Reaction-Energy Diagrams: Activation Energy, ΔH, and Catalysis
The four reaction-coordinate diagrams every kinetics student needs to read: an exothermic reaction, the same reaction catalyzed (lower Ea, same ΔH), an endothermic reaction, and a two-step mechanism with a rate-determining step. Activation energies and ΔH are annotated on each diagram and tabulated below.
Exothermic reaction (uncatalyzed)
Single-step reaction-energy diagram. Reactants at 10 kJ. TS at 35 kJ (activation energy Eₐ = 25 kJ). Products at 5 kJ (enthalpy change ΔH = -5 kJ; exothermic).
The same reaction with a catalyst
Single-step reaction-energy diagram. Reactants at 10 kJ. TS at 20 kJ (activation energy Eₐ = 10 kJ). Products at 5 kJ (enthalpy change ΔH = -5 kJ; exothermic).
Endothermic reaction
Single-step reaction-energy diagram. Reactants at 5 kJ. TS at 45 kJ (activation energy Eₐ = 40 kJ). Products at 30 kJ (enthalpy change ΔH = +25 kJ; endothermic).
Two-step mechanism (step 1 rate-determining)
Multi-step reaction-energy diagram with 2 elementary steps. Reactants at 10 kJ. Step 1: TS₁ at 50 kJ (step activation energy = 40 kJ). Intermediate at 25 kJ. Step 2: TS₂ at 40 kJ (step activation energy = 15 kJ). Products at 0 kJ (overall enthalpy change ΔH = -10 kJ; exothermic). Rate-determining step: step 1 (highest activation energy 40 kJ).
| Diagram | Character | Ea (forward) | Ea (reverse) | ΔH | Step Barriers |
|---|---|---|---|---|---|
| Exothermic reaction (uncatalyzed) | Exothermic | 25 kJ | 30 kJ | −5 kJ | — |
| The same reaction with a catalyst | Exothermic | 10 kJ | 15 kJ | −5 kJ | — |
| Endothermic reaction | Endothermic | 40 kJ | 15 kJ | +25 kJ | — |
| Two-step mechanism (step 1 rate-determining) | Exothermic | 40 kJ | 50 kJ | −10 kJ | step 1: 40; step 2: 15 |
Important Notes
- Reading the diagram: the forward activation energy Ea is the climb from the reactant plateau to the (highest) transition state; ΔH is the difference between product and reactant plateaus; and Ea,reverse = Ea,forward − ΔH.
- A catalyst lowers the transition state, shrinking Ea in both directions and speeding both forward and reverse reactions equally, but it never changes ΔH or the equilibrium constant.
- Exothermic: products below reactants (ΔH negative, heat released). Endothermic: products above reactants (ΔH positive, heat absorbed). The sign of ΔH says nothing about the RATE; that is Ea's job.
- In a multi-step mechanism, each step has its own barrier measured from its own starting plateau; the step with the largest barrier is rate-determining. Valleys between transition states are intermediates: real species with finite lifetimes, unlike transition states.
- The energy values on the schematics are illustrative; ChemWhiz problems supply their own diagram values in the problem statement.
Source: ChemWhiz course diagrams (the catalysis pair is the graded Topic 20 payload set; the endothermic and two-step diagrams are labeled teaching schematics)