A reaction between species X and Y converts X to Z through the two-step mechanism X →…
CHEMISTRY · Multi-part · A Level · Chemistry · Introduction to organic chemistry · Introduction to organic chemistry
Problem
A reaction between species X and Y converts X to Z through the two-step mechanism X → I → Z. A chemist collected calorimetric data and constructed the following verbal description of the potential-energy profile for the forward reaction.
• The reactants start at an assigned reference energy of 0 kJ·mol⁻¹.
• The first transition state (TS₁) peaks at +75 kJ·mol⁻¹.
• The intermediate (I) rests at +20 kJ·mol⁻¹.
• The second transition state (TS₂) peaks at +60 kJ·mol⁻¹.
• The products lie at –40 kJ·mol⁻¹.
Use this information to analyze the reaction.
(a) Sketch a qualitative potential-energy diagram for the forward reaction, clearly labeling the positions of the reactants, TS₁, I, TS₂, and products on the energy axis.
(b) Identify the rate-determining elementary step for the forward direction and justify your answer using features of the energy diagram.
(c) Determine the numerical value of the overall enthalpy change, ΔH_rxn, for the forward reaction and state whether the reaction is exothermic or endothermic.