CHEMICAL EQUILIBRIUM 237 



The Condition for Chemical Equilibrium. As we have 

 seen (p. 231), the characteristic of a system in chemical equilib- 

 rium is that the speeds of the forward and reverse reactions have 

 become equal. Applying this to the case of the dissociation of 

 phosphorus pentachloride discussed above, we see that, when we 

 have an equilibrium mixture of the three substances, where Si = 

 S 2 , then we also have the relationship: 



K! [PC1J = K 2 [PCI,] [C1 2 ] 



This may be written in the form: 



K! [PCI,] [C1 2 ] 



K 2 [PC1J 



The ratio Ki/K 2 is, of course, a constant, since KI and K 2 are 

 both constants. This ratio, which we may write K, is called the 

 equilibrium constant of the reversible reaction. 



The equilibrium constant is a very important quantity. Once 

 we have determined it, by investigating one equilibrium mixture, 

 we can calculate exactly what will happen to any mixture of the 

 three substances concerned at the same temperature. However 

 much we may vary the molecular concentrations of the three 

 substances, whether by changing the pressure or by adding an 

 excess amount of one of them, such as chlorine, the composition 

 of the mixture will adjust itself until, when equilibrium is attained, 

 the ratio [PC1 3 ] [C1 2 ]/[PC1 5 ] has again reached the value K. In 

 large-scale industrial processes, therefore, a knowledge of the 

 equilibrium constant of the reaction involved is often of inesti- 

 mable value. 



In many important reversible reactions, the concentrations 

 of the reacting substances on one side of the equation are, under 

 equilibrium conditions, very much greater than the concentra- 

 tions of those on the other side. Although the reaction is rever- 

 sible, it proceeds much farther towards completion in one direc- 

 tion than the other. This fact may often be indicated, very con- 



