158 



CHEMICAL TRANSFORMATIONS IN 



concentration, of none of the reacting substances on one side can 

 be zero, 1 unless the osmotic pressure of one or more of the reacting 

 substances on the other side also became zero. But a zero value 

 on both sides could only mean that substances taking part in the 

 reaction are absent, and this condition is therefore impossible. 

 Hence there is theoretically an equilibrium 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 indistinguish- 

 able experimentally from that of complete passage into the sub- 

 stances found on one side of the equation of reaction. Hence 



practically reactions may 

 be divided into reversible 

 or incomplete and irre- 

 versible or complete re- 

 actions, and we now 

 proceed to consider the 

 conditions which tend to 

 cause a reaction to be- 

 come practically revers- 

 ible or incomplete. 



We have already seen 

 that in those reactions 

 in which, as a result of 

 the reaction, the number 

 of molecules in solution 

 chan g es > the . concentra- 

 tion of solution changes 

 the equilibrium point, and 

 that such reactions tend 

 to become reversible or incomplete 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 



1 For if any one factor in the product P A .P B .P C , etc., becomes zero, the 

 value of the whole product is zero, and this cannot be equated to any finite 

 value on the other side. 



B 



FIG. 2,-GRAPHic REPRESENTATION OF THE 

 ENERQY CHANGES IN THE COURSE OF A 

 REACTION. 



