Enthalpy (H)

It is the sum of internal energy and product of pressure and volume.

i.e. H=E+PV…..(1)
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.
Now, ∆H=Hp-Hr…………(2)
Where, Hp= enthalpy of product
Hr= Enthalpy of reactant
∆H at constant pressure
We have, from first law of thermodynamics;
Q=∆E+W
Q=∆E+P∆V….(1)
Also, H=E+PV……..(2)
At constant pressure, Q=Qp
So, equation (1) becomes,

Qp=Ep-Er+P(Vp-Vr)
Qp=Ep+PVp-(Er+PVr)
Qp=Hp-Hr
Qp=∆H
It means, heat of reaction at constant pressure is equal to change in enthalpy of reaction.