Standard cell potential, computation

Standard half-cell potentials can be used to compute standard cell potentials, standard Gibbs free energy changes, and equilibrium constants for oxidation-reduction reactions. 

Ve [aq) in a solution of V [aq) is to drop a piece of metallic iron into the storage container. Write the reaction that removes the Fe, and compute its standard cell potential. 

This equation allows one to compute the chemical equilibrium constant from measured standard-state electrochemical cell potentials (usually referred to as standard cell potentials). Some standard half-cell potentials are given in Table 14.6-1. The standard potential of an electrochemical cell is obtained by combining the two relevant half-cell potentials. 

Standard Potential (Em0 Volts) of Ag—AgCl Electrode (Cell I) Computed by the Curve-Fitting Method... 

It would seem that substitution of E and Q values would allow the computation of the standard redox potential E° for the couple, However, a problem arises because the calculation of Q requires not only knowledge of the concentrations of the species involved in the cell reaction but also of their activity coefficients. These coefficients are not usually available, so the calculation cannot be directly completed. However, at very low concentrations, the Debye-Hticke limiting law for the coefficients holds. The procedure then is to substitute the Debye-Hiickel law for the activity coefficients into the specific form of the Nernst equation for the cell under investigation and carefully examine the equation to determine what kind of plot to make of the E b ) data so that extrapolation of the plot to zero concentration, where the Debye-Hucke law is valid, gives a plot intercept that equals Es. See Section 7.8 for the details of this procedure and an example for which the relevant graph involves a plot of E + (2RT/F) In b against bl/2. 

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. 

Ab initio atomic simulations are computationally demanding present day computers and theoretical methods allow simulations at the quantum mechanical level of hundreds of atoms. Since an electrochemical cell contains an astronomical number of atoms, however, simplifications are essential. It is therefore obvious that it is necessary to study the half-cell reactions one by one. This, in turn, implies that a reference electrode with a known fixed potential is needed. For this purpose, a theoretical counterpart to the standard hydrogen electrode (SHE) has been established [Nprskov et al., 2004]. We will describe this model in some detail below. 

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