

CHEMICAL EQUILIBRIUM 239 



surface and better mutual contact, but will not transfer the action 

 to the rapid class. It is chiefly the dissolved part of the substance 

 which interacts, for chemical action takes place between mole- 

 cules, and only the dissolved part is disintegrated in such a way 

 that the molecules are readily accessible. Thus, the action is 

 held back by continual waiting for the slow replenishment, from 

 the " insoluble " solid, of the supply of dissolved molecules. In 

 the cases cited, the restraining influence of the dissolving process, 

 which is part of the whole phenomenon, may be formulated thus : 



Zn (solid) fc? Zn (dslvd.) + 2HC1 -> ZnCl 2 + H 2 . 

 MnO 2 (solid) <= MnO 2 (dslvd.) + 4HC1 - MnCl 2 + 2H 2 + C1 2 . 



Here, again, the mechanical details, depending on physical prop- 

 erties, have more to do with the progress of the action than has 

 the chemical affinity. In terms of the law of concentration, the 

 action is slow, and the apparent activity small, because the con- 

 centration of the acting molecules of one of the substances is very 

 small, and cannot be increased because of low solubility. 



Displacement of Equilibria. We have seen (pp. 234-5) that 

 one way in which a reversible action may be forced nearer to com- 

 pletion, in one direction or the other, is the introduction of an 

 excess of one of the ingredients contributing to the forward action. 

 This method of displacing the equilibrium point, however, cannot 

 be very effective, unless it is possible to introduce an exceedingly 

 large excess of the selected ingredient in a high degree of molecular 

 concentration, since this operation does not in any way affect or, 

 in particular, restrain the reverse action which is continually undoing 

 the work of the forward one. A much more effective means of 

 furthering the desired direction of such actions is found, therefore, 

 in the restraint or practical annulment of the reverse action. A 

 good way of accomplishing this is to allow the products of the 

 direct action to separate into an inhomogeneous mixture. Any 

 agency which could remove the water vapor as fast as it was 

 formed by the interaction of hydrogen chloride and oxygen, for 



