ChemWhiz ReferenceKinetics

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.

01Use this when

Reading activation energy (E⁠a), ΔH, and a catalyst's effect from a reaction-coordinate diagram.

02Conditions

Energy plotted against reaction progress; the peak is the transition state.

03Watch out

A catalyst lowers E⁠a in both directions but does NOT change ΔH (products − reactants).

Compare the first two diagrams on the same 0–40 kJ scale: the catalyst lowers the transition-state energy while the reactant and product levels stay fixed.

Exothermic reaction (uncatalyzed)

Single-step reaction-energy diagramSingle-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).35105Energy (kJ)Reaction coordinateReactants10 kJTS35 kJProducts5 kJEₐΔH
The forward activation energy E⁠a is the climb from the reactant plateau to the transition state; ΔH is the net drop to the products.

The same reaction with a catalyst

Single-step reaction-energy diagramSingle-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).20105Energy (kJ)Reaction coordinateReactants10 kJTS20 kJProducts5 kJEₐ (catalyzed)ΔH
A catalyst provides a lower-energy pathway: the transition state drops, so E⁠a shrinks in both directions, but the endpoints (and therefore ΔH) do not change.

Endothermic reaction

Single-step reaction-energy diagramSingle-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).45305Energy (kJ)Reaction coordinateReactants5 kJTS45 kJProducts30 kJEₐΔH
Products sit above reactants: ΔH is positive, and the reverse activation energy is smaller than the forward one (E⁠a,rev = E⁠a,fwd − ΔH). Illustrative schematic.

Two-step mechanism (step 1 rate-determining)

Multi-step reaction-energy diagramMulti-step reaction-energy diagram with 2 elementary steps. Reactants at 10 kJ. Step 1: TS₁ (RDS) 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).504025100Energy (kJ)Reaction coordinateReactants10 kJTS₁ (RDS)50 kJIntermediate25 kJTS₂40 kJProducts0 kJEₐ,1Eₐ,2ΔH
A two-step mechanism shows one valley (the intermediate) between two transition states. The step with the largest activation barrier (here the first) is rate-determining. Illustrative schematic.

Activation Energy and ΔH by Diagram

Compare the forward and reverse activation energies, the sign of ΔH, and how a catalyst lowers Ea.

Activation Energy and ΔH by Diagram
Exothermic reaction (uncatalyzed)Exothermic25 kJ30 kJ−5 kJ—
The same reaction with a catalystExothermic10 kJ15 kJ−5 kJ—
Endothermic reactionEndothermic40 kJ15 kJ+25 kJ—
Two-step mechanism (step 1 rate-determining)Exothermic40 kJ50 kJ−10 kJstep 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 ΔH = E⁠a (forward) − E⁠a (reverse).
  • 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 teaching values.

Verification references: ChemWhiz kinetics reference diagrams; the endothermic and two-step examples are teaching schematics.

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ThermochemistryKinetics