Energy changes in chemical reactions
A thermochemical description begins by defining the system and the surroundings. Enthalpy, H, is a state function. At constant pressure, the heat transferred to the system is represented by ΔH. An exothermic reaction transfers energy from the system to the surroundings and has ΔH < 0. An endothermic reaction absorbs energy into the system and has ΔH > 0. The activation-energy barrier controls how rapidly a reaction can proceed; it is not the same as the overall enthalpy change.