240 Van Name and Edgar — Velocities of Certain Reactions 



From the point of view of the diffusion theory the mechanism 

 of the reaction is as follows : The weight of bromine or iodine 

 which reaches and reacts with the surface of the metal in the 

 time interval dt is the amount which can diffuse through the 

 adherent layer of liquid in that time, that is, according to 

 Fick's law, it is proportional to the concentration fall across 

 the layer. Owing to the rapidity of the chemical reaction the 

 concentration of the halogen at the surface of the metal is 

 always practically zero. At the outer surface of the layer it is 

 equal to c. the concentration of the main solution. Hence the 

 concentration fall is <?, and if m is the total weight of available 

 halogen in the solution, 



_ dm _ d(cv) 



~ ~dt ~ dt 



Kc, 



where K is the velocity constant. Integrated for constant 

 volume this gives 



K - V In -^ 



h ~ ^i C i 



the expression referred to above. 



To assume that K is proportional to the area of the surface 

 of contact, and to calculate upon this basis its value per unit 

 of surface, is only permissible when the reaction takes place 

 uniformly at every point of the surface. In the following 

 tables this has not been done, since under the conditions of 

 experiment the stirring could not have been equally effective 

 on both sides of the disk. 



Finally, it should be mentioned that the above equation, 

 though a necessary consequence of the diffusion hypothesis, is 

 by no means dependent upon it, since it is nothing but an 

 expression of the a priori very probable assumption that the 

 reaction velocity is proportional to the concentration of the 

 halogen. 



Experiments with Iodine. 



The iodine solutions were initially about 0'03-0'045 equiva- 

 lent normal, and contained in different cases from 100 to 800 

 grams of potassium iodide per liter (0'6 to 4*8 normal with 

 respect to KI), the larger concentrations of potassium iodide 

 being needed with the metals copper and silver to prevent the 

 formation of a coating of insoluble iodide. 



In the experiments with mercury and iodine the procedure 

 was as follows : The gold disk was immersed in clean mercury, 

 a known volume (500-520 cc ) of the solution was placed in the 

 beaker, and the stirrer adjusted to the required speed, after 

 which a 20 cc sample was removed with a pipette and delivered 

 into a glass-stoppered flask for subsequent titration. The disk 



