Special Versions of the Mass Action Equation

Until now, we have described mass action by using functions in which the concentrations c or, more exactly, the ratios c/cq or c/c appear as arguments. Instead of c, it would be possible to introduce any other measure of composition as long as it is proportional to concentration. This is almost always the case at small c values. We will highlight two of these measures here because they are of greater importance. 

When the pressure of a gas is increased, the concentration of the gas particles also increases because they are compressed into a smaller volume. If the temperature remains unchanged, the concentration grows proportionally to the pressure c p, or 

As a result, the concentration ratio in the mass action equation for gases can be replaced by the pressure ratio  

This equation is precise enough to be applied to pressures up to about 10 kPa (1 bar). It also lends itself to estimates up to 10 or even 10 kPa. In anticipation of this, we have applied the equation above to treating the pressure dependence of the chemical potential of gases [cf. Eq. (5.18)]. 

The mass action equation 2 can be generalized somewhat. In the case of gaseous mixtures, one imagines that each component A, B, C,. .. produces a partial pressure which is independent of its partners in the mixture. This corresponds to the pressure that the gaseous components would have if they alone were to fill up the available volume. The total pressure p of the gaseous mixture is simply equal to the sum of the partial pressures of all the components present (Dalton s law)  

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