Chemical standard potential

Dimensionless heterogeneous rate constant Chemical potential Standard chemical potential Sweep rate in CV or LSV (Vs )... 

Figure 4.5 shows the change in the standard chemical potential (=standard molar Gibbs energy, Dtr uc) for the transfer of hydrocarbons from their own medium to water. This change may be established by studying the equilibrium between pure hydrocarbon (HC) and hydrocarbon dissolved in water (mole fraction HC in water, Xgc). At equilibrium, and for an ideal solution of HC in water. 

In thermodynamic terms the equilibrium constant is related to the standard chemical potential by the equation... 

To proceed fiirther, to evaluate the standard free energy AG , we need infonnation (experimental or theoretical) about the particular reaction. One source of infonnation is the equilibrium constant for a chemical reaction involving gases. Previous sections have shown how the chemical potential for a species in a gaseous mixture or in a dilute solution (and the corresponding activities) can be defined and measured. Thus, if one can detennine (by some kind of analysis)... 

For a free energy of fonnation, the preferred standard state of the element should be the thennodynamically stable (lowest chemical potential) fonn of it e.g. at room temperature, graphite for carbon, the orthorhombic crystal for sulfiir. 

The standard chemical potentials are approximately tire same if tire surfactant in each aggregate sees nearly tire same interaction witli tire solvent. This simplifying assumption tlien gives... 

The chemical potential p, of the adsorbate may be defined, following standard practice, in terms of the Gibbs free energy, the Helmholtz energy, or the internal energy (C/,). Adopting the last of these, we may write... 

The chemical potential in the dyebath solution, is defined in equation 1 where is the standard chemical potential in the solution, R is the gas constant, Tis temperature in K, and a is activity. 

However, the chemical potential is given by Eq. (4-341) for gas-phase reactions and standard states as the pure ideal gases at T°, this equation becomes... 

The term 7 7 ln p/p° is clearly the chemical potential of a surface of radius r with respect to a flat surface of the same material as standard state. It follows that the difference in chemical potential between two surfaces, where... 

For a substance in a given system the chemical potential gi has a definite value however, the standard potentials and activity coefficients have different values in these three equations. Therefore, the selection of a concentration scale in effect determines the standard state. 

The standard electrode potentials , or the standard chemical potentials /X , may be used to calculate the free energy decrease —AG and the equilibrium constant /T of a corrosion reaction (see Appendix 20.2). Any corrosion reaction in aqueous solution must involve oxidation of the metal and reduction of a species in solution (an electron acceptor) with consequent electron transfer between the two reactants. Thus the corrosion of zinc ( In +zzn = —0-76 V) in a reducing acid of pH = 4 (a = 10 ) may be represented by the reaction ... 

As with all determinations of thermodynamic stability, we comihehce by defining all stable phases possible, and their standard, chemical, potentials. For inost, metals there are many such phases, including oxides, hydroxides and dissolved ions. For brevity, here, only the minimum number of phases is Considered. The siriiplest system is a metal, ilf, which can oxidise lo form a stable dissolved pro,duct, (qorrpsipn), or to form a stable oxide MO (passivation), lit aqueous environments thfbe equilibria Can thereby be... 

The standard chemical potentials of the elements in the form that they are normally stable at the temperature and pressure under consideration are given an arbitrary zero... 

In the case of ions in solution, and of gases, the chemical potential will depend upon concentration and pressure, respectively. For ions in solution the standard chemical potential of the hydrogen ion, at the temperature and pressure under consideration, is given an arbitrary value of zero at a specified concentration... 

Standard chemical potentials for ions and compounds are given in Chapter 21, Table 21.5. 

Thus the equilibrium constant K can be evaluated from standard electrode potential or from the standard chemical potentials x . 

For ions in solution the standard reference state is the hydrogen ion whose standard chemical potential at = 1 is given an arbitrary value of zero. Similarly for pure hydrogen at Phj = = 0- Thus for the... 

Example 2 Finely divided gold is agitated with an oxygen-saturated alkali-cyanide solution of pH = 12 containing 10 moldm of CN . Calculate the concentration of Au(CN)2 ions in solution at 25°C when the reaction is at equilibrium. The standard chemical potentials of the species involved (in kJ) are CN, 165 HjO, —237 Au(CN)f, 244 OH", —157. Assume that fl jo = 1, and take 2-303RT = 5710. 

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