1. 1
   A student adds 2.00 g of marble chips (calcium carbonate) to 50.0 cm³ of 1.00 mol dm⁻³ hydrochloric acid at 25 °C and collects the carbon dioxide in a gas syringe: CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g). The acid is in excess. In the first 40 s, 60.0 cm³ of carbon dioxide is collected; over the next 40 s only a further 35.0 cm³ is collected.

   (a)

   What is the mean rate of production of carbon dioxide over the first 40 s?

   [1 mark]

   • A0.667 cm³ s⁻¹
   • B2.40 × 10³ cm³ s⁻¹
   • C1.50 cm³ min⁻¹
   • D1.50 cm³ s⁻¹

   (b)

   Which change, made on its own, would NOT increase the initial rate of production of carbon dioxide?

   [1 mark]

   • AUsing 2.00 g of powdered calcium carbonate instead of chips
   • BUsing 100.0 cm³ of 1.00 mol dm⁻³ hydrochloric acid
   • CUsing 50.0 cm³ of 2.00 mol dm⁻³ hydrochloric acid
   • DCarrying out the reaction at 40 °C

   (c)

   Explain why less carbon dioxide is collected in the second 40 s than in the first 40 s.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Hydrogen peroxide decomposes slowly at room temperature: H₂O₂(aq) → H₂O(l) + ½O₂(g), ΔH = −98 kJ mol⁻¹. The activation energy of the uncatalysed reaction is 75 kJ mol⁻¹. When a small amount of potassium iodide is added, iodide ions act as a catalyst and the activation energy falls to 56 kJ mol⁻¹. The temperature is kept at 20 °C throughout.

   (a)

   What is the activation energy of the reverse reaction, H₂O(l) + ½O₂(g) → H₂O₂(aq), without the catalyst?

   [1 mark]

   • A23 kJ mol⁻¹
   • B75 kJ mol⁻¹
   • C173 kJ mol⁻¹
   • D154 kJ mol⁻¹

   (b)

   Which statement about the effect of the iodide ions is correct?

   [1 mark]

   • AThey provide an alternative reaction pathway with a lower activation energy.
   • BThey make ΔH of the reaction less negative.
   • CThey increase the average kinetic energy of the hydrogen peroxide molecules.
   • DThey lower the activation energy of the forward reaction but not of the reverse reaction.

   (c)

   With reference to the Maxwell–Boltzmann energy distribution, explain how adding iodide ions increases the rate of decomposition at 20 °C.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student investigates the reaction between sodium thiosulfate and hydrochloric acid: Na₂S₂O₃(aq) + 2HCl(aq) → 2NaCl(aq) + S(s) + SO₂(g) + H₂O(l). The sulfur formed makes the mixture cloudy. The same volumes and concentrations of both solutions are used each time, and the time taken for a fixed amount of cloudiness to form is recorded at four temperatures: 20 °C, 120 s; 30 °C, 62 s; 40 °C, 31 s; 50 °C, 16 s.

   (a)

   Deduce how the rate changes between 20 °C and 40 °C, and evaluate the claim that the rate of this reaction roughly doubles for every 10 °C rise in temperature.

   [3 marks]

   (b)

   Raising the temperature from 20 °C to 30 °C increases the average kinetic energy of the particles by only about 3%. Explain why the rate nevertheless roughly doubles.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Methanol is made industrially from synthesis gas: CO(g) + 2H₂(g) → CH₃OH(g), ΔH = −91 kJ mol⁻¹. A plant currently operates at 250 °C and 5 MPa, passing the gases over pellets of a copper/zinc oxide catalyst. An engineer proposes three changes to increase the rate of methanol production: (1) raise the pressure to 10 MPa; (2) raise the temperature to 350 °C; (3) replace the catalyst pellets with smaller pellets of the same total mass.

   (a)

   Predict and explain the effect of each of the three proposed changes on the rate of methanol production.

   [6 marks]

   (b)

   Describe, in words, the energy profiles for this reaction with and without the catalyst, and compare how raising the temperature and using a catalyst each change the probability of a successful collision, with reference to the Maxwell–Boltzmann distribution.

   [6 marks]

   Total for question 4: 12 marks

End of questions