26 CHEMICAL TRANSFORMATIONS 



identical with the solvent, so that it causes no change in volume 

 energy when it appears or disappears in the reaction. Such a 

 reaction occurring in aqueous solution, for example, as 



C 12 H 22 O n + H 2 2 C 6 H 12 6 + 6 H 12 6 

 (Maltose) (Dextrose) (Dextrose) 



Using the same notation as before, let P A be the osmotic pressure 

 at any stage in the reaction of the single substance which undergoes 

 change in osmotic pressure on the left-hand side of the equation, and 

 P B the osmotic pressure of either of the two substances produced on 

 the right-hand side, for these being produced in equimolecular pro- 

 portion, their osmotic pressures at any given stage in the reaction 

 will be equal in value. 1 



Here, as a result of the reaction, a grm. molecule of A disappears, 



p 

 lowering the volume energy by RT log ^, and a grm. molecule of 



* 



each of the two substances formed appears at a pressure P B , raising 



-p T> 



the volume energy in each case by RT log ^, or in all by 2 RT log ~' 



r o r o 



Accordingly our equation becomes 



or 



As before, the reaction runs exothermically from either end until 



P 2 



RT log ^ ^- = C, at which point there is equilibrium. 2 



" 



1 If unequal quantities of the two substances formed on the right-hand side 

 are present at the beginning along with the single substance, or a certain 

 amount of one of them, then the osmotic pressures of these two (B and C) will 

 not be equal ; but if they are represented by the pressures P B and P c the only 

 difference is that we will have P B . P c in the final equation instead of P B 2 , so that 



P P 



the equation becomes: H = C RTlog B ' -, which for equilibrium leads to 



PA PO 



Q 



i " B PC ^ -* B * C "RT 



log p p = m, or p'-p = e RT - 



r A *o "* * A *Q 



2 As before, the equil.brium equation can accordingly be written 



r i r 1 



P2 B _ g KT or P*B_p RT 

 PP- ' P F ' 



