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.
△HIonizationofweakacid=ΔHNeutralizationofWA+SB−ΔHNeutralizationofSA+SB=
ΔHionisation of CH3COOH=−55.3−(−57.3)
=2KJ/mol
Hence, 2KJ/mol of heat is absorbed for the ionization of acetic acid.