Strong acids, strong bases and their salts ionize completely in the solution. Consider the neutralization of hydrochloric acid by sodium hydroxide, which is as follows:
NaOH + HCl → NaCl + H2O + Q cals.
On ionization, we have
Na+ + OH- + H+ + Cl- → Na+ + Cl- + H2O + Q cals.
H+ + OH- → H2O + Q cals
This shows that heat of neutralization of a strong acid by a strong base is nothing, but heat of formation of water molecule from hydrogen and hydroxyl ions, hence it is in the same and fixed quantity.
