;5 W 24 Van Name and Hill — Solution of Metals 



mium, nickel, tin, and copper, the differences being either 

 within the experimental error or, at most, exceeding it but 

 slightly. 



(b) The values for silver are in all cases decidedly lower 

 than for the other metals under like conditions. 



(c) The agreement between different metals tends to become 

 closer the higher the acidity. 



Slight deviations from (c) appear in several cases, but are 

 generally smaller than the experimental error, the only clear 

 exception being the one which occurs with silver in chromic 

 acid, due to the unexpectedly low value l - 22. This exception 

 will be discussed on a later page. > 



Following the point of view explained on page 316, the 

 observed reaction velocity is to be regarded as the resultant of 

 the rates of two different simultaneous processes, the diffusion 

 and the chemical reaction. The agreement between different 

 metals in the strongly acid solutions indicates a nearly or 

 wholly complete dependence, under these conditions, of the 

 observed rate upon that of the diffusion process. On the other 

 hand, the systematic differences which appear between the rates 

 for different metals in the less strongly acid solutions, show 

 clearly that in these cases the diffusion process is not the sole 

 controlling factor, and point to the conclusion that the chemi- 

 cal reaction is here slow enough to have a specific influence 

 upon the observed result. Especially significant in support of 

 this inference is the fact that the sequence of the metals, when 

 arranged in the order of their observed reaction velocities, is 

 very nearly the same as that of the electromotive series, the 

 agreement being as close as could be expected when due allow- 

 ance is made both for the normal error of experiment and for 

 the additional uncertainty which attaches to several of the 

 values on account of specific complications already described. 



So far, our explanation is well supported by the results. 

 One important point, however, remains to be considered. It 

 has been shown that the reaction velocity in all normal cases 

 obeys the equation for a reaction of the first order, thus prov- 

 ing that the rate is proportional at every instant to the con- 

 centration of the oxidizing agent. According to the view of 

 Nernst this is due to the high velocity of the chemical reaction, 

 which keeps the concentration of the oxidizer at the surface of 

 the metal at practically zero. The observed reaction velocity 

 is then the rate at which the oxidizer is supplied by diffusion, 

 and this in turn is proportional to the difference in concentra- 

 tion between the two sides of the diffusion layer, which by 

 hypothesis is equal to the concentration in the main body of 

 the solution. It is evident that this explanation holds only 

 when the velocity of the chemical reaction is very great com- 



