478 XJNIVEBSITY OF VIRGINIA PUBLICATIONS 



Part IV. Discussion of Results. 



A\'hat is diffusion — Nernst's "Reststrom" — ^Work of Harry Heymann — ^Relation 



of density and viscosity to the rate of diffusion. 



General confirmation of the diffusion theory of reaction velocity — Summary. 



Acknowledgment. 



PART I. 



THE DIFFUSION THEORY OF REACTION VELOCITY IN HETEROGENEOUS SYSTEMS. 



Work of Noyes and Whitney, 1897. In 1897 Noyes and Whitnej' 

 carried out a series of experiments for the purpose of discovering, if possible, 

 the laws govemiog the rate of solution of salts in water and other solvents. 

 From the results of their work, and those of others, the theory has been 

 advanced by N'ernst that reaction velocity in heterogeneous systems is 

 controlled largely, and in some cases entirely, by the rate of diffusion of 

 the reacting or dissolving substances through a thin unstirred layer of the 

 saturated solution adhering to the surface of the solid (1) (2) (3). That is, 

 we conceive a thin film of saturated solution adhering to the dissolving 

 solid somewhat as a fibn of water adheres to the inside of a pipette, this 

 film is always saturated on the inside, because the rate of solution is very 

 rapid in comparison with the rate of diffusion, which latter in liquids is 

 very slow. 



Thus, if we assume the thickness of the layer to be approximately 

 constant, as seems reasonable with uniform stirring, it should follow that 

 the solution, or reaction velocity, varies directly as the rate of diffiusion. 

 This latter is usually assumed to be proportional to the difference in con- 

 centration on the opposite sides of the thin unstirred layer. This theory 

 was suggested by Noyes and Whitney, and further elaborated by Nernst. 



The rate of reaction for first order reactions in homogeneous systems 

 is expressed by the "Unimolecular formula," 



-^ = k'(a — x) ' 



dt ^ , 



where h is the reaction velocity constant, that is, a number depending on 

 'the so-called "affinity" of the reacting particles for each other, {a is the 

 amount of reacting material originally present, and x the amount trans- 

 formed). 



The rate of reaction in heterogeneous systems is expressed by the 

 formula, 



^r- = k.O. (a — x) 

 dt 



