THE DRIVE TOWARD EQUILIBRIUM 



175 



THE DRIVE TOWARD EQUILIBRIUM 



The Driving Force 



It is a familiar fact that if two mechanical forces of difFerent magnitude 

 oppose each other at a point, the resulting movement will be in the direction 

 of the larger force. Similarly, it seems almost axiomatic that if two systems 

 of different free energy. F, are made to oppose each other, provided they are 

 able to interact, the interaction will proceed in the direction of the larger. 

 For chemical reactions, if the free energies of formation for reactants and 

 products are known, then the free energy of reaction. AF, is simply the dif- 

 ference between the two. This value, AF, represents the maximum amount 

 of work available from the reaction of 1 mole of reactant into product. 

 Since AF = F finaj - /'„„,,,,, a negative value of AF means that the reaction 

 will proceed spontaneously from reactants to products. Such a reaction is 

 said to be exergonic. If (see Figure 7-3) AF is positive, free energy must be 

 supplied from the outside — another reaction perhaps — before reactants will 

 go into products; the reaction is said to be endergonic. The analogy with 

 exothermic (negative AH) and endothermic (positive AH), introduced 

 earlier, is obvious. 



State 



Figure 7-3. Free Energy of Initial and Final 

 States. For exergonic (free energy-producing) 

 processes, AF (= F fin — F in ) is negative; for 

 endergonic (free energy-consuming) processes, 

 AF is positive. 



The energy-producing reactions in the living system are numerous. Nearly 

 all the primary sources are the combustion of food products. By suitable 

 carriers the free energy required by the endergonic syntheses of anabolism 

 is trapped and carried through the blood stream to the locations at which 

 the synthetic processes take place. 



Naturally, free energy is not a driving force, although it is often considered 

 as such. Nor is the partial molal free energy, (dF/dn) T<P , often called 



