Energetics

Enthalpy of formation (ΔH_f^⦵ )

Enthalpy change when one mole of a substance is formed from its constituent elements with all substances in their standard states: Exothermic (-)

e.g. Na₂O(s)

2 Na(s) + ½ O₂(g) → Na₂O(s)

Enthalpy of combustion (ΔH_c^⦵ )

Enthalpy change when one mole of a substance undergoes complete combustion in oxygen with all substances in their standard states: Exothermic (-)

e.g. hydrogen

H₂(g) + ½ O₂(g) → H₂O(l)

Enthalpy of neutralisation (ΔH_neut^⦵ )

Enthalpy change when 1 mole of water is formed in a reaction between an acid and alkali under standard conditions: Exothermic (-)

e.g. H₂SO₄ + NaOH

½ H₂SO₄(aq) + NaOH(aq) → ½ Na₂SO₄(aq) + H₂O(l)

(First) Ionisation enthalpy (ΔH_IE^⦵ )

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

e.g. magnesium

Mg(g) → Mg⁺(g) + e^-

(Second) Ionisation enthalpy (ΔH_IE^⦵ )

Enthalpy change when each ion in one mole of gaseous 1+ ions loses one electron to form one mole of gaseous 2+ ions: Endothermic (+)

e.g. magnesium

Mg⁺(g) → Mg²⁺(g) + e^-

(First) Electron affinity (ΔH_EA^⦵ )

Enthalpy change when each atom in one mole of gaseous atoms gains one electron to form one mole of gaseous 1- ions: Exothermic (-) for many non metals

e.g. oxygen

O(g) + e^- → O^-(g)

(Second) Electron affinity (ΔH_EA^⦵ )

Enthalpy change when each ion in one mole of gaseous 1^- ions gains one electron to form one mole of gaseous 2^- ions: Endothermic (+) as adding negative electron to negative ion

e.g. oxygen

O^- (g) + e^- → O₂^-(g)

Enthalpy of atomisation (ΔH_atm^⦵ )

Enthalpy change when one mole of gaseous atoms is produced from an element in its standard state: Endothermic (+)

e.g. iodine

½ I₂(s) → I(g)

Hydration enthalpy (ΔH_hyd^⦵ )

Enthalpy change when one mole of gaseous ions become hydrated (dissolved in water): Exothermic (-)

e.g. magnesium ions

Mg²⁺(g) → Mg²⁺(aq)

Enthalpy of solution (ΔH_sol^⦵ )

Enthalpy change when one mole of an ionic solid dissolves in an amount of water large enough so that the dissolved ions are separated and do not interact with each other: Varies

e.g. magnesium chloride

MgCl₂(s) → Mg²⁺(aq) + 2 Cl^-(aq)

Bond dissociation enthalpy (ΔH_BDE^⦵ )

Enthalpy change when one mole of covalent bonds is broken in the gaseous state: Endothermic (+)

e.g. I-I bond

I₂(g) → 2 I(g)

Lattice enthalpy of formation (ΔH_LE^⦵ )

Enthalpy change when one mole of a solid ionic compound is formed from its constituent gaseous ions: Exothermic (-)

e.g. magnesium chloride

Mg²⁺(g) + 2 Cl^- (g) → MgCl²(s)

Lattice enthalpy of dissociation (ΔH_LE^⦵ )

Enthalpy change when one mole of a solid ionic compound is broken up into its constituent gaseous ions: Endothermic (+)

e.g. magnesium chloride

MgCl₂(s) → Mg²⁺(g) + 2 Cl^- (g)

Enthalpy of vaporisation (ΔH_vap^⦵ )

Enthalpy change when one mole of a liquid is turned into a gas: Endothermic (+)

e.g. H₂O(l)

H₂O(l) → H₂O(g)

Enthalpy of fusion (ΔH_fus^⦵ )

Enthalpy change when one mole of a solid is turned into a liquid: Endothermic (+)

e.g. Mg(s)

Mg(s) → Mg(l)