It is the enthalpy change accompanying the complete neutralization of an acid by a base or vice versa involving combination of 1 mol of H+ ions (from acid) and 1 mol of 011 ions (fro”} base) to form 1 mol of H p(l) in dilute aqueous solutions. For example, enthalpy change accompanying the neutralization of NaOH and HCI is represented as:
HCl (aq) + NaOH(aq) à NaCl(aq) + H2O(l); ΔnH = 75.1 KJ mol-1
HCl (aq) + 1/2 Ca(OH)2 (aq) à 1/2 CaCl2 (aq) + H2O(l)
Some acid-base reactions and their standard enthalpy of neutralization are given in Table 17.3.
Table 17.3. Standard Enthalpies of Neutralization
of Some Acids and Bases
The enthalpy of neutralization of all strong acids and strong bases is always constant, i.e., -57.1 k]. The explanation to this generalization can be provided on the basis of Arrhenius theory of ionization. The strong acids and strong bases are almost completely ionized in dilute aqueous solutions. The neutralization of strong acid and strong bases simply involve the combination of H+ ions (from acid) and OH- ions (from base) to form water molecules. It bas been found experimentally that when 1 mole of water is formed by the neutralization of 1 mole of H+ ions and 1 mol of OH- in aqueous solutions, 57.1 kJ of energy is released.
H+ (aq) + OH- (aq) à H2O(l) ; ΔH = – 57.1 KJ.
Now, the net process in the neutralization of HC1(aq) and NaOH(aq) can be represented as:
H+(aq) + Cl-(aq) + Na+(aq)
+ OH-(aq) à H2O(l) + Na(aq) + Cl- (aq)
or cancelling out the common spectator ions on both sides of arrow, we get
H+(aq) + OH(-aq) à H2O(l)
In the similar way, the neutralization of HNO3(aq) and KOH(aq) can be represented as
H+(aq) + NO3-(aq) + K+(aq)
Thus, during neutralization of all strong acids and strong bases same reaction takes place. Hence the value of enthalpy of neutralization of strong acid or strong base is constant. However, if either acid or base is weak, then its ionization is not complete in solution. Therefore, a part of the energy liberated during combination of H+ ions and OH- ions is utilised for the ionization of weak acid (or base). Consequently, the value of enthalpy of neutralization of weak acid/strong base or strong acid/weak base is numerically less than 57.1 kJ. For example, neutralization of acetic acid and sodium hydroxide can be represented as
Thus, enthalpy of neutralization of acetic acid and sodium hydroxide is -56.1 kJ. Similarly, enthalpy of neutralization of ammonium hydroxide (weak base) and hydrochloric acid (strong acid) is -51.5 kJ.