Enthalpy Change Terms

Enthalpy Change of Reaction

The enthalpy change that accompanies a reaction in the molar quantities shown in a chemical equation under standard conditions, and with everything in its standard state.

Enthalpy change of formation

Enthalpy change when one mole of a substance is formed from its constituent elements under standard conditions, with all the reactants and products in their standard states

2 Na (s) + ½ O²(g) → Na²O (s)

Enthalpy change of combustion

Enthalpy change when one mole of a substance reacts completely with oxygen under standard conditions, with all the reactants in their standard states

H² (g) + ½ O² (g) → H²O (l)

Enthalpy change of neutralisation

Energy change that accompanies the reaction of an acid by a base to produce 1 mole of water (l), under standard conditions with all the reactants in their standard states.

Enthalpy change of atomisation

The enthalpy change that takes place for the formation of 1 mole of gaseous atoms from the element in its standard state under standard conditions.

½ Br2(l) → Br(g)

First Ionisation enthalpy

The enthalpy change required to remove one electron from each atom in 1 mole of gaseous atoms to form one mole of gaseous 1+ ions

Second ionisation enthalpy

The enthalpy change required to remove one electron from one mole of gaseous 1+ ions to produce one mole of gaseous 2+ ions.

First Electron affinity

The enthalpy change that occurs when 1 electron is added to each atom in one mole of gaseous atoms to form one mole of gaseous 1- ions

Second electron affinity

The enthalpy change that occurs when 1 electron is added to each atom in one mole of gaseous 1- ions to form one mole of gaseous 2- ions.

Enthalpy change of solution

The enthalpy change that takes place when one mole of solute dissolves in a solvent.

NaCl(s) + aq → Na+(aq) + Cl-(aq)

Enthalpy of Hydration

Enthalpy change that accompanies the dissolving of gaseous ions to form one mole of aqueous ions.

X+/X- + aq → X+/X- (always exothermic)