Why protons combined with electrons will release heat?
If the process of uniting protons and electrons is assumed to occur isothermically in a vessel of constant volume, then the heat released will be equal to the change in internal energy of the system: 

$$q_v=(\Delta U)_v$$ 

Now if the mixture were to behave as an ideal gas, the change in internal energy would be zero and therefore so would the heat. But we can expect a mixture of oppositely charged species such as protons and electrons to behave as an extremely non-ideal mixture, consistently with the large Coulombic potential between the particles. Since $\Delta U$ for the process of uniting protons and electrons (and thereby also minimizing repulsions) is negative, the process is then expected to be exothermic ($q&lt;0$).