Entropy Changes of Matter

Let us consider a closed system undergoing a reversible process with zero shaft work, the first law of thermodynamics leads us to:

Since entropy is a state function (depends only on the initial and final states), equation 3 holds true for any kind of process and is valid for all substances. 

The same can be done for enthalpy:

Equation 6 is similarly valid for all substances.

a) Entropy Change of an Ideal Gas

For an ideal gas, the following equations can be considered:

Let us now consider the enthalpy of an ideal gas.

b) Entropy Change of a Solid or a Liquid

For solids and liquids, density is almost independent of pressure and temperature. Thus, eq. 3 reduces to: