Activation energy and catalysisEdexcel International A Level Chemistry: Flashcards

Exam questions on Activation energy and catalysis

1. The Arrhenius equation, ln k = ln A − Ea/RT, relates the rate constant k of a reaction to the absolute temperature T, where Ea is the activation energy and R is the gas constant, 8.31 J K⁻¹ mol⁻¹. A student plots ln k on the y-axis against 1/T on the x-axis for one reaction, with T in kelvin.
   The gradient of the line is −6.50 × 10³. Calculate the activation energy in kJ mol⁻¹.2 marks
2. In an experiment to find the activation energy of a reaction, a student mixes sodium thiosulfate solution with dilute hydrochloric acid and measures the time t for a cross viewed through the flask to disappear behind the sulfur precipitate. The experiment is repeated at several temperatures using the same volumes and concentrations each time.
   Explain, using collision theory, why the time for the cross to disappear is shorter at higher temperatures.2 marks
3. The rate constant of a first-order decomposition is 2.40 × 10⁻⁴ s⁻¹ at 300 K and 8.80 × 10⁻⁴ s⁻¹ at 320 K. Use R = 8.31 J K⁻¹ mol⁻¹.
   Calculate the activation energy of the reaction in kJ mol⁻¹, using ln(k₂/k₁) = (Ea/R)(1/T₁ − 1/T₂).3 marks