Thermodynamics - Enthalpy Change Definitions

Enthalpy of Formation

Energy change when one mole of a substance is formed from its constituent elements with all substances in their standard states.

e.g. 2Na(s) + ½O₂(g) → Na₂O(g)

Exothermic Reaction (-ve)

Enthalpy of Combustion

Energy change when one mole of a substance undergoes complete combustion in oxygen with all substances in their standard states.

e.g. H₂(g) + ½O₂(g) → H₂O(l)

Exothermic Reaction (-ve)

Enthalpy of Neutralisation

Energy change when one mole of a water is formed in a reaction between an acid and an alkali under standard states.

e.g. ½H₂SO₄(aq) + NaOH(aq) → ½Na₂SO₄(aq) + H₂O(l)

Exothermic Reaction (-ve)

Ionisation enthalpy

(First Ionisation) Enthalpy change when each atom in one mole of gaseous atoms loses one electron to form one mole of gaseous 1+ ions.

e.g. Mg(g) → Mg⁺(g) + e⁻

Endothermic Reaction (+ve)

Electron affinity

(First Electron Affinity) Enthalpy change when each atom in one mole of gaseous atoms gains one electron to form one mole of gaseous 1- ions.

e.g. O(g) + e⁻ → O⁻(g)

Endothermic Reaction (+ve) [Second EA is endothermic(+ve)]

Enthalpy of Atomisation

Enthalpy change when one mole of gaseous atoms is produced from an element in their standard state.

e.g. ½I₂(s) → I(g)

Endothermic Reaction (+ve)

Hydration Enthalpy

Enthalpy change when one mole of gaseous ions becomes hydrated (dissolved in water).

e.g. Mg²⁺(g) + aq → Mg²⁺(aq)

Exothermic Reaction (-ve)

Enthalpy of Solution

Enthalpy change when one mole of gaseous ions becomes hydrated (dissolved in water).

e.g. Mg²⁺(g) + aq → Mg²⁺(aq)

Exothermic Reaction (-ve)

Bond Disscociation Enthalpy

Enthalpy change when one mole of covalent bonds is broken in the gaseous state.

e.g. I₂(g) → 2I(g)

Endothermic Reaction (+ve)

Lattice Enthalpy of Formation

Enthalpy change when one mole of a solid ionic compound is formed into its constituent ions in the gas phase.

e.g. Mg²⁺(g) + 2Cl⁻(g) → MgCl₂(s)

Exothermic Reaction (-ve)

Lattice Enthalpy of Dissociation

Enthalpy change when one mole of a solid ionic compound is broken up into its constituent ions in the gas phase.

e.g. MgCl₂(s) → Mg²⁺(g) + 2Cl⁻(g)

Endothermic Reaction (+ve)

Enthalpy of Vaporisation

Enthalpy change when one mole of a liquid is turned into a gas.

e.g. H₂O(l) → H₂O(g)

Endothermic Reaction (+ve)

Enthalpy of Fusion

Enthalpy change when one mole of a solid is turned into a liquid.

e.g. Mg(s) → Mg(l)

Endothermic Reaction (+ve)