Activation energy and catalysisEdexcel International A Level Chemistry: Subtopic test

1. 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.

   (a)

   What is the gradient of the straight line obtained?

   [1 mark]

   • AEa/R
   • B−Ea/R
   • C−Ea × R
   • Dln A

   (b)

   What are the units of the gradient?

   [1 mark]

   • AJ mol⁻¹
   • BK⁻¹
   • CK
   • DJ K⁻¹ mol⁻¹

   (c)

   The gradient of the line is −6.50 × 10³. Calculate the activation energy in kJ mol⁻¹.

   [2 marks]

   Total for question 1: 4 marks

2. 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.

   (a)

   Which quantity is proportional to the rate of the reaction?

   [1 mark]

   • At
   • Bt²
   • Cln t
   • D1/t

   (b)

   Which graph should give a straight line with a negative gradient?

   [1 mark]

   • Aln(1/t) against 1/T, with T in kelvin
   • B1/t against T
   • Ct against 1/T
   • Dln t against T

   (c)

   Explain, using collision theory, why the time for the cross to disappear is shorter at higher temperatures.

   [2 marks]

   Total for question 2: 4 marks

3. 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.

   (a)

   Calculate the activation energy of the reaction in kJ mol⁻¹, using ln(k₂/k₁) = (Ea/R)(1/T₁ − 1/T₂).

   [3 marks]

   (b)

   Using an activation energy of 51.8 kJ mol⁻¹, calculate the rate constant at 310 K.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A school chemistry department runs a project on activation energy. Students use the reaction between sodium thiosulfate solution and dilute hydrochloric acid, timing how long a cross takes to disappear. On a visit to an ammonia plant they also learn that nitrogen and hydrogen react in the gas phase over a solid iron catalyst: N₂(g) + 3H₂(g) ⇌ 2NH₃(g).

   (a)

   Describe how the students could use the thiosulfate and acid reaction to determine the activation energy, including how they would process their results.

   [6 marks]

   (b)

   Explain how a solid catalyst such as iron increases the rate of a gas-phase reaction, such as the formation of ammonia, in terms of its surface, and explain why the catalyst is used as small pellets or a finely divided solid.

   [6 marks]

   Total for question 4: 12 marks

End of questions