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.

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