24 PHYSICOCHEMICAL BASIS OF PHYSIOLOGICAL PROCESSES 



condition is reached in which the changes in one direction are exactly 

 offset by those in the other. An equilibrium is said to have become estab- 

 lished between the reacting substances. Bearing in mind that the ions 

 and molecules entering into these reactions are constantly moving about 

 and coming in contact with one another, it is easy to see that if we were 

 to add an additional quantity of one kind of molecule or ion, there would 

 be a change all along the line until a new equilibrium was established. 

 If, on the other hand, we were to remove one kind of molecule or ion 

 as fast as it is formed, the equilibrium could never be established, and 

 the reaction would proceed until all of this material had disappeared. 

 The natural rate at which any chemical reaction proceeds is dependent 

 upon a number of conditions, such as chemical affinity, temperature, 

 catalysis, and concentration. Of these conditions that of concentration 

 is most readily measured. If we maintain all of the conditions other 

 than that of concentration unchanged, and designate this combined in- 

 fluence as K (constant), we shall find that the speed of the reaction is 

 proportional to the molecular concentration of the reacting substances 

 (i. e., the number of gram-molecular weights per liter). In other words, 

 the speed with which two substances, a and b, unite to form other sub- 

 stances, c and d, will be expressed by the equation, 



k (a)x(b) * k' (c)x(d);* 



which means that, when the reaction is complete, the composition of 

 the mixture will be dependent upon the ratio between k and k'. Since 

 however these are both constants, their quotient is also constant (K), and 



we have the equation, -y-r ~f = K, indicating that no matter how 



\ / ^ \ ^*- / 



the concentrations a, b, c, and d are varied, reaction will take place in 

 one direction or the other until the concentrations have become adjusted 

 so that K remains unchanged. 



As an example of the application of these laws, let us take the reaction 

 which occurs between alcohols and organic acids to form the substances 

 called esters a reaction which is analogous to that between mineral 

 alkalies and acids to form neutral salts, and which is of special interest 

 to us because it is the reaction involved in the splitting of animal fats. 

 The equation for the reaction is: 



C 2 H 5 OH + CH S COOII ? C 2 H B OOCCH 8 + H 2 O. 

 (ethyl (acetic (ethyl acetate, 



alcohol) acid) an ester) 



Or expressed according to the law of mass action: 



[C 2 H 5 OH] x [CH 3 COOH] 

 [C 2 H 6 OOCCH 3 ] x 



*The brackets indicate that gram molecular quantities are used. 



