What is Enthalpy of a System?

Let us suppose that the state of a system changes at constant Pressure. Hence there will be change in volume of the system.
Suppose volume changes from V₁ to V₂ at constant pressure P.

Therefore work done w, by the system will be:

w = – P (V₂ – V₁)

As we know that internal energy i.e. ΔU is equal to q + w.
ΔU = q + w

Substituting value of w in this equation we get:
ΔU = q + {- P (V₂ – V₁)}
As ΔU = U₂ – U₁

Therefore
U₂ – U₁ = q – P (V₂ – V₁)

Or (U₂ + P V₂) – (U₁ + P V₁) = q

The quantity U + PV is known as the enthalpy of the system. It is denoted by H. It represents the total energy stored in the system.

Thus
H = U + PV
Where U is a definite property
P and V are also definite properties
Hence H is also definite property i.e. it depends on the state of the system.

We can rewrite as H₁ – H₂ = ΔH = q
Where ΔH represents the increase in the enthalpy of a system when it changes from state 1 to state 2.

Like internal energy, enthalpy is also an extensive property as well as a state function. The absolute value of enthalpy cannot be determined. However the change in enthalpy can be experimentally determined.
ΔH = ΔU + PΔV