Enthalpy of neutralization

The enthalpy of neutralization (ΔH_n) is the change in enthalpy that occurs when one equivalent of an acid and one equivalent of 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 it is called the standard enthalpy of neutralization (ΔH_n^o).

The heat (Q) released during a reaction is $Q = mc_{p} \Delta T$

where m is the mass of the solution, c_p 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.

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

The standard enthalpy change of neutralization for a strong acid and base is -55.8 kJ/mol.

The standard enthalpy of neutralization for organic acids is slightly less exothermic than that of mineral acids because of the partially ionizing property of weak organic acids. The bond between the proton and its conjugate base requires energy to be broken, hence the lower measured value enthalpy change.

For weak acids and bases, heat of neutralization is different because they are not dissociated completely and during dissociation some heat is absorbed - total heat evolved during neutralization will be smaller.

e.g. $HCN + NaOH \rightarrow NaCN + H_{2}O; \Delta H = -12 kJ/mol$

Heat of ionization in this reaction is equal to (–12 + 57.3) kJ/mol = 45.3 kJ/mol

Enthalpy change of a reaction of neutralization an acid and a base

When a powerful acid and a powerful base react, enthalpy change is unique. It may be more clear in the example below,

$NaOH_{(aq)} + HCl_{(aq)} \rightarrow NaCl_{(aq)}+H_{2}O_{(l)}$

when we consider the above reaction it is clear the only reaction we can see inside this is,

$H^{+}_{(aq)} + OH^{-}_{(aq)} \rightarrow H_{2}O_{(l)}$

Enthalpy change of this reaction is -57.7 kJ mol⁻¹ When a weak acid reacts with a powerful base the enthalpy change must be a lesser enthalpy change than -55.8 kJ mol⁻¹, as weak acids in solution do not fully dissociate to their respective anions and cations. To dissociate these bonds some amount of energy is needed; the enthalpy change reduces.