Enthalpy changes
From Chemistry Resource
For a reaction occur at constant pressure, the transfer of heat between the system and surroundings can be determined from the change in enthalpy of the system, ΔH.
ΔH = HProducts - HReactants
If the enthalpy of the reactants is greater than the enthalpy of the products then ΔH is negative and heat is released to the surrondings. So the surroundings get warmer. In this case the reaction is said to be exothermic.
If the enthalpy of the reactants is less than the enthalpy of the products then ΔH is positive and heat flows out of the surrondings. So the surroundings get colder. In this case the reaction is said to be endothermic.
