Enthalpy is the heat content of the system at constant pressure.Enthalpy (H) is defined as the sum of the internal energy (E) and the product of pressure (P) and volume of the system (V).The exchange of heat between the system and surrounding is enthalpy change ∆H.The change in enthalpy for a reaction is given above.Hp is the enthalpy of products in the final state of the system.Hr is the enthalpy of reactants in the initial state of the system.For exothermic reaction, Hp is lesser than the Hr, so change in enthalpy is negative.For endothermic reaction, Hp is greater than Hr, so change in enthalpy is positive.When no heat is lost or gained, Hp is equal to Hr, so change in enthalpy is equal to zero.