Topic 1 of 5
Enthalpy changes and reaction profiles
Keep heat flow, activation energy and the defined process separate.
A-Level 9476 (2026-2027)
Breaking bonds costs energy; forming bonds releases it
The overall enthalpy change compares products with reactants, not the height of the barrier.
The reacting chemicals are the system; the solution, vessel and room can act as surroundings. At constant pressure, heat absorbed by the system corresponds to an increase in enthalpy. An exothermic reaction releases heat and has ΔH < 0; an endothermic reaction absorbs heat and has ΔH > 0. Bond breaking absorbs energy and bond formation releases it, so the difference between these contributions determines the overall sign.
Exothermic reaction profile
Reactants rise to a transition-state maximum before falling to lower-enthalpy products. Forward activation energy is measured upward from the reactant level to the maximum. Delta H points downward from reactants to products.
Endothermic reaction profile
The transition-state maximum lies above both endpoints. Products have higher enthalpy than reactants, so delta H is positive. Activation energy still measures from the reactant level to the maximum.
Worked example
Construct a profile from two energy measurements
A reaction has ΔH = -65 kJ mol-1 and forward activation energy 90 kJ mol-1. Sketch and label its profile, then determine the reverse activation energy.
- Draw axes labelled enthalpy, H / kJ mol-1, and reaction progress. Choose the reactant level as zero; this is a relative reference, not an assertion that reactants contain no energy.
- Draw a short horizontal reactant level at 0 and a product level at -65. Connect them by a smooth curve whose maximum is at +90, because forward activation energy is measured above the reactants.
- Draw the forward activation-energy arrow upwards from 0 to +90. Draw the enthalpy-change arrow downwards from 0 to -65. Label the two endpoints reactants and products.
- For the reverse reaction, start at the product level: draw its activation-energy arrow upwards from -65 to the same +90 maximum. The vertical difference is 90 - (-65) = 155.
- If asked to include a catalyst with forward activation energy 45 kJ mol-1, add a second curve peaking at +45 with the same endpoints. Its reverse barrier is 45 - (-65) = 110; the enthalpy change remains -65.
Ea,reverse = 155 kJ mol-1. Check the uncatalysed drawing using 90 - 155 = -65 = ΔH; both activation-energy arrows point upwards, even though the forward reaction is exothermic.
A catalyst provides a different mechanism with a lower activation barrier. It does not change the enthalpies of the initial and final states, so it leaves ΔH unchanged. In a multistep profile, minima between peaks represent intermediates; the highest absolute peak is not automatically the step with the largest barrier measured from its preceding intermediate.
An enthalpy value belongs to an exact equation and set of states
One mole means one mole of the specified event, not always one mole of the same substance.
Standard enthalpy changes refer to substances in their standard states at a specified temperature, commonly 298 K, with standard pressure 100 kPa. School calculations conventionally use 1 mol dm-3 for specified aqueous standard conditions. Temperature must still be stated: the standard symbol does not itself mean 298 K. Changing a state, multiplying an equation or reversing it changes the corresponding enthalpy value.
| Term | Process for one mole |
|---|---|
| Enthalpy change of reaction | Enthalpy change for the reaction as written, under stated conditions; coefficients define the amount of reaction. |
| Formation | One mole of a compound forms from its constituent elements in their standard states. Example: C(graphite) + O2(g) → CO2(g). |
| Combustion | One mole of substance is completely burnt in oxygen, with all products in specified states. |
| Hydration | One mole of gaseous ions becomes hydrated aqueous ions. Example: Na+(g) → Na+(aq). |
| Solution | One mole of solute dissolves in sufficient solvent that further dilution produces no appreciable enthalpy change. |
| Neutralisation | An acid and a base react to form one mole of water. |
| Atomisation | One mole of gaseous atoms forms from the element in its stated standard state. Example: 1/2 Cl2(g) → Cl(g). |
| Bond energy | Energy required to break one mole of a specified covalent bond in gaseous species; always positive for bond breaking. Tabulated mean bond energies average different environments. |
| Lattice energy in this syllabus | One mole of ionic solid forms from its constituent gaseous ions. Example: Na+(g) + Cl-(g) → NaCl(s); negative. |
For a strong acid and strong base in dilute aqueous solution, the main reaction is H+(aq) + OH-(aq) → H2O(l), so neutralisation enthalpies are similar. A weak acid or base also undergoes ionisation, changing the measured total. Do not assume every acid-base combination has exactly the same enthalpy.