36 BIOLOGICAL CHEMISTRY 



centration of acid. Hence the true acidity must always be 

 expressed in terms of the concentration of hydrogen ions. 



LAW OF MASS ACTION 



In order to follow this subject further we must divert our 

 attention to the Law of Mass Action.* This law states that 

 the rate of a chemical reaction depends upon the effective f 

 concentration of the reacting substances. The law can be 

 explained on the Kinetic Theory mentioned above as the 

 rate of reaction depends upon the number of collisions and 

 the latter depends upon the number of molecules per unit 

 volume of reacting substances. 



Let us consider a dilute solution in which a reaction takes 

 place between A and B with the formation of C and one 

 molecule of water. The reaction can also be considered as 

 reversible so that on the one hand we have a dehydration, 

 and on the other an hydrolysis. 



dehydration 



A + B + '"* C + water, 



hydrolysis 



The rate of formation of C depends on the molecular con- 

 centration of A multiplied by the molecular concentration 

 of B multiplied by a factor k called the velocity constant. 



Therefore = &'C A -C B where C A = molecular concentration 

 at 



of A and C B , the molecular concentration of B. 

 The rate of formation of A and B from C can be expressed 



in a similar way as ~ = &"C C 'C H . but as we are dealing 

 ctt 



with a dilute solution and the concentration of water is so 

 great that small changes in the amount of water make no 



* C. M. Guldberg and P. Waage, Journ. prakt. Chem., 1879, vol. 

 19, P- 69. 



f The law of Mass Action deals with the number of reacting particles 

 in unit volume. In the case of substances which exist in solution as 

 single molecules it is the molecular concentration, i.e. the molecular 

 weight in grams in a litre. In the case of substances which dissociate 

 in solution such as the complete dissociation of a dibasic acid it is the 

 number of hydrogen ions that pass to be considered ; the normal solu- 

 tion is the half of the molecular concentration as each molecule yields 

 two hydrogen ions. If a substance associates in solution the effective 

 concentration is some sub-multiple of the molecular concentration. 

 The correct application of the law therefore depends on a knowledge 

 of the actual reacting particles. 



