Rate equations, orders and rate constantsAQA A-Level Chemistry: Subtopic test

1. 1
   A student investigates the reaction between substances A and B at constant temperature. The reaction has the rate equation rate = k[A][B]², where square brackets show concentrations in mol dm⁻³.

   (a)

   What is the overall order of the reaction?

   [1 mark]

   • AFirst order
   • BSecond order
   • CThird order
   • DZero order

   (b)

   The concentration of B is doubled and the concentration of A is kept constant. What happens to the rate?

   [1 mark]

   • AIt doubles
   • BIt increases by a factor of 3
   • CIt stays the same
   • DIt increases by a factor of 4

   (c)

   State what is meant by the statement that the reaction is first order with respect to A.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   The initial rate of the reaction between substances P and Q was measured at constant temperature in three experiments. Experiment 1: [P] = 0.10 mol dm⁻³, [Q] = 0.10 mol dm⁻³, initial rate = 2.0 × 10⁻⁴ mol dm⁻³ s⁻¹. Experiment 2: [P] = 0.20 mol dm⁻³, [Q] = 0.10 mol dm⁻³, initial rate = 8.0 × 10⁻⁴ mol dm⁻³ s⁻¹. Experiment 3: [P] = 0.20 mol dm⁻³, [Q] = 0.20 mol dm⁻³, initial rate = 1.6 × 10⁻³ mol dm⁻³ s⁻¹.

   (a)

   What is the order of the reaction with respect to P?

   [1 mark]

   • ASecond order
   • BZero order
   • CFirst order
   • DThird order

   (b)

   What is the value and unit of the rate constant, k?

   [1 mark]

   • A0.020 dm⁶ mol⁻² s⁻¹
   • B0.20 dm⁶ mol⁻² s⁻¹
   • C0.20 dm³ mol⁻¹ s⁻¹
   • D0.20 mol dm⁻³ s⁻¹

   (c)

   Define the term rate constant, k, and state one factor that changes its value.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   In acid solution, propanone reacts with iodine: CH₃COCH₃ + I₂ → CH₃COCH₂I + HI. The initial rate was measured in four experiments, with the concentrations of propanone, H⁺ and I₂ in mol dm⁻³. Experiment 1: 1.00, 0.50, 0.0050, rate = 1.35 × 10⁻⁵ mol dm⁻³ s⁻¹. Experiment 2: 2.00, 0.50, 0.0050, rate = 2.70 × 10⁻⁵. Experiment 3: 1.00, 1.00, 0.0050, rate = 2.70 × 10⁻⁵. Experiment 4: 1.00, 0.50, 0.0100, rate = 1.35 × 10⁻⁵.

   (a)

   Deduce the order of reaction with respect to each of propanone, H⁺ and I₂. Give your reasoning.

   [3 marks]

   (b)

   Write the rate equation and calculate the value of the rate constant, including its units. Calculate the initial rate when the concentrations of propanone, H⁺ and I₂ are 0.40, 0.25 and 0.020 mol dm⁻³.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Nitrogen monoxide reacts with hydrogen: 2NO(g) + 2H₂(g) → N₂(g) + 2H₂O(g). Initial rates were measured at constant temperature. Experiment 1: [NO] = 0.0040 mol dm⁻³, [H₂] = 0.0020 mol dm⁻³, rate = 8.0 × 10⁻⁵ mol dm⁻³ s⁻¹. Experiment 2: [NO] = 0.0080 mol dm⁻³, [H₂] = 0.0020 mol dm⁻³, rate = 3.2 × 10⁻⁴ mol dm⁻³ s⁻¹. Experiment 3: [NO] = 0.0040 mol dm⁻³, [H₂] = 0.0040 mol dm⁻³, rate = 1.6 × 10⁻⁴ mol dm⁻³ s⁻¹.

   (a)

   Deduce the orders of reaction with respect to NO and H₂. Write the rate equation and calculate the rate constant, including its units.

   [6 marks]

   (b)

   (i) State the factor by which the rate changes when [NO] is halved and [H₂] is unchanged, and explain your answer. (ii) Calculate the rate when [NO] = 0.0060 mol dm⁻³ and [H₂] = 0.0030 mol dm⁻³. (iii) State and explain what happens to the value of k when the concentrations are changed, and when the temperature is increased.

   [6 marks]

   Total for question 4: 12 marks

End of questions