The heat of neutralization between a strong acid and a strong base is a constant. This is because;
the heat of neutralization is the energy released when one mole of hydrogen ions (H+) from the acid reacts with one mole of hydroxide ions (OH-) from the base to form one mole of water. The reaction is exothermic, meaning it releases heat energy.
The heat of neutralization is constant because it depends only on the strength of the acid and base involved and not on their individual concentrations or volumes. Strong acids and bases completely dissociate in water, releasing the same amount of energy when they react with each other. The specific values for the heat of neutralization for different acid-base pairs have been experimentally determined and are found to be similar under standard conditions.
Therefore, regardless of the initial concentrations or volumes of the acid and base, the energy released during the neutralization reaction will always be the same, resulting in a constant heat of neutralization.