IN LIVING MATTER 23 



/. C = H+RTlog| B 



r 'A 



/. H = C - RT log |X 



"A 



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 

 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 



p 

 log ^ has a large negative value, and this being prefixed by a 



* A 



negative sign, it follows that the heat of reaction l at this stage has 



a large positive value. As the reaction proceeds P B continuously 



p 

 increases and log =p decreases in negative value, accordingly the heat 



^A 

 of reaction diminishes. As P B still increases and P A diminishes, 



p 

 log ^ becomes positive in sign and progressively increases in value, the 



CA 



p 



heat of reaction accordingly still diminishes until when RT log =? = 0, 



^A 



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, 

 p p 



and RT log ^ becomes RT log ^, so that the equation runs 



*A " B 



H = - C - RT log |*. 



*B 



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



p 

 small, RT log p^ has hence a large negative value. Accordingly 



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



p p 



mically until RT log ^= - C, or RT log ^ = C, when the heat of 



__ _ *JB _ "A _ 



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

 set free as heat energy ; it is merely for simplicity that energy set free in the 

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



