Van Name and Edgar — Velocities of Certain Reactions. 237 



Art. XIX. — On the Velocities of Certain Reactions between 

 Metals and Dissolved Halogens ; by E. G. Van Name 

 and Gkaham Edgar. 



[Contributions from the Kent Chemical Laboratory of Yale Univ. — ccvii] 



In a reaction between a liquid and a solid, according to the 

 so-called diffusion theory of reaction velocity, a thin layer of 

 liquid adhering to the solid remains unaffected by stirring, 

 and the reaction is maintained by the transport of dissolved 

 substances across this layer by diffusion. When the diffusion 

 is sufficiently slow compared with the other stages of the reac- 

 tion, the velocity of the whole will be determined by the rate 

 of diffusion alone. 



This theory was proposed by A. A. JSToyes and W. R. 

 Whitney," in 1897, for the special case of the solution of a 

 solid in a liquid, but Nernst and E. Brunnerf were the first to 

 suggest its general applicability to the various types of hetero- 

 geneous reactions. It was tested by Brunner for cases of 

 simple solution, neutralization, action of acids on metals, and 

 for several electrolytic processes, and has since been accepted 

 by various other investigators as giving the best explanation 

 of the facts in the case of a number of other types of reac- 

 tions,^: including even gas and enzyme reactions. 



On the other hand, the validity of the diffusion principle in 

 the case of reactions between metals and acids has been dis- 

 puted by Ericson-Auren and Palmaer, § and R. Marc || has 

 shoAvn that the crystallization of supersaturated solutions of 

 certain salts seems to follow a different mathematical relation. 

 Recently the theory has been vigorously attacked by M. 

 Wilderman,^f who holds that the hypothesis of a diffusion layer 

 is improbable and unnecessary, and shows that the same velocity 

 equation can be derived without it. Furthermore, Wilderman 

 has found that the rate of solution of gypsum is not constant 

 as the theory would require, but varies widely with its physical 

 state, and is different on different surfaces of the same crystal. 



The experiments to be described deal with the rate of reac- 

 tion between dissolved iodine and the metals mercury, copper, 

 silver, cadmium, and zinc, also with those between bromine 

 and mercury, and between cupric bromide and mercury. The 

 work was undertaken with a view to testing the applicability 



* Zeitschr. phys. Chem., xxiii, 689. flbid., xlvii, 52 and 56. 1904. 



JSenter, ibid., li, 696; Teletew, Dissertation, Heidelberg, 1906; Jabl- 

 czinsky, Zeitschr. phys. Chem., lxiv, 748: Spear, Jour. Am. Chem. Soc, xxx, 

 195 ; Bodensteln and Fink, Zeit. phys. Chem., lx, 1 ; Schleuderberg, Jour. 

 Phvs. Chem., xii, 583. 



£ Zeitschr. phys Chem., lvi, 089. ||Ibid., Ixi, 885 ; Ixvii, 470. 



If Ibid., lxvi, 445. 



