WILLIAM D. HARKINS 141 



LAW OF MASS ACTION 



Suppose that two substances, A and B, react to form the substances E and F, and 

 that the reaction has proceeded until equilibrium has been attained. The chemical re- 

 action may be expressed: 



aA+bB = cE+fF. (i) 



It was found by Guldberg and Waage that in such a case a definite law, known as the 

 "mass law," determines the condition of the system at equilibrium. If the substances 

 are gases this may be expressed 



Pe-Pf j^ / X 



Pa-Pb 

 in which Kp is a constant. 



In the case of the simple dissociation 



A^B^C, (3) 



the mass law becomes 



-^ '^P' (4) 



Pa ^ 



In the ordinary development of the mass law it is assumed that the gas law {pv = 

 NRT) is true, so the mass law fails to hoM in any case in which the gas law is invalid. 

 The gas law may be written 



p^K^ = CRT. (S) 



V 



If this is substituted in (4) the following expression is obtained: 



^^ = ^ (6) 



C^ RT ' ^ ' 



but since the temperature is constant and R is the gas constant, Kp/RT is a constant, 

 or 



^^ = K,. (J) 



^A 



DEVELOPMENT OF THE LAW OF MASS ACTION 



The law of mass action may be developed very simply by a consideration of the 

 amount of work necessary to compress a gas reversibly, i.e., in such a way that the 

 same amount of work may be regained when the gas expands. The amount of work 

 done when a force (F) acts through a distance (S) is 



W = F.S. (i) 



If a gas expands in a cylinder provided with a piston, the force (F) is equal to the 

 pressure (p) times the area (A) of the piston, so 



W=pAS=p-Av. (2) 



