174 VELOCITY OF REACTION, AND 



substance and the constant 1 which represents the reciprocal of the 

 resistance to change. For (as is shown in deducing the conditions 

 of equilibrium) the energy set free by the reaction will vary with the 

 osmotic pressure, and the resistance is inversely proportional to the 

 constant, and accordingly the product of these two is proportional 

 to the velocity of reaction. 



In all cases the tendency to react is proportional to the osmotic 

 pressure or molecular concentration of each substance in solution, 

 and this, the fundamental law of chemical kinetics, is called the 

 law of mass action. When two or more substances tend to unite 

 to form a single substance, the tendency on the part of each sub- 

 stance is by the law of mass action proportional to its pressure 

 or concentration in the solution, and hence the velocity of forma- 

 tion of the combined substance will be proportioned to the product 

 of the concentrations of the uniting constituents. Accordingly, 

 in any equation of velocity, the velocity of formation of a substance 

 may be set down as equal to the product of the concentrations of its 

 constituents in the solution, multiplied by a constant (k) which 

 represents, and is the reciprocal of, the resistance to the reaction. 

 The value of k will vary with the amount of catalyst present, 

 with the temperature and other factors which alter the resistance ; 

 but these factors being supposed kept constant, the value of k 

 will remain constant throughout the reaction. 



We are now in a position to state equations for the velocity of 

 reactions. 



Let two substances, A and B, in solution react to form two others, 

 C and D, according to the equation 



A+B C + D. 



Further, let the molecular concentrations of the four substances (or 

 their osmotic pressures, which are proportional to these concentra- 

 tions) be represented by C A , C B , c c , c u , and the constant of reaction 

 for conversion of A and B into C and D (as explained above) be k lt 

 and the constant for reaction in the opposite direction of C and D 

 into A and B be & 2 . Then the rate of formation of C and D is given 

 by k 1 .c A .c ] ,, and the rate of formation of A and B that is, of dis- 

 appearance of C and D is given byk 2 .c c .c D . Hence the net velocity 



1 It will be shown later that the assumption that the resistance is constant 

 throughout the reaction is only an approximation, and that the resistance 

 really varies somewhat with the ratio between enzyme and substratum. 



