146 CHEMICAL TRANSFORMATIONS IN 



is constant, being the chemical energy set free by the passage of a 

 grm. molecule from the form A into the form B, and the variable 

 value H expresses the heat energy set free by the reaction when a 

 grm. molecule changes form at those particular pressures. 



If we begin at the point where nearly all the substance is present 

 in the form A, then P B has a very low value, and consequently log 

 p 

 p 5 has a large negative value, and this being prefixed by a negative 



sign, it follows that the heat of reaction 1 at this stage has a large 

 positive value. As the reaction proceeds P B continuously increases 



P 



and log p B decreases in negative value ; accordingly the heat of reaction 



* A 



p 



diminishes. As P B still increases and P A diminishes, log p B becomes 



*- A. 



positive in sign and progressively increases in value ; the heat of reaction 



p 

 accordingly still diminishes until, when RT log p* = C, it becomes zero. 



Up to this point the reaction has been exothermic that is to say, giving 

 out heat but from this point on it absorbs heat, the sign of H becoming 

 negative, and the reaction is said to be endothermic. 



If we take the reaction as running from B into A the same sequence 

 of events in the reverse order occurs. Here C is negative, and RT 



P P 



log p- becomes RT log p*, so that the equation runs 



*A *B 



H = -C-RTlog^ A . 



^B 



Starting with nearly all the substance in the Form B, P A is very 



p 

 small; RT log p A has hence a large negative value. Accordingly 



*a 



p 



- C - RT log P A has a positive value, and the reaction runs exother- 

 * 



p p 



mically until RT log ~ = - C, or RT log p- B = C, when the heat of 



*B *A 



reaction is zero, and from this on the reaction is endothermic. It is 

 hence evident that the reaction runs exothermically from either end 



p 

 down to the same point, where C = RT log -p 5 , and at this point where 



"-A 



H = 0, energy is neither given up to nor taken up from the surround- 



1 That is, the energy set free in the reaction, which need not necessarily 

 all be set free as heat energy; it is merel/ for simplicity that energy set free 

 in the reaction is spoken of in the text as heat of reaction. 



