FREE ENERGY CHANGES 



15 



terms introduced by Coryell (1940) to describe reactions involving loss or gain of free 

 energy. When aF is zero the system is in equihbrium and no reaction occurs ; 

 when aF is positive the reaction cannot occur unless external energy is supphed ; 

 but if aF is negative a spontaneous reaction is possible on thermodynamic grounds. 

 Actually, of course, the liberation of free energy is not the only criterion of whether a 

 reaction can occur under any given conditions since the molecules concerned must be 

 in a reactive state or made so by the presence of a catalyst or enzyme. 





A 4 



L. 



c 



UJ 

 0/ 



Fig. 3 

 The function of a catalyst or enzyme 



In the diagram although the reaction A->B would result in the liberation of free 

 energy the reaction does not occur spontaneously because of the hump, but if a suitable 

 catalyst is present it will supply a little free energy (X — A) to a small quantity of A so 

 that it will reach the peak. Having reached the peak there will be spontaneous 

 conversion to B. In sliding down, the free energy liberated will be X — A and A — B. 

 The free energy X— A will be given back to the catalyst which will transport more of 

 A over the hump and so on. The net result will be that the catalyst will have given 

 up X — A of free energy and received back X — A so that it will be in the same state 

 finally as at the beginning. During the reaction A-»B the free energy liberated in the 

 main reaction will be A — B regardless of the humps negotiated with the help of 

 catalyst or enzyme. 



To repeat therefore, the important feature of a chemical reaction is not the total 

 heat change but the change in free energy. It is possible to calculate the free energy 

 change (aF) from the equilibrium constant (K) of the reaction : — 



(46) aF = — RT In K 



(R being the gas constant, and T the absolute temperature). 



When the reactants form part of a galvanic cell the change in free energy is 

 equal to the electrical work done ; so that : — 



(47) — aF = n F E 



where n = number of electrons involved in the reaction ; 

 F = the Faraday ; 

 E = the electrode potential. 



Incidentally, the temperature coefficient of the electrode potential gives a 

 measure of the change in entropy of the reaction. 



(48) 



AS 



nF — 

 dT 



