Determining rate equations experimentallyAQA A-Level Chemistry: Mind map

Exam questions on Determining rate equations experimentally

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.
   Calculate the value of the rate constant, k, using Experiment 1, and state its units.2 marks
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.
   The student states that the reaction is first order with respect to H₂O₂. Use the concentration data to justify her conclusion.2 marks
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.
   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