Application to Ideal Gases at Equilibrium

All the applications in this chapter will be to the gas phase, in particular to ideal gases. In later chapters the discussion will treat isotope effects on equilibria for condensed phase systems. 

At very low pressure all gases obey the ideal gas law, PV = nRT, where P is the pressure, V is the volume and, T is the temperature. The gas constant, R, is a universal constant, and T is the absolute temperature in kelvin. A mixture of gases at sufficiently low pressure also behaves ideally 

From Equation 4.30, one can write for a gas consisting of one molecular species 

Here G°(T) refers to an n mole ideal gas system at a standard pressure designated as P° (usually 1 bar). The chemical potential of a one component i ideal gas system is then 

Here fif (T) is the Gibbs free energy per mole of an ideal gas at temperature T and standard pressure P°. Thus the condition of equilibrium for a gas phase system subject to a chemical reaction (Equation 4.36), whether at constant T and P or constant T and V, is given by 

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