Multistep Reaction Energy Profile (College Board AP Chemistry)

• 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