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:
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