Multistep Reaction Energy Profile (College Board AP® Chemistry)
Study Guide
Written by: Oluwapelumi Kolawole
Reviewed by: Stewart Hird
Multistep Reaction Energy Profile
The energy changes that occur in reactions involving a multistep mechanism can be shown in a graph
From this graph, we can:
Identify the number of elementary reactions
Determine / calculate the activation energy of each step
Consequently, identifying the rate-limiting step
Identify the number of intermediates involved in the reaction
Determine / calculate the overall energy change or enthalpy change
For example, consider a hypothetical reaction given by the equation:
A + 2B → D ΔH = +ve
If the proposed mechanism for this reaction involves a two-step elementary reaction given as:
A + B ⇋ C (fast)
C + B → D (slow)
The energy profile for this reaction may look like this:
Multistep Energy Profile
An energy profile diagram for a two-step reaction showing the activation energy, overall enthalpy change, and the intermediate
From the graph above, we observe that:
The number of steps involved in the reaction is indicated by the number of peaks
Each peak also represents an activated complex/transition state
The step with the largest difference in energy, activation energy, is the rate-limiting step
The number of intermediates is indicated by the number of “troughs”
The overall enthalpy change, ΔH, is indicated by the difference in energy between the products and the reactant
Examiner Tips and Tricks
Careful: Do not confuse the curve with the highest peak as the step with the highest activation energy
The activation energy is usually the distance between one trough (energy of intermediate or reactant) and another peak (transition state/activated complex)
Worked Example
Consider the energy profile diagram for the hypothetical reaction whose equation is given below and answer the following questions
X + 2Y → P + Q
Describe the overall reaction as exothermic or endothermic.
Suggest the number of intermediates involved in the overall reaction.
Suggest the number of elementary steps involved in the reaction mechanism.
Identify the rate-limiting step.
Answers:
Answer a
The energy of the product is greater than the energy of the reactants
Therefore, the reaction is endothermic
Answer b
The are two intermediates in the reaction
These are indicated by the troughs labeled B and D
Answer c
There are three elementary steps in the reaction
These are indicated by the peaks labeled A, C and E
Answer d
The rate-limiting step is the reaction with the highest activation energy
Therefore, the rate-limiting step is the second step from B → D
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