Enthalpy
The key idea
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.
Why it matters
A negative reaction enthalpy is associated with an exothermic process under those conditions; a positive value with an endothermic one.
A picture to remember
A negative reaction enthalpy is associated with an exothermic process under those conditions; a positive value with an endothermic one.