144 



ATOMS, IONS, SALTS, AND SURFACES 



Now, e-\-J—a—b is simply the increase (A n) in the number of molecules when the 

 given reaction takes place, so 



P%P-i 



Et^F 



W = RTln'^^-RTln 

 (Term i) (Term 2) 



P'^P'J 





(18) 



(Term 3) 



If the reaction takes place at a constant temperature, term 2 is a constant, since 

 the pressures designated by primes are fixed values. Term 3 is also constant, sine • 

 An, the increase of the number of mols of gas, is fixed by the reaction. 



The work done in a series of reversible changes depends only on the initial anrl 

 final states, and is therefore a fixed quantity for the given reaction. Equation (18) 

 may now be written: 



K, = RTln 



KPi 



or 



RT 



P%pfn 



4^5 



pU'^"'^"' 



■ Ki — K2 — K^ = K^ , 



Therefore 



pipV 



K, 



(19) 



(20) 



(21) 



which is the law of mass action. This equation gives the mass law in a general form. 

 For a reaction in which only i mol of each substance is involved, 



it takes the form 



A+BZE+F , 

 PeXPf 



Pa^Pi 



K 



Thus, according to the mass law, the product of the partial pressures in the equi- 

 librium mixture of the substances formed in a chemical reaction, divided by the prod- 

 uct of the partial pressures of the substances from which they are formed, is equal to 

 a constant at any given temperature. If more than i mol of any of the substances is 

 involved in the reaction, the corresponding partial pressure must be raised to a power 

 equal to the number of mols. 



ESCAPING TENDENCY OR ACTIVITY 



According to the mass law, the effect of any substance present upon the chemical 

 equilibrium is proportional to its concentration. Thus, the activity (a) of a substance 

 may be expressed by its concentration, or 



CbCc 



= K 



(8) 



