The Equilibrium Constant for a Gas-Phase Reaction

Recall that our reference state for the gas phase is a pressure low enough that the gas behaves as an ideal gas. For the sake of chemical reaction calculations, we usually choose the standard state pressure to be 1 bar (or 1 atm). For most gases, intermolecular interactions are negligible at 1 bar and the ideal gas assumption is valid. However, if the gas is not ideal at this pressure, we go to a low enough pressure that it is an ideal gas, then extrapolate back to a pressure of 1 bar, assuming the gas is ideal. In this case, the standard state represents that of a hypothetical ideal gas at 1 bar where we have turned off the intermolecular interactions of the real gas. In either case, the standard state fiigacity becomes  

The pressure in Equation (9.25) is written in bar since the reference state fugacity has units of bar. From this point on, these units will be implicit. 

Recall from Section 7.3 that there are three levels of rigor to solve for the fugacity 

This is the rigorous solution, where the fugacity coefficient depends on the concentration all the species in the mixture. 

In this case, we approximate the fugacity coefficient of species i in the mixture by the pure species fugacity coefficient, which is therefore independent of concentration. 

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