60 PHYSICAL CHEMISTRY IN MEDICINE. 



The chemical reaction 

 CH 3 COOH + Ca 



Acetic acid Ethyl alcohol Ethyl acetate Water 



furnishes a picture of a simple reaction which may take 

 place in either direction. If we begin with a mixture of 

 chemically equivalent amounts of acetic acid and ethyl 

 alcohol, the reaction takes place from left to right, with 

 the formation of ethyl acetate and water; and conversely, 

 if the latter are added to each other, the reaction takes 

 place in the reverse direction. Whatever the starting- 

 point, the final result is a definite state of equilibrium 

 between all the end products. Depending upon the 

 direction .toward which the equilibrium-point is pushed, 

 at one time the one reaction, at another time the other, 

 may have the upper hand. The manner in which such 

 an equilibrium-point can be displaced is illustrated by 

 the change that systems such as the above suffer under 

 the influence of an increase in temperature. According 

 to a well-known law, an increase in temperature pushes 

 the equilibrium toward the side of the endothermic reac- 

 tion. 



The following equations, in which letters have been 

 used in place of different chemical formulas, illustrate 

 some types of such reactions which in practice take 

 place only in the one or in the opposite direction: 



A+m 

 B 



M +m= 



+m = B I 

 -m = A ) A 



=M ) 



. . . } 



