452 



principles is demanded. The basis referred to, which should now 

 be re-read as a preliminary to what follows, is contained in the 

 discussion of chemical equilibrium in general (pp. 230-241). 

 One important point may be briefly recapitulated here, as an 

 aid to the reader. 



Equilibrium in Reversible Reactions. We saw (p. 237) 

 that the point of equilibrium in the dissociation of phosphorus 

 pentachloride : 



is dependent upon the molecular concentrations of the reacting 

 substances, and may be represented by the equation: 



[PC1 3 3 [GUI 

 [PCU] 



In this equation the quantities within the brackets represent 

 the equilibrium concentrations of the respective substances, and 

 K is the equilibrium constant of the reversible reaction. How- 

 ever much we may vary the molecular concentration of any one 

 substance at any fixed temperature (say, by adding excess of 

 C1 2 to the gaseous mixture), the only effect will be a re-adjustment 

 of the equilibrium point such that the final ratio [PC1 3 ] [C1 2 ] / 

 [PC1 5 ] is unchanged. In other words, the equilibrium constant 

 K is a fixed quantity. 



Application to lonization. Exactly the same principles 

 can be applied to the ionic dissociation of an electrolyte. The 

 behavior of acetic acid, a weak, slightly ionized acid, will serve 

 as an illustration. 



In normal solution (60 g. in 11.) acetic acid is only 0.004 ionized 

 (p. 190), so that, in the equation for the equilibrium, 



(0.996) HC 2 H 3 2 =H+ (0.004) + C 2 H 3 2 - (0.004), 



the relative proportions are as shown by the numbers in paren- 

 theses. If the whole of the acid (60 g.) were ionized, there would 



