A two-step mechanism for the reaction is summarized below

CHEMISTRY · Single correct · AP · Chemistry · Kinetics · Kinetics

Problem

A two-step mechanism for the reaction A→D\text{A} \rightarrow \text{D} is summarized below. Step 1: A → B Ea,1=60 kJ mol−1,  ΔH1=+10 kJ mol−1E_{\text{a,1}} = 60\,\text{kJ}\,\text{mol}^{-1},\;\Delta H_{1} = +10\,\text{kJ}\,\text{mol}^{-1} Step 2: B → D Ea,2=50 kJ mol−1,  ΔH2=−80 kJ mol−1E_{\text{a,2}} = 50\,\text{kJ}\,\text{mol}^{-1},\;\Delta H_{2} = -80\,\text{kJ}\,\text{mol}^{-1} Which statement about the complete potential-energy diagram for this reaction is correct? (a) Because the overall reaction is exothermic, the transition state of Step 2 must lie below the energy level of intermediate B. (b) The intermediate B lies below the reactants because the energy absorbed in Step 1 is less than the energy released in Step 2. (c) Both transition states are at the same potential energy, and each is the highest point on the diagram. (d) The highest point on the diagram is 110 kJ mol−1110\,\text{kJ mol}^{-1} above the reactants, equal to the sum of the two activation energies.

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