Enthalpy Profiles and Activation Energy: GCSE Chemistry

What is enthalpy and what does ΔH mean?

Enthalpy (H) is a measure of the total energy stored in the chemical bonds of a substance. You cannot measure absolute enthalpy, but you can measure enthalpy change (ΔH) — the difference in enthalpy between the reactants and the products of a reaction.

ΔH = H(products) − H(reactants)

• If products have less stored energy than reactants: energy is released to surroundings → exothermic reaction → ΔH is negative (by convention)
• If products have more stored energy than reactants: energy is taken in from surroundings → endothermic reaction → ΔH is positive

The SI unit of enthalpy change is kJ/mol (kilojoules per mole of reaction).

What is an enthalpy profile diagram?

An enthalpy profile diagram (also called a reaction profile or energy level diagram) plots enthalpy (on the y-axis) against reaction progress (on the x-axis, sometimes called the reaction coordinate). It shows:

1. The energy level of the reactants (starting material, on the left)
2. A peak representing the transition state (the highest-energy intermediate state during bond breaking and forming)
3. The energy level of the products (on the right)

The difference in height between the reactants level and the peak is the activation energy (Eₐ). The difference in height between the reactants level and the products level is ΔH.

What does an exothermic profile look like?

For an exothermic reaction (e.g. combustion, neutralisation, most oxidation reactions):

• The products line is lower than the reactants line on the y-axis.
• The difference (products − reactants) is negative → ΔH < 0
• The activation energy hump still exists above the reactants level — you must put energy in to start the reaction.

Worked example — combustion of methane:

CH₄ + 2O₂ → CO₂ + 2H₂O ΔH ≈ −890 kJ/mol

On the profile: reactants (CH₄ + 2O₂) are higher than products (CO₂ + 2H₂O) by 890 kJ/mol. There is a hump above the reactants representing the activation energy (approximately 528 kJ/mol for uncatalysed gas-phase combustion — much lower in a flame where temperatures provide the needed energy).

What does an endothermic profile look like?

For an endothermic reaction (e.g. thermal decomposition, photosynthesis, dissolving ammonium nitrate in water):

• The products line is higher than the reactants line.
• ΔH is positive (energy is absorbed from surroundings → the reaction mixture cools).
• The activation energy hump rises above the reactants level and the products level.

Worked example — thermal decomposition of calcium carbonate:

CaCO₃ → CaO + CO₂ ΔH ≈ +178 kJ/mol

On the profile: products (CaO + CO₂) are 178 kJ/mol higher than the reactant (CaCO₃). The reaction requires continuous heating to supply this energy.

Comparison table: exothermic vs endothermic profiles

How do catalysts change the enthalpy profile?

A catalyst provides an alternative reaction pathway with a lower activation energy. On the enthalpy profile diagram:

• The hump (transition state peak) is lower with a catalyst than without one.
• The positions of the reactants and products do not change — ΔH is the same with or without catalyst.
• The reaction is faster because more collisions have enough energy to exceed the (lower) activation energy.

This is a critical GCSE point: a catalyst changes the rate but not the energy released or absorbed. The equilibrium position is also unaffected — a catalyst speeds up both the forward and reverse reactions equally.

On the diagram: draw two humps over the same reactants and products — one high (uncatalysed) and one lower (catalysed). The products and reactants levels remain the same for both.

Frequently asked questions

Why do reactions need activation energy if they are going to release energy overall?

Even in an exothermic reaction, the reactant bonds must be broken before new bonds can form. Breaking bonds requires energy input. The activation energy represents the energy needed to begin breaking the existing bonds in the reactants to reach the transition state. Only after the transition state can the atoms rearrange and form new, stronger bonds in the products, releasing more energy than was put in. The net result is exothermic, but the reaction still needs a "push" (activation energy) to get started — this is why many combustible materials need a spark or flame to ignite.

What is the difference between activation energy and bond energy?

Bond energy (or bond dissociation energy) is the energy required to break one mole of a specific type of bond in the gas phase (e.g. C−H bond energy ≈ 413 kJ/mol). It is a property of an individual bond type. Activation energy is the minimum kinetic energy that colliding particles must have for a reaction to occur — it depends on the specific reaction, not just one bond. Activation energy is typically calculated from enthalpy profile diagrams or from the Arrhenius equation (A-level). For GCSE, activation energy is the height from the reactants level to the peak of the energy profile.

Can the activation energy ever be zero?

Theoretically, some reactions can proceed with extremely low activation energy (approaching zero), but true zero activation energy is rare in normal chemistry. Some radical reactions and barrierless reactions in the gas phase can be nearly barrierless. In practice, all reactions in which stable molecules must rearrange have a positive activation energy. If activation energy were zero, the reaction would proceed instantaneously whenever reactants were mixed, with no need for heat or a catalyst — which would make most chemistry far more explosive than it is.

How do I draw an enthalpy profile from given data?

Step 1: Draw labelled axes — "Enthalpy" (y-axis), "Reaction progress" (x-axis). Step 2: Mark the reactants level as a horizontal line on the left. Step 3: If exothermic (ΔH negative), draw the products line lower by the given ΔH value; if endothermic, draw it higher. Step 4: Draw a smooth hump from the reactants level, rising to the transition state peak, then falling to the products level. Label the peak "transition state". Step 5: Mark a vertical arrow from the reactants level to the peak — label it Eₐ (activation energy). Mark another vertical arrow from reactants to products — label it ΔH (with correct sign). Step 6: Label the reactants and products with their formulae.

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Professor Curie at aitutors.me can walk you through drawing energy profiles, quiz you on exothermic versus endothermic, and help you master every style of activation energy question.