, [I⁻] = 0.010 mol dm⁻³, rate = 2.4 × 10⁻⁶ mol dm⁻³ s⁻¹. Experiment 3: [H₂O₂] = 0.020 mol dm⁻³, [I⁻] = 0.030 mol dm⁻³, rate = 3.6 × 10⁻⁶ mol dm⁻³ s⁻¹. Deduce the order with respect to each of H₂O₂ and I⁻, write the rate equation, calculate the rate constant with its units, and state what the rate equation shows about the rate-determining step.

   [6 marks]

   Total for question 4: 12 marks

End of questions