Energy Profiles and Transition States in Multistep Reactions HL Only
Specification Reference R2.2.7
Quick Notes:
- Energy profiles show energy changes as reactants turn into products.
- Multistep reactions have multiple peaks and valleys:
- Each peak = a transition state
- Each valley = an intermediate
- The highest peak = rate-determining step (RDS).
- Activation energy (Ea) is the energy difference between reactants and the peak of the step.
- Profiles help link kinetic data (reaction rate) to the mechanism.
Full Notes:
For an introduction to energy profiles and the basics, please see here.
In reactions that occur in multiple steps, the energy profile helps visualize the activation energies and transition states for each elementary step.
Key Features of Multistep Energy Profiles

- Each peak represents a transition state — a high-energy, unstable configuration.
- Valleys between peaks correspond to intermediates, which exist for a short period between steps.
Rate-Determining Step (RDS)
The tallest peak corresponds to the step with the highest activation energy (Ea).

- This is the slowest step (rate-determining step, RDS), controlling the overall rate.
- Even if it’s not the first step, the slowest step dominates the kinetics.
See here for more detail.
Summary
- Energy profiles show the kinetic pathway of reactions.
- Each peak = transition state and each valley = intermediate.
- The highest peak is the rate-determining step.
- Profiles must match both kinetic data and the proposed mechanism.