Free Energy and Potential of Half-Reactions

Free Energy and Potential of Half-Reactions Our calculation has involved the overall redox reaction and thus far we have done nothing different from our previous thermodynamic treatment of other types of equilibria. For the sake of convenience it would be profitable to be able to examine any two half-reactions and judge from them independently, in which direction a reaction could proceed spontaneously if they were combined. 

One way to do this is to determine the AG° values for each half-reaction and then add them to determine the value of AG° for the complete redox reaction. Let us examine one of the steps of the Winkler method for determining dissolved oxygen (DO) as an example of this technique. The last step of the Winkler method is the titration of iodine (Igoq)) to iodide (I ) using thiosulfate, 8203 , which is itself oxidized to tetrathionate, 8406 . The reaction takes place in acid solution. The two half-reactions are 

In both of these equations we need to know the value of the free energy of formation of the electron AG .e-. Thinking one step ahead to the addition of Eqs. 7-2 and 7-3 to produce overall redox equation, we can see that AGf.e- will cancel out so that it does not really matter what value AG/,e-has. By convention, however, AG ,e- is assigned a value of zero and this assignment permits us to examine half-reactions independent of the overall reaction.  

As soon as we connect the platinum wires, the voltmeter will register a voltage reading showing (1) that a difference in potential exists between the two electrodes, and (2) that electrons are flowing in the external circuit between the two compartments, assuming that the reactions in question can take place on the platinum surface We have formed a galvanic cell 

The E° values for various half-cells, like the AG/ values for various species, cannot be determined absolutely. Like AG/ values they are measured with reference to a particular half-reaction that is assigned an E° value of zero at 25°C and I atm. This reaction is the reduction of hydrogen ion to hydrogen gas  

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