Enthalpy

Enthalpy is a property of a system and is equal to E + PV, where E is the internal energy of the system, P is the pressure, and V is the volume. In a chemical reaction, the enthalpy change is equal to the total enthalpy of the products minus the enthalpy of the reactants. The following equation is used to determine the change in enthalpy in a chemical reaction:

H = H_products - H_reactants
When the products have a greater enthalpy than the reactants, H ends up positive. In these situations, the higher enthalpy in the products results in the absorbtion of heat by the system, thus an endothermic reaction. Also, if H is negative, that means that the reactants have more enthalpy, and must "release" it on its way to becoming products. How do the reactants "release" their enthalpy? They generate heat, and the reaction is exothermic. So, when H is positive, the reaction is endothermic, and when H is negative, the reaction is exothermic.