Reaction before Complete Equilibrium. 481 



Reverting to " the velocity of chemical reaction in hetero- 

 geneous systems," by such an expression we now understand 

 the velocity with which one or more substances of one part 

 of the heterogeneous system transform indirectly (l>y means 

 of its vapour or dissolved molecules) into other substances 

 of another part of the heterogeneous system. In the above 

 system NH 4 C1, NH 3 , HC1 we see two reactions taking place 

 simultaneously: The first reaction is solid NE^Clr*" or ■*- gase- 

 ous NH 4 C1 is regulated by the equation 



(j T ) = C%r(po-pr) 



(we omit for convenience' sake the instability constant ; we 

 ought in more strict calculations to always write Xr + K instead 

 of Sr), where X r is the surface of the solid NH 4 C1, p is the 

 vapour-pressure of the solid IS T H 4 C1 at the real point of 

 equilibrium, p T is the vapour-pressure at the time t. The 



second reaction is gaseous KH 4 C1^NH 3 gas + HCl gas ; it is 

 regulated by the equation 



where p T ' is the vapour-pressure of XH 3 gas as well as of 

 HC1 gas at the time r. 



If the reaction is now begun, e. g., with solid NH 4 C1, a state 

 will soon be arrived at in the system at which in the unit of time 

 as muck of the solid NH 4 C1 is transformed into NH 4 C1 gas as 

 the gaseous NH 4 C1 dissociates into NH 3 and HC1 ; because the 

 gaseous NH 3 and HOI can be formed continuously from 

 gaseous XH 4 C1 only because gaseous NH 4 C1 is simultaneously 

 formed from solid NH 4 C1. Now the quantity of gaseous 

 !NH 4 C1 which is transformed in the unit of time into NH 3 

 and HC1 is given by the total velocity of reaction in the 



homogeneous system, i. e., by I — ); therefore 



cXr(p —pr) = c'p T — c"p T ' 2 , 



and from this 



_ ct T po + c" P T 2 ,. . 



lT ~ 7+cS~r ' [ } 



i. e.j before equilibrium has been arrived at, the concentration 

 of the gaseous XH 4 C1 is of no constant value, as it is a function 

 of the surface of the solid XH 4 (J1 and of the concentrations 

 of the gaseous XH 3 and H(.:l at the time r; since these 

 vary during the reaction, p r must also change (as given by i.). 



