160 CHEMICAL TRANSFORMATIONS IN 



p 



X, and points close to it, - RT log tr nas a large positive value, which 



is to be added to the constant C in order to give the energy set free. 

 Hence no value of C positive or negative can cause the equilibrium 

 point to lie quite at X. As the reaction proceeds, however, and more 

 and more of B is present, P B rises in value, and the positive value of 



p 

 - RT log p B rapidly drops. If now the value of C is negative that is, 



if chemical energy is absorbed on change of substance A into substance 

 B then an equilibrium will be reached as soon as the value of - RT 



p 

 log p B equals C. This will occur nearer X, as at E I} if C has a large 



**A 



negative value than if C has a small negative value, as at E 2 . Beyond 



p 

 the point of equilibrium so denned the positive value of - RT log p B 



p 



becomes still smaller, and hence H, which is C - RT log p B , becomes 



*-A 



negative that is, energy is absorbed, the reaction is endotheraiic, arid 

 cannot proceed without external energy being added, which is excluded 

 under the conditions we are considering. But if C has a positive value 

 the reaction will run farther towards X' before the equilibrium point 

 is reached. As it so runs P B continually increases and P A decreases. 



p 

 So long as P A is greater than P B , the fraction p B is less than unity, its 



*A 



P 



logarithm is negative, and hence - RT leg p B has a positive value; 



*ji 

 p 

 but at the position where P B = P A , log p B = log 1=0, and the curved 



*4 



line representing the change in the osmotic energy crosses the base-line, 



for the osmotic energy set free at this point in the reaction is zero. 



From this point onward osmotic energy is absorbed instead of being 



p 

 given out in the reaction, for log p B now becomes positive and goes on 



increasing in value, at first slowly, and later very rapidly as P A becomes 

 very small in the neighbourhood of X' and the curved line becomes 

 asymptotic to the ordinate. Hence at a certain point the distance of 

 the curved line below the base-line becomes equal to the distance above 

 the base-line of the horizontal line representing the positive value of C. 

 Also the smaller the positive value of C the farther from the end point 

 will be the point of equilibrium. 



The same reasoning applies if we start at X' with the substance 

 all in the form B, and proceed towards X. The diagram to suit 



