Why do some reactions get hot?
Enthalpy is a thermodynamic state function. At constant pressure, under the usual conditions for pressure-volume work, its change is especially useful for discussing heat transferred.
The short answer
Enthalpy is a thermodynamic state function. At constant pressure, under the usual conditions for pressure-volume work, its change is especially useful for discussing heat transferred.
What is really happening?
Enthalpy is a thermodynamic state function. At constant pressure, under the usual conditions for pressure-volume work, its change is especially useful for discussing heat transferred.
A negative reaction enthalpy is associated with an exothermic process under those conditions; a positive value with an endothermic one.
At constant temperature and pressure, Gibbs energy provides a criterion for the direction of spontaneous change in a closed system under the usual constraints.
A useful mental picture
Chemical reactions rearrange bonds and redistribute energy. If the reacting system transfers energy to its surroundings as heat at constant pressure, its enthalpy change is negative.