Why is a neutralisation reaction exothermic?

Neutralisation reactions are not always exothermic. I will illustrate this with some examples:

When an acid is neutralised by an alkali the reaction is exothermic.

eg. 1.

#HCl_((aq))+NaOH_((aq))rarrNaCl_((aq))+H_2O_((l))# for which #Delta H=-57kJ.mol^(-1)#

eg.2

#HNO_(3(aq))+KOH_((aq))rarrKNO_(3(aq))+H_2O_((l))# for which #DeltaH=-57kJ.mol^(-1#

You will notice that the enthalpy changes for these two reactions are the same. This is because they are essentially the same reaction namely:

#H_((aq))^++OH_((aq))^(-)rarrH_2O_((l)) #

The other ions are spectators. Bond formation is an exothermic process hence the reaction is exothermic because bonds are being formed.

eg.3

Citric acid can be neutralised by sodium hydrogen carbonate. Citric acid is a tribasic acid which means that it has three available protons per molecule which can be donated to a base. To keep things simple I will give it the formula #H_3Cit#. It is neutralised by sodium hydrogen carbonate:

#H_3Cit_((s))+3NaHCO_(3(aq))rarrNa_3Cit_((aq))+3CO_(2(g))+3H_2O_((l))#

The value of #DeltaH# is positive i.e heat is taken in from the surroundings.

Reactions like this are used as "cold packs" to reduce swelling in injuries. When you drop the anti - acid tablet "alkaseltzer" into water this reaction happens. It is also used in confectionary products to give a "fizz" effect.

Spontaneous endothermic reactions like this this are not very common. In this case it is driven by the large increase in entropy as a result of the high entropy values of the products compared to the reactants. That, however, is another topic.

So, in summary, neutralisation reactions are not always exothermic.