Enthalpy Profiles & Activation Energy
Flip a reaction between exothermic and endothermic, drop in a catalyst, and watch the reaction-coordinate diagram redraw — activation energy, transition state and ΔH all in one picture.
An enthalpy profile (or reaction profile) is a single picture that captures three of the most tested ideas in energetics: the activation energy a reaction must climb, the transition state at the summit, and the overall enthalpy change ΔH. Toggle the reaction below and watch the diagram rebuild.
Energy along a reaction
The horizontal axis is the reaction coordinate — progress from reactants on the left to products on the right. The vertical axis is energy (enthalpy). Reactants sit at one level; before they can become products they must first climb an energy hill, then settle at the product level.
Reactions don’t roll straight downhill. Even one that releases energy has to climb the activation-energy hill first — which is why a spark can be needed to light a fire that then burns on its own.
Build the profile
Switch between exothermic and endothermic, add a catalyst, and run the reaction to send a marker rolling over the barrier. Keep your eye on two measurements: Ea (reactants up to the peak) and ΔH (reactants across to products).
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An interactive reaction profile plotting energy against the reaction coordinate. A curve runs from a reactant level, up over an activation-energy barrier (with the transition state marked at the peak), and down to a product level. A toggle switches between exothermic (products lower than reactants, ΔH negative) and endothermic (products higher, ΔH positive). A catalyst checkbox lowers the peak — a smaller activation energy — while leaving the reactant and product levels, and hence ΔH, unchanged. A double-headed arrow marks Eₐ (reactants to peak) and another marks ΔH (reactants to products). A marker can be animated rolling from reactants over the barrier to products. Readouts give the activation energy, the ΔH value and sign, and whether the reaction is exothermic or endothermic.
Reading the diagram
Note that every reaction has an activation-energy barrier, including exothermic ones — the energy released refers to the difference between the start and end levels, not to the absence of a hill.
What a catalyst changes
A catalyst offers an alternative route with a lower activation energy, so the peak of the curve drops. Because the barrier is smaller, a greater fraction of molecules can get over it, and the reaction speeds up.
Worked example
A reaction has an activation energy of 60 kJ mol⁻¹ and an enthalpy change of ΔH = −40 kJ mol⁻¹. Describe the profile.
ΔH is negative, so the reaction is exothermic: the product level sits 40 kJ mol⁻¹ below the reactants. From the reactant level the curve rises 60 kJ mol⁻¹ to the transition state, then falls 100 kJ mol⁻¹ down to the products (60 up, then 60 + 40 = 100 down). The activation energy for the reverse reaction would be 60 + 40 = 100 kJ mol⁻¹.
Your turn
Adding a catalyst lowers a reaction’s activation energy from 75 to 50 kJ mol⁻¹. What happens to ΔH?
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Nothing. ΔH depends only on the reactant and product energy levels, which the catalyst does not change. Only the activation energy (the barrier height) is reduced.
Common mistakes
Frequently asked questions
What does an enthalpy profile diagram show?+
What is the difference between an exothermic and endothermic profile?+
What is the transition state?+
How does a catalyst change the enthalpy profile?+
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Written and reviewed by ScholarsGate tutors who teach A-Level and undergraduate chemistry. Every explainer is checked against the AQA, Edexcel, OCR and CIE specifications.
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