In chemistry and thermodynamics, the enthalpy of neutralization (ΔnH) is the change in enthalpy that occurs when one equivalent of an acid and a base undergo a neutralization reaction to form water and a salt. It is a special case of the enthalpy of reaction. It is defined as the energy released with the formation of 1 mole of water. When a reaction is carried out under standard conditions at the temperature of 298 K (25 °C) and 1 bar of pressure and one mole of water is formed, the heat released by the reaction is called the standard enthalpy of neutralization (ΔnH⊖).

The heat (Q) released during a reaction is

Q = m c p Δ T {\displaystyle Q=mc_{p}\Delta T}

where m is the mass of the solution, cp is the specific heat capacity of the solution, and ∆T is the temperature change observed during the reaction. From this, the standard enthalpy change (∆H) is obtained by division with the amount of substance (in moles) involved.

Δ H = − Q n {\displaystyle \Delta H=-{\frac {Q}{n}}}

When a strong acid, HA, reacts with a strong base, BOH, the reaction that occurs is

H + + OH − ⟶ H 2 O {\displaystyle {\ce {H+ + OH^- -> H2O}}}

as the acid and the base are fully dissociated and neither the cation B+ nor the anion A− are involved in the neutralization reaction. The enthalpy change for this reaction is -57.62 kJ/mol at 25 °C.

For weak acids or bases, the heat of neutralization is pH-dependent. In the absence of any added mineral acid or alkali, some heat is required for complete dissociation. The total heat evolved during neutralization will be smaller.

e.g. HCN + NaOH ⟶ NaCN + H 2 O ;   Δ H {\displaystyle {\ce {HCN + NaOH -> NaCN + H2O}};\ \Delta H} = -12 kJ/mol at 25 °C

The heat of ionization for this reaction is equal to (–12 + 57.3) = 45.3 kJ/mol at 25 °C.