Enthalpy of neutralization

In chemistry and thermodynamics, the enthalpy of neutralization ($ΔHn$) 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 degrees Celsius) and 1 atm of pressure and one mole of water is formed, the heat released by the reaction is called the standard enthalpy of neutralization ($ΔHn⊖$).

The heat ($Q$) released during a reaction is
 * $$ Q = mc_p \Delta T $$

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.


 * $$ \Delta H = - \frac{Q}{n} $$

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

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. at 25°C

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