5.6 Reaction Energy Profile

Reading the energy profile

A reaction energy diagram plots potential energy vs. reaction progress. Reactants start at one energy level, rise to a peak called the transition state, then drop to products. The height from reactants to the peak is the activation energy (Ea) — the energy barrier that must be overcome for a collision to lead to reaction.

Endothermic vs. exothermic profiles

If products sit higher in energy than reactants, the reaction is endothermic (ΔH > 0), absorbing energy. If products sit lower, the reaction is exothermic (ΔH < 0), releasing energy. ΔH is the difference between product and reactant energy levels, regardless of the size of the activation energy hump.

Catalysts and multi-step mechanisms

A catalyst provides an alternate pathway with a lower activation energy, speeding up the reaction without changing ΔH or being consumed. Multi-step mechanisms show multiple peaks (transition states) and valleys (reaction intermediates), with the tallest peak corresponding to the rate-determining step, the slowest step overall.

Key terms

Activation energy (Ea)

The minimum energy needed for reactants to reach the transition state and react.

Transition state

The highest-energy, unstable peak on the energy diagram where bonds are breaking and forming.

Exothermic reaction profile

Products are lower in energy than reactants; ΔH is negative; energy is released.

Endothermic reaction profile

Products are higher in energy than reactants; ΔH is positive; energy is absorbed.

Rate-determining step

The slowest step in a multi-step mechanism, corresponding to the highest peak (largest Ea) on the energy diagram.

Try an AP-style question

What are ΔH and Eₐ?

1. ΔH = −210 kJ; Eₐ = 60 kJ
2. ΔH = −150 kJ; Eₐ = 60 kJ
3. ΔH = +150 kJ; Eₐ = 60 kJ
4. ΔH = −150 kJ; Eₐ = 210 kJ