Cell potentials Gibbs function from

Dickson et al. [5], calculated the Gibbs function for the ionization of the bisulfate ion by measurement of cell potentials in the temperature range from 50° to 250°C. They found that the Gibbs function could be represented by the equation... 

The method of determination from measurements of cell potentials depends on the possibility of carrying out a transformation reversibly in an electrical cell. (See Fig. 7.2.) In this case, the spontaneous tendency of the transformation wUl be opposed by an opposing potential just sufficient to balance the potential in the electrical ceU produced by that spontaneous tendency. The potential observed under such circumstances is related to the change of the Gibbs function for the reaction by Equation (7.84)... 

Each reactant and product appears in the Nemst equation raised to its stoichiometric power. Thermodynamic data for cell potentials have been compiled and graphed (3) as a function of pH. Such graphs are known as Pourbaix diagrams, and are valuable for the study of corrosion, electrodeposition, and other phenomena in aqueous solutions.From the above thermodynamic analysis, the cell potential can be related to the Gibbs energy change... 

Since Ecell is defined to be positive for a spontaneous reaction, this equation correctly expresses a decrease in Gibbs function, which is the thermodynamic criterion for a spontaneous reaction at constant T and P. It is evident that if AGreact can be calculated from AG j- data, the potential of a cell arranged for reversible operation can be determined conversely, experimental measurements of Ecen permit calculation of AGreact. Both types of calculations are useful in electrochemical work and, thus, in the analysis of corrosion. 

Standard Gibbs energy of formation of NiO was found to be the following function of the temperature AfG°(7) = (-233.651 + 0.085 (77K)) kJ-mol". The accuracy calculated from the maximum deviation from the computed least-squares line is equal to 0.209 kJ mol . This corresponds to the maximum deviation in cell potential (emt) of 1.0 mV. 

It can be seen from these relations that the reversible potential is dependent on temperature and pressure since the Gibbs free energy is a function of temperature and the activity coefficients are dependent on temperature, pressme for gases and ionic strengths for ionic electrolytes. The Nemst equation (1.7) is used to derive a formula for calculating the reversible cell potential as follows Anode electrode potential ... 

The relation between the standard cell potential and the standard reaction Gibbs energy is a convenient route for the calculation of the standard potential of a couple from two other standard potentials. We make use of the fact that G is a state function and that the Gibbs energy of an overall reaction is the sum of the Gibbs energies of the reactions into which it can be divided. In general, we cannot combine the E values directly because they depend on the value of v, which may be different for the two couples. 

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