6.2 Energy Diagrams

Reading an energy diagram

An energy diagram plots the potential energy of a system as reactants convert to products along the reaction progress axis. Reactants and products sit at their own energy levels, connected by a curve that rises to a peak — the transition state — before descending (or continuing to rise) to the products.

Activation energy and enthalpy

The activation energy (Ea) is the energy difference between the reactants and the peak of the curve — the minimum energy needed for a successful collision. The overall ΔH is the energy difference between products and reactants: if products are lower, the reaction is exothermic (ΔH < 0); if higher, it's endothermic (ΔH > 0).

Catalysts and reverse reactions

A catalyst lowers the activation energy by providing an alternate pathway (often a different mechanism/transition state), speeding up the reaction without changing ΔH or the identity/energy of reactants and products. The reverse reaction's Ea equals the forward Ea plus |ΔH| in the direction needed to return to reactants — the curve is simply read right to left.

Key terms

Transition state

The high-energy, unstable peak on an energy diagram where old bonds are breaking and new bonds forming.

Activation energy (Ea)

The energy difference between reactants and the transition state; minimum energy needed for a reaction to occur.

Exothermic vs. endothermic on a diagram

Exothermic: products lower than reactants (ΔH < 0). Endothermic: products higher than reactants (ΔH > 0).

Effect of a catalyst

Lowers activation energy via an alternate pathway; does not change ΔH or reactant/product energy levels.

Reverse reaction activation energy

Read the diagram right to left: the reverse Ea is measured from the product energy level up to the transition-state peak.

Try an AP-style question

An energy diagram shows products lower than reactants. The reaction is…

1. at equilibrium
2. exothermic
3. endothermic
4. impossible