A reaction that gives off heat when it occurs is said to be exothermic. The change in enthalpy, ΔH, of an exothermic reaction is negative. This may seem counter-intuitive, but remember that the properties of a system are seen from the systems point of view. Since the system looses heat (by giving it off to the surroundings) the change in enthalpy is negative. A negative ΔH value means that the enthalpy of the products is lower than the enthalpy of the reactants.
Figure 1. The energy diagram of endothermic and exothermic reactions.
The opposite is the case for a reaction that consumes heat when it occurs. Such a reaction is said to be endothermic. The enthalpy change, ΔH, of an endothermic reaction is positive, because heat is applied to the system. A positive ΔH value means that the enthalpy of the products is higher than the enthalpy of the reactants.
If a chemical reaction is exothermic going in one direction, it will be endothermic going in the opposite direction. The magnitude of the change in enthalpy, ΔH, will be the same, but with reversed number sign.