CATALYSIS. 53 



III. CATALYSIS. 



When two bodies which can act chemically upon each other are 

 brought together the reaction generally takes place so fast that it 

 cannot be measured. In other cases, by special means, we can observe 

 how the reaction gradually proceeds. When cane sugar is inverted 

 by weak acid, the decrease in the rotation of the solution can be fol- 

 lowed with the polariscope; and when an ester is decomposed by alkali 

 the quantity of still free alkali can be determined by titration. The 

 quantity of substance measured in gram-molecule per liter (mole) 

 which is decomposed in the unit of time, is called the reaction velocity 

 of the system. The so-called law of mass action, as proposed by 

 GULDBERG and WAAGE, states that the reaction velocity is every 

 moment proportional to the molecular Concentration of the reacting 

 bodies. A mixture of alcohol and acetic acid is transformed into acetic 

 ether and water, especially in the presence of some mineral acid. If 

 the molecular concentration, of the alcohol and acid be designated by 

 C A and C s , then according to the law of mass action the reaction velocity 

 is vi = ki.C A .C s , where ki, indicates a constant which is independent 

 of the quantity of reacting substances and the time limit is so short 

 that the concentration can be considered as constant. This reaction, 

 like many others, is reversible, i.e., two reactions occur simultaneously: 

 one between the alcohol and acetic acid, producing acetic ether and 

 water, and second, between acetic ether and water, reforming alcohol 

 and acetic acid. This is expressed as follows: 



C 2 H 5 .OH + HO.CO.CH 3 <=* C 2 H 5 .O.CO.CH3 + H 2 O. 



\ "* 



The velocity of reaction when it passes from left to right is called 

 Vi. If the velocity in the reverse reaction is called v 2 and the molec- 

 ular concentration of the acetic ether and water is called C E and C^-, 

 then we obtain V2 = k 2 .C E .C w . At the beginning when C E as well as 

 C W = Q } the velocity of the" ester formation is expressed by the formula 

 vi = ki.C A .C 8 ; afterward it is expressed by the difference Vi v 2 or 

 k\.C A .C s k 2 .C E .C w . When ki.C A .C s = k2.C E .C w is attained, then the 

 velocity of both reactions is the same; no measurable decomposi- 

 tion occurs and the system is in equilibrium. The equilibrium condi- 

 tion is the same irrespective of whether we start from alcohol + 

 acetic acid or from the corresponding quantity of acetic ether + water. 

 On equilibrium it is 



ki.CA.Cs = k 2 .CE.Cw or -~ -^ = = K. 

 CE-^W ki 



K is called the equilibrium constant; as is apparent it can be determined 

 in two ways either from the concentration of the reacting bodies when 



