Reaction before Complete Equilibrium. 483 



in the air-space A mol. NH 3 or HC1 are present, and we start 

 the reaction with the solid NH 4 01, then (i.) becomes 



ct 7 p<> + e"pT , (pT' + A) 



and (i/) becomes 



— -el (» e ^P' + e "^'(P' ! + A ) \-^ / C J Pc-o"Pr(Pr' + A) \ 



dT - C *V° 7T7lr ) - M J + Clr J' ' } 



i. e., the velocity of dissociation of solid, NH 4 C1 becomes smaller 

 when one of the products of dissociation is introduced. 



As is to be already seen from this comparatively simple 

 case, there is no wonder that to the present the equations for 

 the velocity of chemical reaction in heterogeneous systems 

 could not be found. It is only natural that a simple connexion 

 between the velocity of reaction and the concentrations, or 

 the concentrations and the surface of the solid, has always been 

 sought for : the processes which are taking place here and 

 the remoteness of p T from p escaped the attention of the 

 experimenters, since the equations for the velocity of evapora- 

 tion or condensation, <fcc, were quite unknown to them, and 

 without them there is no possibility of getting to the true 

 equations in this very complicated region of phenomena. 



As we shall see in some other examples, the equations for 

 the velocity of chemical reaction in heterogeneous systems 

 easily become of a very complicated form; we now know them, 

 nevertheless, with complete certainty, since they follow 

 directly from well-established and known equations, so that 

 the investigation of the former is reduced to the investigation 

 of the latter. 



Let us now take another system in order that we may 

 further follow up the points which come into consideration 

 in the solution of the problems of this kind : — 



nA 1 gas^??zBi gas + rn'O^ gas. 

 { or { { or | 



A x solid Bj solid, 

 and nAi in sol ^mB 1 in so\. + m'C 1 in sol. 



{ or f I or f 



Ai solid B x solid, 

 e. g., the dissociation of CaO^ into CaO and C0 2 , 



CaC0 3 gas 'Z CaO gas + C0 2 gas 

 { or { \ or \ 



CaC0 3 solid CaO solid. 



