REACTIONS IN HOMOGENEOUS SYSTEMS 37 



difference to the rate of reaction, we can write the equation as 



_ = k"C c . In other words the concentration of water is 

 at 



constant and this constant figure is included in k"* 



During the union of A and B to form C the rate of reaction 

 decreases because the concentrations of A and B are diminish- 

 ing and the reaction in the opposite direction is accelerated 

 as the amount of C is increasing. 



At the equilibrium point the rate of formation of C exactly 

 equals that of its decomposition and the composition of the 

 mixture is shown by the equations 



&'C A .C B k"C = O or 'C- A C B = k"C 



Thus CA ' CB = ^ = K and this means that the composi- 

 C c k 



tion at the equilibrium point is proportional to the^ratio of 

 the velocity constants of the reaction and this is expressed 

 by the symbol K or equilibrium constant. 



These simple equations are of immense importance in 

 chemistry as they can be used to express quantitatively the 

 chemical changes which follow the mixing of the substances in 

 varying proportions. A proper appreciation of their value 

 will help the biological chemist to understand much that is 

 otherwise difficult and it is necessary that every biological 

 chemist should be able to deal with his subject by means of 

 this law. 



The constants k' and k" depend upon the nature of the 

 reaction, but once the reaction has been studied and the con- 

 stants determined it is possible to foretell the result of mixing 

 the substances in any proportions. 



The reaction of breaking up of C is called a mono-molecular 

 reaction as only one variable is concerned, and the combina- 

 tion of A and B is called a bi-molecular reaction as two 

 variables are concerned. More complicated reactions occur 

 but they are unusual and can be dealt with by introducing 

 extra terms into the equations. If two molecules of the same 

 substance are required, the corresponding power of the con- 

 centration must be used. For instance, if two molecules of 



* For reasons which depend upon the mathematical treatment of the 

 subject the rate of reaction is represented as a differential and the symbol 



- is used. means the small change dx which takes place in the 



at at 



quantity of x during the short interval of time, dt. 



