Enthalpy

Why is the Enthalpy of neutralisation identical no matter which acid or alkali is used?

It's only the same for strong acids and alkalis, it's generally slightly more (less negative) for weak acids and alkalis. It's because for strong acids/alkalis it is assumed the ions have fully dissociated so the only reaction that's actually taking place to form water is H+ + OH- = H2O and this reaction is the same no matter which acid or alkali is used. The other ions would only be spectator ions and not take part on the reaction. So since the enthalpy is the energy change when one mole of water is formed, this would be the same for any strong acid or alkali as it is the same reaction happening every time (hope this makes sense)