Demonstrate that the straight line `1-5` corresponding to the isothermal-isobaric phase isotherm so that areas `I` and `II` are equal (Fig. 2.5).
.

Explanation

We apply the relation `(T = constant)`
`T oint dS = oint d U + oint p d V`
to the cycle `1234531`
Here `oint dS = oint dU = 0`
So `oint p d V = 0`
This implies that the areas `I` and `II` are equal. This reasoning is inapplicable to the cycle `1231`, for example. This cycle is irreverisble because it involves the irreverible transition from a single phase to a two-phase state at the point `3`.
.