In exothermic reactions, heat is given out, so the enthalpy of products is less than the enthalpy of the reactants, and the enthalpy change is negative. In endothermic reactions, heat is absorbed, which increases the enthalpy of the products. In such a case enthalpy change of the reaction is positive.
Changes in Enthalpy
A system that releases heat to the surroundings, an exothermic reaction, has a negative ΔH by convention because the enthalpy of the products is lower than the enthalpy of the system’s reactants. The enthalpies of these reactions are less than zero and are therefore exothermic reactions. A system of reactants that absorbs heat from the surroundings in an endothermic reaction has a positive ΔH because the enthalpy of the products is higher than the enthalpy of the system’s reactants.