An exothermic reaction is one in which heat is released to the surroundings and hence can take place spontaneously. In comparison, an endothermic reaction is one which needs energy to take place and hence cannot take place spontaneously.
To determine if a given chemical equation represents an exothermic reaction or an endothermic reaction, we need to know the enthalpy of formation of products and reactants. Enthalpy is the total heat involved in the reaction. If the products have a lower enthalpy than the reactants, the change in enthalpy of the reaction would be less than zero (`DeltaH < 0`). Such a reaction, by convention, is termed an exothermic reaction and heat will be released when such reactions take place. In comparison, when the enthalpy of products is more than that of the reactants, the change in enthalpy associated with the reaction is greater than zero (`DeltaH > 0`) and the reaction is termed endothermic.
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