The gas-phase conversion of compound A to product C is proposed to occur by the…

CHEMISTRY · Multi-part · A Level · Chemistry · Energetics · Energetics

Problem

The gas-phase conversion of compound A to product C is proposed to occur by the two-step mechanism shown below, where B is an intermediate. Step 1: A→BΔH1=+20 kJ mol−1A \rightarrow B \quad \Delta H_1 = +20\,\text{kJ mol}^{-1} Step 2: B→CΔH2=−85 kJ mol−1B \rightarrow C \quad \Delta H_2 = -85\,\text{kJ mol}^{-1} Assume that the activation energy for Step 1 is 95 kJ mol−195\,\text{kJ mol}^{-1} and for Step 2 is 40 kJ mol−140\,\text{kJ mol}^{-1}. Answer the following. (a) On axes of potential energy versus reaction coordinate, sketch a qualitative energy profile consistent with the data above. Label all reactants, intermediates, products, transition states, each ΔH\Delta H, and each activation energy. (b) Identify which elementary step is rate-determining and justify your choice with reference to the sketch. (c) The experimentally measured overall ΔH\Delta H for the reaction A→CA \rightarrow C is −65 kJ mol−1-65\,\text{kJ mol}^{-1}. Discuss whether the mechanism and numerical data given are thermodynamically consistent with this experimental value.

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