Chemistry · Unit 5: Kinetics · 14 min read · Updated 2026-05-11
Multistep Reaction Energy Profile — AP Chemistry
AP Chemistry · Unit 5: Kinetics · 14 min read
1. Core Definition and Structural Features★★☆☆☆⏱ 3 min
A multistep reaction energy profile (or reaction coordinate diagram) plots the potential energy of all species along a reaction pathway against the reaction coordinate, a qualitative measure of reaction progress from initial reactants to final products. Unlike single-step reactions (which have only one energy peak), multistep profiles have one energy peak per elementary step, with valleys between peaks corresponding to reaction intermediates.
The overall enthalpy change of the reaction is independent of the number of steps in the mechanism, calculated as:
If $
Delta H_{\text{rxn}} < 0$, the reaction is exothermic (products have lower energy than reactants); if $
Delta H_{\text{rxn}} > 0$, it is endothermic.
Exam tip: Never count the initial reactant or final product as an intermediate. Only energy minima between the start and end points count as intermediates, regardless of their energy value.
2. Identifying the Rate-Determining Step★★★☆☆⏱ 4 min
The rate-determining step (RDS, or rate-limiting step) is the slowest elementary step in a multistep mechanism, and it dictates the overall rate of the entire reaction, analogous to a bottleneck controlling traffic flow on a highway.
To find the RDS on an energy profile, calculate the activation energy $E_a$ for each individual step. Activation energy for a step is the difference between the energy of the step's transition state (peak) and the energy of the starting species for that step:
The step with the highest activation energy is always the rate-determining step, because a higher $E_a$ means fewer molecules have enough kinetic energy to overcome the barrier, so the step proceeds slower.
Exam tip: Do not just pick the peak highest relative to the initial reactant y-axis as the RDS. Always calculate $E_a$ relative to the starting point of the individual step, because intermediates can be higher in energy than the original reactants.
3. Connecting Profiles to Mechanisms and Catalysis★★★☆☆⏱ 4 min
A core AP Chemistry skill is matching an energy profile to a proposed mechanism, and vice versa. Every elementary step in a mechanism corresponds to exactly one energy peak (transition state) on the profile, so the number of peaks directly tells you the number of steps in the mechanism. Intermediates, which are formed in one step and consumed in another, are never part of the overall balanced reaction.
Catalysts modify multistep energy profiles by providing an alternate reaction mechanism with a lower activation energy for the rate-determining step. Catalysts do not change the overall energy of reactants or products, so $
Delta H_{\text{rxn}}$ remains identical for catalyzed and uncatalyzed reactions.
Exam tip: If you are asked to draw an energy profile from a mechanism, always draw the RDS peak as the highest peak, regardless of where it falls in the reaction sequence. AP exam graders look for this key feature.
4. AP-Style Worked Practice Problems★★★★☆⏱ 3 min
Common Pitfalls
Why: Students confuse any energy minimum on the graph with an intermediate, forgetting intermediates are formed and consumed during the reaction.
Why: Students see the tallest peak relative to the y-axis origin and automatically pick it, forgetting intermediates can be higher in energy than reactants.
Why: Students mix up the definitions of unstable transition states and short-lived but detectable intermediates.
Why: Students confuse activation energy of the RDS with overall enthalpy change of the reaction.
Why: Students associate lower energy barriers with lower product energy, forgetting catalysts work by changing the mechanism, not the starting or ending energy.
Why: Students match the number of intermediates to the number of steps, instead of steps minus one.