LIVING MATTER AND ITS PRODUCTS 147 



ings. This point is accordingly the equilibrium point of the reaction. 



p 

 The equation for equilibrium is accordingly RT log p? = C = const. 1 



*"4 



If the chemical energy involved in the passage from the form A 

 to the form B, or vice versa, is so small as to be negligible, as is usually 

 the case in the passage of a substance from one isomeric form to another, 



P P P 



we can write RT log p B = C = 0, and hence log p^ = 0, p B = l, or P B = P A 



* a * A * 4 



that is, under such conditions equilibrium is attained when the 

 osmotic pressures, and hence the molecular concentrations in solution, 

 are equal. 



If we write p A and p B for the osmotic pressures of the two sub- 

 stances at the equilibrium point, and P A and P B as before for the 

 corresponding pressures at any given point in the reaction, another 

 form can be given to the fundamental equation for the heat of reaction 

 at any given point in the reaction. 



/+j 



For now C = RT log , and hence on substituting this value we 

 obtain 



H = RT 



(log g-log ), or H = RT log , 



2. Let us take next the cases where two substances A and B 

 interact to form reversibly two other substances C and D, and 

 let P, with the appropriate suffix of the letter denoting the sub- 

 stance, represent the osmotic pressure of each substance. 



Then since A and B disappear from solution and diminish the 

 volume energy or osmotic pressure energy, and C and D appear and 

 increase the osmotic pressure energy, the equation becomes 



H = C-RT (log ^ + log 5- log ? A -log p B ), 



V *(> o o *' 



or 



The reaction, as before, can be shown to run exothermally from 



P P 



either end until C = RT log fr~W> at which point energy is neither 



-* A -* B 



given to nor taken up from the surroundings ; hence at this point there 



P P c 

 1 This equation may also be written log p B = iTm* or p B=eRI 



