Cells, electrochemical Gibbs function

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. 

Most of the methods we have described so far give the activity of the solvent. Often the activity of the solute is of equal or greater importance. This is especially true of electrolyte solutions where the activity of the ionic solute is of primary interest, and in Chapter 9, we will describe methods that employ electrochemical cells to obtain ionic activities directly. We will conclude this chapter with a discussion of methods based on the Gibbs-Duhem equation that allow one to calculate activities of one component if the activities of the other are known as a function of composition. 

If thermodynamic variables are temperature and pressure, the main characteristic for understanding whether the system (process, reaction) is in equilibrium or not is Gibbs energy (AG). Electrochemical cells commonly work at well-defined temperature and pressure, and therefore, AG is the thermodynamic function showing how far the electrochemical cell is from equilibrium. When an electrochemical cell is at equilibrium, AG is simply zero. Two other important thermodynamic functions to understand an electrochemical cell behavior are enthalpy (AH) and entropy (AS), which are connected to AG via a weU-known fundamental equation ... 

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