482 Dr. Meyer Wi I derm an on the Velocity of 



This can be clearly understood also from the following : 

 should at the time t more of the gaseous NH 4 C1 dissociate 

 into NH 3 and HC1 in the unit of. time, than solid NH 4 C1 

 transforms into gaseous NH 4 C1, p T would become smaller 

 and po—pr greater, i.e., the reaction of the dissociation 

 of the gaseous NH 4 C1 into NH 3 and HC1 would become 

 slower and the reaction of evaporation of the solid NH 4 C1 

 quicker. Again, should in the unit of time at the time 

 t more of the solid _NH 4 C1 evaporate than of the gaseous 

 NH 4 C1 dissociate into NH 3 and HCl,jo r will become greater 

 and thep — p T smaller, i. e., the reaction of evaporation will 

 become slower and that of dissociation quicker. The vapour- 

 pressure of the solid NH 4 C1 has thus the fund ion of a regulator 

 which keeps the velocities of the two reactions equal. As to the 

 mode in which the reaction takes place it may be further added 

 that if we start the reaction with solid NH 4 C1, p T is <^ , the 

 concentration of the gaseous NH 4 C1 is smaller than that of 

 its saturated vapour at equilibrium, i. e., only the process of 

 evaporation of the solid NH 4 C1 is possible. On the contrary. 

 if we start the reaction with NH 3 and HCl, solid NH 4 C1 can 

 be formed from the gaseous NH 4 ' 1 only when the latter is in 

 a supersaturated state, consequently, p T >p and only the re- 

 action of condensation of the gaseous ^s H 4 C1 to solid NH 4 C1 

 can occur. 



If we put the above value of p T from equation (i.) into 



(J) = c2 r („ r )o,Mnto(J)' 



we set 



&' 



± = r2j„- C M 



. c 2- ( V - CS ^° + C P T '"~ \ - cl (P° C '- C "Pt" \ (V ) 



dr C2 "V J + Ctr )- CZ \ C' + Clr )' ■ ( ' 



i. e., the velocity with which solid NH 4 C1 evaporates or gaseous 

 NH 3 and HC1 are indirectly formed from solid NH 4 C1 is a function 

 of the surface of the solid NH 4 C1 {of the surface of contact of 

 reacting parts of the system) and of the ioncentratiojis of the 

 gaseous NH 3 and HC1. The same is the case with the indirect 

 formation of the solid NH 4 C1 from NH 3 and HC1. 



If the surface of the solid NH 4 C1 remain during the whole 

 time of the reaction constant, we have instead of (i/) 



i. e., the velocitv of reaction is a function of the concentra- 

 tions of NH 3 and HOI. 



If one of the products of dissociation be introduced, e. g., if 



