It is the sum of internal energy and product of pressure and volume.
Here, E, P & V all are state function so H is also the state function.
Being a state function it depends upon initial and final state and also its absolute value cannot be determined however change can be measured.
Where, Hp= enthalpy of product
Hr= Enthalpy of reactant
∆H at constant pressure
We have, from first law of thermodynamics;
At constant pressure, Q=Qp
So, equation (1) becomes,
It means, heat of reaction at constant pressure is equal to change in enthalpy of reaction.