Enthalpy Change

Specification Reference Physical Chemistry, Energetics 3.1.4.1

Quick Notes

Enthalpy change (ΔH) is the heat energy change of a reaction or process measured at constant pressure.
Exothermic reactions release energy (ΔH is negative).
- For Example: Combustion, neutralisation.
Endothermic reactions absorb energy (ΔH is positive).
- For Example: Thermal decomposition.
Standard conditions: 298 K (25°C), 100 kPa, 1 mol dm⁻³ solutions.
Types of enthalpy change:
- ΔH_r (Reaction) – Enthalpy change for a reaction in molar quantities of balanced reaction equation.
- ΔH_f (Formation) – Enthalpy change when 1 mole of a compound formed from its elements (all in standard states).
- ΔH_c (Combustion) – Enthalpy change when 1 mole of a substance is burned completely in oxygen.
- ΔH_neut (Neutralisation) – Enthalpy change when acid and base react to form 1 mole of water.

Full Notes

Enthalpy and Enthalpy Change has been outlined with more background theory here.

This page is just what you need to know for AQA A-level Chemistry :)

Enthalpy (H) is the total heat content of a system. ΔH (Enthalpy change) is the overall heat energy transferred to or from a system during a reaction (at constant pressure).

Exothermic Reactions

If ΔH is negative, the reaction is described as exothermic. Heat energy is released to the surroundings.

AQA A-Level Chemistry enthalpy diagram showing an exothermic reaction with products at lower energy than reactants, negative ΔH.

Products have less energy than reactants.

Examples:
Combustion (CH₄ + O₂ → CO₂ + H₂O)
Neutralisation (acid + base → salt + water)

Endothermic Reactions

If ΔH is positive, the reaction is described as endothermic. Heat energy is absorbed from the surroundings.

AQA A-Level Chemistry enthalpy diagram showing an endothermic reaction with products at higher energy than reactants, positive ΔH.

Products have more energy than reactants.

Example: Thermal decomposition of calcium carbonate (CaCO₃→CaO + CO₂)

Standard Enthalpy Changes

To enable values can be compared for different processes, enthalpy changes are measured under standard conditions.

When standard conditions are used, the enthalpy changes are referred to as 'Standard Enthalpies', shown with the plimsol symbol ^⦵.

Standard Conditions:

Temperature = 298 K (25°C)
Pressure = 100 kPa
Solutions at 1 mol dm⁻³

Summary Table of Standard Enthalpy Changes

Enthalpy Change	Definition	Example Reaction	ΔH (kJ mol⁻¹)
Enthalpy of Reaction (ΔH_r)	Energy change in a reaction under standard conditions	H₂ + ½O₂ → H₂O	−286
Enthalpy of Formation (ΔH_f)	Energy change to form 1 mole of a compound from its elements	C + 2H₂ → CH₄	−75
Enthalpy of Combustion (ΔH_c)	Energy released when 1 mole of a substance burns in oxygen	CH₄ + 2O₂ → CO₂ + 2H₂O	−890
Enthalpy of Neutralisation (ΔH_neut)	Energy change when acid and base react to form 1 mole of water	HCl + NaOH → NaCl + H₂O	−57

3. Factors Affecting Enthalpy Change

Amount of Reactants: More reactants = larger enthalpy change.
State changes of reactants and products:
- Gas → Liquid → Solid changes releases energy (exothermic).
- Solid → Liquid → Gas changes absorbs energy (endothermic).
Temperature and Pressure: Standard conditions ensure fair comparisons.

Summary

Enthalpy change (ΔH) = heat energy change of a reaction at constant pressure.
Exothermic: ΔH negative (energy released). Endothermic: ΔH positive (energy absorbed).
Standard enthalpy changes: ΔH_r, ΔH_f, ΔH_c, ΔH_neut.
Standard conditions: 298 K, 100 kPa, 1 mol dm⁻³.