16 



OXIDATION-REDUCTION POTENTIALS 



As given above (expression 45) the change in entropy equals the difference between 

 the total energy of the reaction and the change in free energy. 



The importance of the expressions (46) and (47) is that we are now able to 

 calculate the change in free energy of a reaction from the equilibrium constant and 

 also to calculate the standard electrode potential of the system from the change in 

 free energy. It is important in this connection to note the units in which the various 

 quantities must be measured in order to get the calculations to agree. The free energy 



TABLE 1 



STANDARD OXIDATION-REDUCTION POTENTIALS 



Referred to normal hydrogen electrode at 25°C (oxidising S3^stems more positive) 



is generally calculated in calories and the electrode potential in volts ; the gas 

 constant E is then 1-987 calories per degree per mole and F is 23,070 calories per volt. 

 It should be noted that in the general electrode equation (10) R was given as 8-3 

 joules and F as 96,500 coulombs, but the value of the expression RT is not thereby 

 affected. F 



As already mentioned the potential at a single electrode cannot be measured, but 

 this has to be combined with another half cell to complete the electrical cell. Also 



