Entropy of an Ideal GasThe entropy S of a monoatomic ideal gas can be expressed in a famous equation called the SackurTetrode equation. where
One of the things which can be determined directly from this equation is the change in entropy during an isothermal expansion where N and U are constant (implying Q=W). Expanding the entropy expression for V_{f} and V_{i} with log combination rules leads to For determining other functions, it is useful to expand the entropy expression to separate the U and V dependence. Then making use of the definition of temperature in terms of entropy: This gives an expression for internal energy that is consistent with equipartition of energy. with kT/2 of energy for each degree of freedom for each atom. For processes with an ideal gas, the change in entropy can be calculated from the relationship Making use of the first law of thermodynamics and the nature of system work, this can be written
This is a useful calculation form if the temperatures and volumes are known, but if you are working on a PV diagram it is preferable to have it expressed in those terms. Using the ideal gas law then But since specific heats are related by C_{P} = C_{V} + R, Since entropy is a state variable, just depending upon the beginning and end states, these expressions can be used for any two points that can be put on one of the standard graphs. 
Index Entropy concepts Reference Schroeder Ch 2  

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