In Chapters 2 and 3, numerous property relations were presented for ideal gas and idealized solids. The latter are characterized by constant coefficients of thermal expansion and compressibility and obey the equation of state given by Eq (2.18). For these substances,
• the specific heats (and hence the internal energy and enthalpy) are functions of temperature but are independent of pressure or specific volume
• the entropy of the ideal gas varies with T and v (or p ) according to Eqs (3.9) and (3.10). The entropy of the idealized solid is much more sensitive to temperature than to pressure
• For an ideal gas, the difference between CP and CV is equal to the gas constant. For the ideal solid, CP = 3R.
While the simplified representations above are reasonably applicable to most materials at moderate pressures and temperatures, more exact descriptions are occasionally needed. The objective of this section is to express the deviations from ideality in terms of the equation of state of the substance and its heat capacity.
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