Reaction hefore Complete Equilibrium. 469 



A. In Complete Equilibrium. — Let us take the most general 

 equation for the velocity of reaction ; for this let our reac- 

 tion be 



v 1 a 1 + v 2 a 2 + . . . w, A d + n 2 A 2 + . . . ^ v^al -f vja 2 ' + . . , 02/ A/ + n 2 'A 2 . 



where a iy a 2x . . . a/, a 2 ' are solid or liquid substances, and A l5 

 A 2 . . ., A/, A 2 ' . . . gaseous substances (or substances in 

 solution) ; v v v 2 . . . j>/, v 2 . . . n 1? w 2 . . . n{, n 2 . . . are the 

 number of molecules with which every substance takes part 

 in the equation of the reaction. Since every solid or liquid 

 substance has a vapour- or solution-pressure, our general 

 equation for equilibrium is according to Van't HofF 



hKpwjy . . . p^pf* . . . = Vtt^tt^ . . . p^pj*, . • (*•) 



where 77-/, vr 2 f . . . ir u ir 2 . . . are the partial vapour-pressures (or 

 concentrations in solution) of the solid or liquid substances^ 

 and p u p 2 ...p/, pj . . . those of the gaseous substances. 

 Since, according to the law of Dalton, the vapour-pressure 

 (or solubility) of a solid or of a liquid has for every tempera- 

 ture a certain constant value, we have instead of (i.) : 



Kpi> 2 ^ . . . = KfaWpJ*', (ii.) 



i. e., u the active mass of a solid (or liquid) substance is/' as 

 Guldberg and Waage found empirically, u necessarily con- 

 stant, independent of the quantity of the solid.' 1 Thus, what 

 Guldberg and Waage found empirically, Van't Hoff explained 

 to be a necessary consequence of the law of action of mass. 

 The mutual chemical action between the solid or liquid and 

 gaseous substances is, however, conceived by Van't Hoff in 

 the same light as it was by Guldberg and Waage, namely, as of 

 a reciprocal nature (and he extends this even to physical 

 reactions), representing them all as follows : before equili- 



brium, when a reaction is still going on, water aqueous 



vapour, K (solid substance) "*~ cpf\pf* (gaseous substance or 



substances in solution) ; at equilibrium, water = saturated 



aqueous vapour, K = cp^pf*. . ., i. e., at equilibrium the two 



opposite reactions still continue, but become equal, i. e., just as 

 much (counting in mass or in molecules) of every one of the 

 substances is formed in the unit of time as disappears. No 

 variation in the quantities of the substances is thus taking 



