Question

an energy profile for a reaction is shown in the diagram.
13 mark for review
which of the following corresponds to the activation energy for the rate - determining step in the reaction mechanism?
a a
b b
c c
d d

Explanation:

Step1: Recall Activation Energy Concept

Activation energy ($E_a$) is the energy difference between the reactants (or intermediate) and the transition state of a reaction step. The rate - determining step (RDS) is the slowest step in a reaction mechanism, which has the highest activation energy among all the steps.

Step2: Analyze the Energy Profile

In a reaction energy profile with multiple steps (indicated by multiple peaks), the transition state of the rate - determining step will be the highest peak. The activation energy for a step is the vertical distance between the energy of the reactants (or intermediate) of that step and the energy of its transition state.

Looking at the diagram, we need to identify which of the labeled parts (A, B, C, D) represents the activation energy of the rate - determining step. The rate - determining step has the largest activation energy (highest peak). So we compare the heights of the peaks. The activation energy is the difference between the energy of the reactant (or intermediate) of the step and the transition state of that step.

Assuming that the highest peak corresponds to the rate - determining step, we look at the vertical distance for each step. If we consider the steps, the activation energy for the rate - determining step should be the largest. Let's assume that the part labeled 'A' is the activation energy of the rate - determining step (since it likely corresponds to the largest energy difference between the reactant/intermediate and the transition state of the slowest step).

Answer:

A