22 CHEMICAL TRANSFORMATIONS 



mally from pressure P x to pressure P 2 the well-known expression 



This amount of work may be expressed as heat energy by giving the 

 proper value to the constant RT, which for a grm. molecule of a sub- 

 stance which obeys the gas law works out to 5-8 K calories 1 at 0. 



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 



p 



being, RTlogp-. 

 "o 



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 solution 



p 

 at the pressure P the volume energy increases by RTlog -=5-, and conversely 



*9 



when 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, 2 



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

 is so large that a grm. molecule of A can change into B without 

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

 pressure of B. 



The disappearance of A causes a diminution of volume energy 



p 

 given by RT log -p A , and the appearance of B an increase of volume 



M) 



p 



energy given by RT log -fp. Therefore the net increase in volume 

 r o 



(P P \ P 



lo g W 8 - !og -p- ), which is RT log ~. Accordingly we 

 ^0 -M) / * A 



have the equation 



1 The K (calory) is the amount of heat necessary to raise one grm. of water 

 from to 100 C., and is approximately equal to 100 grm. calories. 



2 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. 



