4 JOHN H. NORTHROP 



Theoretical proof of this statement was furnished by van't Hoff, who 

 showed by thermodynamic reasoning that a substance which is not 

 changed during the course of a reaction cannot affect the equilibrium 

 condition (cf. Taylor, 1925, p. 318).^ 



This peculiarity of catalytic reactions also furnishes the most useful 

 method of determining experimentally whether a reaction is catalytic 

 or not. If the substance under investigation acts as a catalyst (enzyme) 

 the addition of increasing concentrations of the substance will not 

 change the final products formed, but only the time required for the 

 reaction to take place. If, on the other hand, the substance takes part in 

 the reaction, the nature or quantity of the products, as well as their rate 

 of formation, will change as the quantity of substance added is increased. 



Since the catalyst does not change the equilibrium, it follows that if 

 any one catalyst causes the reaction to proceed from A to B then any 

 other catalyst which affects the reaction must also cause the reaction to 

 proceed in the same direction and to the same extent. The assumption of 

 "hydrolyzing" and "synthetic" catalysts for the same reaction under 

 the same conditions is therefore in contradiction to theory (cf. Borsook, 



1935)- 



It is true, however, that under some conditions the addition of a 



catalyst can greatly increase the concentration of one or more reaction 

 products and it is for this reason that enzymes are so important in cel- 

 lular reactions and catalysts so useful in industry. Assume the presence 

 of a substance A, which decomposes "spontaneously," i.e. with the liber- 

 ation of energy, to form a series of substances, B, C , D, E — and that 

 at equilibrium all substances are present in equal concentration. Since 

 they are all in equilibrium with A they are also in equilibrium with each 

 other. \i A \s, then allowed to decompose spontaneously the system at 

 the end of the reaction will contain equal concentrations of A, B, C, D, 

 and E, and no catalyst can cause any further change to occur. Now sup- 

 pose that A alone is present and a catalyst is added which accelerates the 



closely to the experimental facts. He defined catalysts as substances which do not appear 

 in the reaction products in simple stoicheiometric relationship. 



3 This conclusion is strictly true only when the catalyst is present in low concentra- 

 tion and does not combine with any of the components of the reaction. If the catalyst 

 is present in high concentration it may affect the activity of the components in the 

 same way as would any indifferent substance and so change the equilibrium (Taylor, 

 1925). If the catalyst combines with one or more components the equilibrium will also 

 be changed (Euler, 1925, p. 305). Enzymes in general are present in such low con- 

 centration that effects on the activity are negligible. Many enzymes combine with the 

 products of reaction, however, and in such cases this combination may affect the 

 equilibrium point, although the principal result is to slow down the rate of reaction 

 so that experimentally it may be very difficult to reach final equilibrium. 



