CHEMICAL TRANSFORMATIONS 



is therefore impossible. Hence there is theoretically an equi- 

 librium point in all reactions, and every reaction is reversible. 

 But the value of the constant may be, and in many reactions is, 

 such that the point of equilibrium lies so near one of the end 

 points that the position is indistinguishable experimentally from 

 that of complete passage into the substances found on one side 

 of the equation of reaction. Hence practically reactions may be 

 divided into reversible or incomplete and irreversible or complete 



reactions, and we now 

 proceed to t consider the 

 conditions which tend 

 to cause a reaction to 

 become practically re- 

 versible or incomplete. 



We have already 

 seen that in those re- 

 actions in which, as a 

 result of the reaction, 

 the number of molecules 

 in solution changes, the 

 concentration of solu- 

 y n tion changes the equili- 



B, 



A, 

 X, 

 -A, 



FIG. 2. Graphic Representation of the Energy 

 Changes in the Course of a Reaction. 



_o 



i brium point, and that 

 such reactions tend to 

 become reversible or in- 

 complete in concentrated 

 solutions and in dilute 



solution to become irreversible in that phase in which the number 

 of molecules in the solution is largest, and the osmotic energy 

 accordingly at a maximum. It follows for all reactions of this 

 type, which involves by far the greatest number of reactions, 

 that reversibility or the presence of the two phases is impossible 

 in sufficiently dilute solution. This follows because in the equa- 

 tion P A = K . P B n , where n is greater than unity, that as P A and P B 

 diminish together P A must get relatively smaller compared to 

 P B , and with sufficient dilution tends to become infinitely small 

 relatively to P B . 



The other factors which affect the position of the point of 

 equilibrium and determine whether a reaction shall be practically 

 reversible or not are the value of C, the chemical energy involved 



