5.10 Multistep Reaction Energy Profile

Energy profiles show every step

A multistep reaction energy profile plots potential energy vs. reaction progress, showing a hump (activation energy peak) for each elementary step. Each hump's height above the previous valley is that step's Ea. Valleys between humps represent intermediates, which are stable enough to exist briefly but aren't the final product.

Finding the rate-determining step

The rate-determining step (slow step) is the elementary step with the highest activation energy, shown as the tallest hump in the diagram. This step controls the overall reaction rate, and its rate law (based on its reactants/intermediates) often matches the experimentally observed rate law for the overall reaction.

Overall energetics

The overall ΔE (or ΔH) of the reaction is the energy difference between the initial reactants and final products, regardless of intermediate humps. If products are lower in energy than reactants, the reaction is exothermic; if higher, it's endothermic. Catalysts lower humps (Ea) without changing the reactants' or products' energy levels.

Key terms

Multistep energy profile

A graph of potential energy vs. reaction progress showing a hump for each elementary step in a mechanism.

Rate-determining step

The slowest elementary step, identified as the hump with the highest activation energy in the profile.

Intermediate (in energy profile)

A species shown as a valley between two humps; formed in one step and consumed in a later step.

Effect of a catalyst on energy profile

Lowers activation energy humps (provides alternate pathway) without changing the energy of reactants or products.

Overall ΔE of a multistep reaction

The energy difference between initial reactants and final products, independent of intermediate humps.

Try an AP-style question

An energy profile has three peaks. How many elementary steps does the mechanism have?

1. 2
2. 4
3. 1
4. 3