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;
Also,      H=E+PV……..(2)
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.

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