8 3 This question is about reaction kinetics. (a) The best way to describe the activation energy of a reaction is (1) A the average energy of the particles when they react B the difference in energy between the reactants and the products C the minimum energy required to make the particles collide D the minimum energy required for a reaction to occur (b) The diagrams show two reaction profiles for the same reversible reaction involving gaseous reactants. Shown on each diagram are the reaction profiles for the pathway without a catalyst and the pathway catalysed by a heterogeneous catalyst. (i) In which diagram does the arrow represent the activation energy for the backward reaction when a catalyst is present? (1) A B 500 400 300 200 100 Enthalpy / kJ mol−1 reactants products Reaction pathway 500 400 300 200 100 Enthalpy / kJ mol−1 reactants products Reaction pathway C D 500 400 300 200 100 Enthalpy / kJ mol−1 reactants products Reaction pathway 500 400 300 200 100 Enthalpy / kJ mol−1 reactants products Reaction pathway 9 Turn over (ii) Estimate, using the diagram, the decrease in the activation energy for the forward reaction when a catalyst is added. (1) 500 400 300 200 100 Enthalpy / kJ mol−1 reactants products Reaction pathway 450 350 250 150 50 A 75 kJ mol−1 B 100 kJ mol−1 C 175 kJ mol−1 D 200 kJ mol−1 (c) State why a solid (heterogeneous) catalyst is suitable for a reaction in the gas phase. (1) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... 10 (d) The diagram shows a Maxwell‑Boltzmann distribution of molecular energies for gaseous molecules. Number of molecules with a given energy Ea (catalyst) Ea Z Y Energy, E (i) Which is the area of the graph corresponding to the number of molecules with sufficient energy to react when a catalyst is present? (1) A Y B Y − Z C Y + Z D Z (ii) Which would always result in a decrease in the number of molecules contained within area Y? (1) A decreasing the temperature of the gas B increasing the pressure of the gas C putting the gas in a smaller container D removing a quarter of the catalyst (Total for Question 3 = 6 marks)