Cathodic-reactant half-cell potential

Describe the role of non-fVwork in electrochemical systems. Define the roles of the anode, cathode, and electrolyte in an electrochemical cell. Given shorthand notation for an electrochemical cell, identify the oxidation and reduction reactions. Use data for the standard half-cell potential for reduction reactions, E°, to calculate the standard potential of reaction E. Apply the Nernst equation to determine the potential in an electrochemical cell given a reaction and reactant concentrations. 

Electrode reactions are inner-sphere reactions because they involve adsorption on electrode surfaces. The electrode can act as an electron source (cathode) or an electron sink (anode). A complete electrochemical cell consists of two electrode reactions. Reactants are oxidized at the anode and reduced at the cathode. Each individual reaction is called a half cell reaction. The driving force for electron transfer across an electrochemical cell is the Gibbs free energy difference between the two half cell reactions. The Gibbs free energy difference is defined below in terms of electrode potential,... 

Under open circuit conditions, the PEVD system is in equilibrium after an initial charging process. The equilibrium potential profiles inside the solid electrolyte (E) and product (D) are schematically shown in Eigure 4. Because neither ionic nor electronic current flows in any part of the PEVD system, the electrochemical potential of the ionic species (A ) must be constant across both the solid electrolyte (E) and deposit (D). It is equal in both solid phases, according to Eqn. 11, at location (II). The chemical potential of solid-state transported species (A) is fixed at (I) by the equilibrium of the anodic half cell reaction Eqn. 6 and at (III) by the cathodic half cell reaction Eqn. 8. Since (D) is a mixed conductor with non-negligible electroific conductivity, the electrochemical potential of an electron (which is related to the Eermi level, Ep) should be constant in (D) at the equilibrium condition. The transport of reactant... 

Plan When an aqueous solution of an ionic compound is electrolyzed, the possible reactants are H2O and the ions of the solute (in this case Ag" " and F ). Because the products are Ag and O2, the reactants must be Ag" " and H2O. Writing the half-reactions for these processes reveals which is oxidation and which is reduction and therefore which occurs at the anode and which at the cathode. The minimum emf is found by using Equation 20.8 to calculate the standard cell potential. 

According to mixed-potential theory, any overall electrochemical reaction can be algebraically divided into half-cell oxidation and reduction reactions, and there can be no net electrical charge accumulation [J7], For open-circuit corrosion in the absence of an applied potential, the oxidation of the metal and the reduction of some species in solution occur simultaneously at the metal/electrolyte interface, as described by Eq 14, Under these circumstances, the net measurable current density, t pp, is zero. However, a finite rate of corrosion defined by t con. occurs at anodic sites on the metal surface, as indicated in Fig. 1. When the corrosion potential, Eco ., is located at a potential that is distincdy different from the reversible electrode potentials (E dox) of either the corroding metal or the species in solution that is cathodically reduced, the oxidation of cathodic reactants or the reduction of any metallic ions in solution becomes negligible. Because the magnitude of at E is the quantity of interest in the corroding system, this parameter must be determined independendy of the oxidation reaction rates of other adsorbed or dissolved reactants. 

The ideal cell potential between the two half-reactions for H2/O2 is 1.23 V at 25 °C. The actual cell voltage will be lower due to losses in the cell. As the current is increased from the open circuit condition, catalytic activation losses (dominant at the cathode) will initially reduce the voltage to 1V. Ohmic losses due to ionic resistance through the membrane and electrical resistance in the assembly (to a lesser extent) will further reduce the voltage in a linear fashion with current. The most common polymer electrolyte is Nation (trademark of DuPont), which is a perfluorinated ionomer that must be humid to maintain good ionic conductivity. A limiting current will then be achieved, beyond which the mass transport of reactants (typically oxygen at the cathode) becomes insuffi-... 

Since cell potential depends not only on the half-reactions occurring in the cell, but also on the concentrations of the reactants and products in those half-reactions, we can construct a voltaic cell in which both half-reactions are the same, but in which a difference in concentration drives the current flow. For example, consider the electrochemical cell shown in Figure 18.12 , in which copper is oxidized at the anode and copper ions are reduced at the cathode. The seeond part of Figure 18.12 depicts this cell under nonstandard conditions, with [Cu ] = 2.0 M in one half-cell and [Cu ] = 0.010 M in the other ... 

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