Section III., 1908. [ 24S ] Tkans. R. S. C, 



XXII. — Harcourt and Esson's Idea in Chemical Mechanics. 

 By W. Lash Miller. 



(Read May 27, 1908.) 



I think that I wiU make the best of the limited time at my disposal 

 by defining at once the fundamental problem of chemical kinetics, for it 

 is with the rates of chemical change that I propose to deal. This pro- 

 blem is, then, to find how the rate of a chemical reaction depends on the 

 circumstances under which that reaction takes place; and in the simple 

 case with which alone we shall be concerned, the simple case that the re- 

 action takes place in a homogeneous solution, the only circumstances that 

 affect the rate are: — the temperature, and the concentrations of the 

 chemicals in the solution. 



The kindness of those in charge of this laboratory puts me in the 

 position to illustrate what I say by a concrete case. Here are samples 

 of the very chemicals used bty Haroourt and Esson in their experiments 

 forty years ago;^ a solution of oxalic acid, water, sulphuric acid, and a 

 solution of potassium permanganate, Wlien these are mixed together, 

 the whole solution looks pink from the presence of the permanganate; 

 on standing awhile, however, the colour wUl gradually fade, owing to 

 the reduction, or destruction, of the red permanganate; and what is 

 meant by the " rate " of the reaction is the number of grammes of per- 

 manganate that are reduced, or destroyed, or fade per second. 



Thait this rate depends on the temperature, may easily be seen if I 

 pour a little of the mixture into a test-tube and heat it over the flame. 

 As the solution gets warmer, the colour fades more rapidly, until now it 

 is all gone, while the part left at the temperature of the room is still 

 deep red. 



The rate depends alsoi on the concentrations of the chemicals dis- 

 Bolved in the solution; so that 



Eaite depends on a, b, c, d, t 



and the fundamental problem of chemical kinetics is to determine quanti- 

 tatively the relation between the rate, the concentrations and the tem- 

 perature. 



Kow for the fundamental difficulty. This lies in the fact that as 

 the reaction proceeds, the temperature changes, owing to the liberation 

 CT absorption of heat by the reaction ; and the various concentrations on 



»Jour. Chem. Soc, 20 460 (1866). 



