← Thermochemical principles and the properties of particles and substances
For bonds: “Breaking Burns energy; Forming Frees it.” Bond breaking is endothermic (costs energy); bond forming is exothermic (releases it). That’s why ΔH = Σ(broken) − Σ(formed).
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3 Enthalpy (ΔH)
Enthalpy change (ΔH) is the heat energy exchanged with the surroundings at constant pressure. It is measured in kJ mol⁻¹. ✓Mnemonic: exo vs endo, and bonds
“Exo = exits (energy leaves, ΔH negative). Endo = enters (energy absorbed, ΔH positive).”For bonds: “Breaking Burns energy; Forming Frees it.” Bond breaking is endothermic (costs energy); bond forming is exothermic (releases it). That’s why ΔH = Σ(broken) − Σ(formed).
The two signs
- Exothermic, ΔH < 0, energy released to the surroundings; products lower in energy than reactants (e.g. combustion, neutralisation).
- Endothermic, ΔH > 0, energy absorbed from the surroundings; products higher in energy than reactants (e.g. thermal decomposition, photosynthesis).
- Bonds breaking absorbs energy (endothermic); bonds forming releases energy (exothermic).
| Symbol | Name | Definition (per mole, standard conditions) |
|---|---|---|
| ΔHf° | Formation | Formed from elements in their standard states |
| ΔHc° | Combustion | Completely burned in excess O₂ |
| ΔHr° | Reaction | For the reaction as written |
| ΔHvap / ΔHfus | Vaporisation / Fusion | Liquid→gas / solid→liquid |