Enthalpy: the amount of heat evolve or consumed in the course of a reaction. A measure of the strength of bonding in products and reactants. Reactions favor products with lowest enthalpy.
Exothermic reaction: negative value of ?H0; weaker bonds are broken and stronger bonds are formed. ?H makes a favorable negative contribution to ?G0.
Endothermic reaction: positive value of ?H0; Stronger bonds are broken and weaker bonds are formed. ?H makes an unfavorable positive contribution to ?G0.