Action betiveen Metals and Acids. 847 



These tubes forming simultaneously and close together 

 will interact, yielding a molecule of the salt MX in solution 

 and a neutral equivalent of H deposited on M. 



The number of interactions of this kind taking place 

 per sq. cm. of surface in the unit of time will depend upon 

 the frequency with which such tubes form side by side, i. <?., 

 upon the number of molecules of HX in solution per cc. on 

 the one hand and upon some specific property of the metal 

 (determining the rate at which tubes of the second kind form) 

 on the other. For the " velocity " of the reaction 



M + HX ^ H + MX, 



we may therefore write 



where k is a constant at given temperature, c h is the concen- 

 tration of the hydrogen ions in solution, and C OT is a specific 

 constant of the metal M. 



As the result of this action, hydrogen will accumulate upon 

 the metal and MX will be formed in solution. A reverse 

 action, similar in kind to the first, now becomes conceivable. 

 This reaction, which may be represented by 



H + MX s- M + HX, 



will proceed with a velocity 



where c m represents the ionic concentration of the metal in 

 solution and h is a specific constant of hydrogen deposited 

 at given pressure upon M. 



If a steady state is reached, after a certain quantity of 

 hydrogen has been displaced and the equivalent quantity 

 of M has dissolved, it will be denned by the condition v=v' or 



kcJC h = VcJC m . 



AVe can thus deduce kinetically a result identical with that 

 obtainable by application of the logarithmic formula of 

 Nernst and contained as a particular case in the general 

 equation of § 4. 



Since there is no effective transfer of electricity across the 

 border layer, a possible contact difference of potential 

 between metal and solution would not affect the available 

 work equation of § 4. Similarly it would not affect the final 

 equation of equilibrium deduced kinetically. For if a 

 potential-difference existed it would have the same relative 

 effect upon v' as upon v. 



