LIVING MATTER AND ITS PRODUCTS 145 



From this expression the amount of energy required to transform 

 a solute from the minimum or zero pressure (P ) to any given pressure 

 (P), or vice versa, may be expressed as heat energy, the expression 



being, RTlog?-. 

 M) 

 When a chemical reaction takes place and molecules appear and dis- 



appear as a consequence, the change in volume energy is the algebraic 

 sum of the changes in volume energy of each kind of molecule so appearing 

 or disappearing. When a grm. molecule of a substance appears in 



p 

 solution at the pressure P the volume energy increases by RT log p-, and 



2 o 

 con versely ivhen a grm. molecule of a substance disappears from solution 



the volume energy decreases by a like amount. 



This may now be applied to the energy changes taking place 

 in different types of reaction. 



1. The simplest case is that in which a grm. molecule of a 

 substance A is formed and a grm. molecule of substance B disappears, 

 as, for example, when a substance changes into an isomeric body, 

 according to the equation, 1 



A;B. 



To simplify matters, we shall assume that the volume of solution 

 is so large that a grin, molecule of A can change into B without causing 

 any appreciable differences in P A , the pressure of A or P B , the pressure 

 ofB. 



The disappearance of A causes a diminution of volume energy 



p 

 given by RT log W^, and the appearance of B an increase of volume 



P 



energy given by RT log p?. Therefore the net increase in volume 



*i 



/ P P \ P 



energy is RT (log j^-log WM, which is RT log jr. Accordingly we 



have the equation 



.-. C = H + KTlogpX 



*A 



.-. H = C-RTlog| B . 



In this equation P B is the osmotic pressure at which B appears, 

 and P A the osmotic pressure at which A disappears. The value of C 



1 The double arrow in a chemical equation is used, instead of the sign of 

 equality, to signify that the reaction may proceed in either direction. 



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