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)

Exothermic 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 an ionic solid dissolves in an amount of water large enough so that the dissolved ions are well separated and do not interact with each other.

e.g. MgCl₂(s) + aq → Mg²⁺(aq) + 2Cl⁻(aq)

Exo/Endothermic Reaction (Varies)

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)