At constant pressure, the amount of energy released or consumed during a reaction is called the enthalpy of a reaction.
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It is an important factor as it helps in calculating the amount of thermal energy liberated or consumed in a chemical reaction in the given set of conditions . Change in enthalpy is negative if heat flows out of the system, and it is positive if energy flows into the system. Hence, for exothermic reactions, dH is negative, and dH is positive for endothermic reactions.