CHEMICAL AFFINITY 



13 



If the system be at equilibrium there will be no tendency for any change. In 

 this case let the ratio (37) at equilibrium be W-^^ = k (system I, fig. 2). 



Now let some other system be considered, not at equilibrium, in which the ratio 



V eei ^^ greater than k (system II). In this case there will be a tendency for 

 [re J 



I;[Fer1[Fer] 



II;LFe, jLFe, J 



Fig. 2 

 Ferrous -ferric ion system 



Pe®®© iq change to Fe®®. Since this process would result in electronic equiUbrium 

 being disturbed the reaction will not proceed but the tendency will exist. Let now 

 platinum electrodes be placed in the two systems and these electrodes be joined and 

 the two systems be brought into contact, e.g., by a semi-permeable membrane, to 

 complete the circuit. The unattackable electrodes will not participate in the systems, 

 but will act as inert conductors of electrons. The tendency of Fe®®® to be reduced 

 to Fe®® will now become an actuality in system II, and this reduction in system II 

 will be accompanied by oxidation in system I, so that electrons will pass through the 

 electrodes from systems I to II ; this electronic migration will produce an electro- 

 motive force, say E, between the electrodes. 



FTTe ®®®1 

 The left-hand half-cell (I) is at equilibrium, - ^^-^ ^ « -" 



so 



that 



] 



right-hand half-cell is not at equilibrium and 



[Fe^®®®] 

 [Fei®® ] 



[Feo^ 

 is greater than k. 



the 



The 



volumes in the two half-cells are considered to be so large that there will be no 

 appreciable change in concentrations during the experiment. When the electrodes 

 are joined, current will flow. Now let it be supposed that the current flows long 

 enough for one mol of Fe®®® in the right-hand half-cell to be reduced to Fe®®. 

 At the same time one mol of Fe®® will be oxidised to Fe®®® in the left-hand 

 half-cell. Thus one mol of Fe®®® will disappear from II and appear in I. The 

 work done will be equivalent to the isothermal and reversible transportation of 

 one mol of Fe®®® from concentration [Fci®®®] to concentration [Fco®®®] through 

 the semi-permeable membrane. The " osmotic " work done in this transportation, 

 is : — 



[Fei®®®] . rj^e®®®! 



ET 



/ 



[Fco®®®] 



[jTe®®©] 



d[Fe®®®] = RT In 



[Fco®®®] 



