What are the differences between exothermic and endothermic reactions?

Reactions are all about the formation and the breaking of bonds between atoms. The formation of bonds within a system causes energy to be released whilst the breaking of bonds requires energy to be applied to the system. In endothermic reactions more bonds are broken than formed during the reaction process. This means that the amount of energy applied to the system is greater than the amount of energy released by it, resulting in a positive enthalpy change for the reaction. In exothermic reactions it's quite the opposite: More bonds are formed than broken during the reaction process. This means that the amount of energy released by the system is greater than the amount of energy applied to it, resulting in a negative enthalpy change for the reaction.