rKESlOENTIAL ADDRESS- 887 



If the reaction -were started with a at double unit concentration, then twice 

 as much a would have to be added per unit time to keep the reaction velocity 

 constant at the double rate it would have started at. 



And with higher concentrations proportionally more A would have to be 

 added. It is therefore shown that the amount of chemical change going on in 

 unit time is proportional to the concentration. This is a most fundamental 

 principle of chemical mechanics, known as the law of mass, and it may be stated 

 thus : the nmount of chemical chancje takinr/ 2^laco at ani/ time is alwai/s propor- 

 tional to the amount of actively reacting substance (or substa7ices) present. 



To carry out experiments by the procedure given above is in practice very 

 dithcult and the velocities of reactions are never measured by the chemist in this 

 way. In a living organism this continual bringing up of new supplies of material 

 to maintain a constant rate of change is the ordinary way of life, but in the 

 chemical laboratory procedure is different. There, definite amounts of substances 

 are initially mixed in a vessel and the reaction is allowed to progress by itself 

 without further additions. In this case there is a continual falling off of the 

 concentration of the substance, and so a .corresponding diminution of the actual 

 reaction-velocity. 



In this procedure the diminution of the initial amount of substance can be 

 actually measured by withdrawing small samples at intervals of time and 

 analysing them. Let us consider a definite example. Cane-sugar can be 

 hydrolysed, under various conditions, to give two molecules of hexose, according 

 to the equation 



CiJI,,0„ + H.,0 = 2C6H,,,0,, 



This reaction goes on, though extremely slowly, when an aqueous solution 

 of cane-sugar is kept very hot in a beaker. Suppose we started with, say, 

 128 grammes dissolved in a litre of water and traced the diminution of this 

 amount down towards zero by withdrawing samples at intervals of time and 

 analysing them. If we plotted the sugar-content of these successive samples 

 against the times when they were taken we should get the curve given in fig. 1. 

 If we call n minutes the time taken for the sugar to diminish from 128 grammes 

 to 64 grammes, we should find that in the second n minutes the sugar had fallen 

 to 32 grammes, after 3>i minutes to 16 grammes, and so on, the amount halving 

 itself every n minutes. Thus the amounts of cane-sugar hydrolysed in successive 

 equal intervals are 6-t, 32, 16, 8, 4, 2, 1 grammes, amounts "in each case just 

 exactly proportional to the quantity of cane-sugar then remaining in solution, 

 thus exemplifying the law of mass. 



Such a curve as A in fig. 1, which changes by a constant multiple for suc- 

 cessive units of time (here halving itself every n minutes) is known as a logarithmic 

 curve ; the velocity of reaction at any moment is exactly indicated by the steepness 

 of the curve at that moment; the velocity is greatest "at first and "it declines to 

 almost zero as the curve approaches the horizontal at the end of the reaction. 



When instead of the decomposition of a single substance we deal with two 

 dissolved substances, a and :n, reacting together, then as both of them go on 

 being thus used up, tlae amount of change must be ever proportional to the mass or 

 amount of A present multiplied by the mass of B present. 



There is a special important case when the amount of, say, B is in very great 

 excess of that amount required to unite with the Avhole of a. Then all through 

 the slow progress of the reaction the amount of b never becomes reduced enough 

 to make appreciable difference to its mass, and it may be considered as practically 

 constant all along. In such a case the rate of the reaction is found to be pro- 

 portional simply to the amount of A present, and we get again the curve A, fig. 1. 

 Here the amount of a may be considered as a limiting factor to the amount of 

 reaction; b being in such great excess never falls low enough to fake a 

 practical part in determining the velocity. 



The case of the hydrolysis of cane-sugar in aqueous solution is just such a case. 

 The water itself enters into the reaction, but so little is used up in relation to the 

 eqorijious e.vcess present that the aujouat remains practically constant and the 



