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.^[1]

The standard enthalpy of neutralization^[2] for organic acids is slightly less exothermic than that of mineral acids because of the partial ionization of weak organic acids. The bond between the proton and its conjugate base requires energy to be broken, hence the lower measured magnitude of the enthalpy change.

For weak acids and bases, the 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. $HCN + NaOH \rightarrow NaCN + H_{2}O; \Delta H = -12 kJ/mol$

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

Enthalpy change of a neutralization reaction between acid and base

When a strong acid and a strong base react, the enthalpy change is unique. It may be clearer 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 in this is

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

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

References

1. University of Waterloo. , 30 September 2014.
2. "Enthalpy of Neutralization" (PDF). Community College of Rhode Island. Retrieved 24 February 2014.