Heat of neutralisation of a strong acid and weak base is less than \(57.3 \mathrm{~kJ} \mathrm{~mol}^{-1}\). This is because (a) all the acid is not ionised (b) all the base is not ionised (c) the ionisation is complete but further reaction does not take place (d) some of the heat is transferred to the surroundings

A neutralisation reaction occurs when an acid reacts with a base and produces a salt and water. The heat released during this reaction is called the heat of neutralization. When a strong acid reacts with a strong base, the heat of neutralisation is always -57.1 kJ/mol because the reaction is complete and results in the formation of water in a very exothermic reaction.

Let's break down each of the given alternatives. Option a suggests that not all the acid is ionised. But the problem statement mentions that it's a strong acid. By definition, strong acids are completely ionised in solution. Hence, option a is incorrect. Now let's explore option b - not all the base is ionised. A weak base does not ionise completely in solution. So, this option seems possible. Option c - complete ionisation but no further reaction. This goes against the fundamental principle of neutralization reaction where heat is generated. So, this is not correct. Option d suggests that some heat is transferred to the surroundings. This is true for all reactions but doesn't account for the heat of neutralisation being less.

From the analysis, option b seems to be the most valid choice. Not all of the weak base is ionised in the solution, which means that not all base molecules can react with acid molecules. This would cause less heat to be released than the standard heat of neutralization for a strong acid and strong base reaction.