Determining rate equations experimentallyAQA A-Level Chemistry: Subtopic test

1. 1
   The gas-phase reaction 2NO + 2H₂ → N₂ + 2H₂O was studied at constant temperature. Experiment 1: [NO] 0.0020 mol dm⁻³, [H₂] 0.0040 mol dm⁻³, initial rate 8.0 × 10⁻⁷ mol dm⁻³ s⁻¹. Experiment 2: [NO] 0.0040, [H₂] 0.0040 (both mol dm⁻³), initial rate 3.2 × 10⁻⁶ mol dm⁻³ s⁻¹. Experiment 3: [NO] 0.0020, [H₂] 0.0080 (both mol dm⁻³), initial rate 1.6 × 10⁻⁶ mol dm⁻³ s⁻¹.

   (a)

   What is the order of reaction with respect to NO, using the data in Experiments 1 and 2?

   [1 mark]

   • AZero
   • BFirst
   • CThird
   • DSecond

   (b)

   Which is the rate equation for the reaction?

   [1 mark]

   • Arate = k[NO]²[H₂]
   • Brate = k[NO]²[H₂]²
   • Crate = k[NO][H₂]
   • Drate = k[NO][H₂]²

   (c)

   Calculate the value of the rate constant, k, using Experiment 1, and state its units.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student studies the decomposition of hydrogen peroxide, 2H₂O₂ → 2H₂O + O₂, using a catalyst, at constant temperature. She works out the concentration of H₂O₂ remaining at intervals: 0.800 mol dm⁻³ at 0 s, 0.400 mol dm⁻³ at 150 s, 0.200 mol dm⁻³ at 300 s and 0.100 mol dm⁻³ at 450 s. A tangent drawn to her smooth concentration–time curve at t = 0 passes through the point (0 s, 0.800 mol dm⁻³) and meets the time axis at 215 s.

   (a)

   If the reaction is first order with respect to H₂O₂, which graph of rate against [H₂O₂] is expected?

   [1 mark]

   • AA straight line through the origin
   • BA horizontal straight line
   • CA straight line with a negative gradient
   • DA curve with a gradient that increases as concentration increases

   (b)

   Use the tangent at t = 0 to find the initial rate of decomposition of H₂O₂.

   [1 mark]

   • A−3.7 × 10⁻³ mol dm⁻³ s⁻¹
   • B2.7 × 10² mol dm⁻³ s⁻¹
   • C3.7 × 10⁻³ mol dm⁻³ s⁻¹
   • D1.9 × 10⁻³ mol dm⁻³ s⁻¹

   (c)

   The student states that the reaction is first order with respect to H₂O₂. Use the concentration data to justify her conclusion.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   In the reaction S₂O₃²⁻(aq) + 2H⁺(aq) → S(s) + SO₂(g) + H₂O(l), a student mixes sodium thiosulfate solution, hydrochloric acid and water in a flask placed over a cross marked on paper. She records the time for the cross to disappear when viewed from above because of the sulfur formed. In three runs the total volume and the hydrochloric acid concentration are identical. The thiosulfate concentrations are 0.100, 0.050 and 0.025 mol dm⁻³ and the times are 25 s, 50 s and 100 s respectively.

   (a)

   Describe how the student should vary the concentration of thiosulfate ions so that the experiment is a fair test, and how the results can be used to show the order with respect to thiosulfate.

   [3 marks]

   (b)

   Deduce the order of reaction with respect to thiosulfate ions from the data, showing your working. Explain why 1/t can be used as a measure of the initial rate.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   The reaction in which propanone is iodinated in acid solution, CH₃COCH₃ + I₂ → CH₃COCH₂I + HI, is studied at constant temperature. Iodine solution is orange-brown, while propanone, hydrochloric acid and the organic product are colourless.

   (a)

   Describe how the order of reaction with respect to iodine, and with respect to propanone, can be found by continuous monitoring with a colorimeter.

   [6 marks]

   (b)

   A series of initial rate experiments gave these results. Experiment 1: [propanone] 0.50 mol dm⁻³, [H⁺] 0.20 mol dm⁻³, [I₂] 0.0020 mol dm⁻³, initial rate 1.4 × 10⁻⁵ mol dm⁻³ s⁻¹. Experiment 2: [propanone] 1.00, [H⁺] 0.20, [I₂] 0.0020 (all mol dm⁻³), initial rate 2.8 × 10⁻⁵ mol dm⁻³ s⁻¹. Experiment 3: [propanone] 0.50, [H⁺] 0.40, [I₂] 0.0020 (all mol dm⁻³), initial rate 2.8 × 10⁻⁵ mol dm⁻³ s⁻¹. Experiment 4: [propanone] 0.50, [H⁺] 0.20, [I₂] 0.0040 (all mol dm⁻³), initial rate 1.4 × 10⁻⁵ mol dm⁻³ s⁻¹. Deduce the order with respect to each reactant and hence the rate equation. Calculate the value of the rate constant, with units, and the initial rate when [propanone] = 0.80, [H⁺] = 0.30 and [I₂] = 0.0010 mol dm⁻³.

   [6 marks]

   Total for question 4: 12 marks

End of questions