Electron flow reverse, thermodynamics

Thermodynamics and Reverse Electron Flow Thermogenic Tissues... 

Of course, since AG and AH are used in the definition (3.16), the theoretical efficiency of a fuel cell depends on the redox reaction on which it is built. In any case the theoretical efficiency, calculated from thermodynamic quantities, corresponds to an operative condition of infinitesimal electronic flow (by definition of reversible process), which practically means no current drawn from the converter. As it is shown in the following sections, also at open-circuit (no current through the external circuit) the voltage of real fuel cells is slightly lower than °, and the main problem of the electrochemical energy conversion is to obtain potentials in practical conditions (when current is drawn) as near as possible the open-circuit voltage, in order to maximize the real efficiency of the device. 

Control of Forward and Reverse Electron Flow in Mitochondria Thermodynamic versus Kinetic Considerations Robert A. Mitchell... 

With its reactant and product connections isolated, but equilibrium concentrations remaining in the cell, the cell voltage is a maximum, namely V , at open-circuit, zero-flow equilibrium (Table A.1). Equilibrium entails the presence of a thermodynamically reversible, symmetrical or balanced exchange current, described by Marcus (1964 1982) as being an equilibrium exchange between electrode electrons and vibrating... 

Figure 4-3. Electrochemical techniques and the redox-linked chemistries of an enzyme film on an electrode. Cyclic voltammetry provides an intuitive map of enzyme activities. A. The non-turnover signal at low scan rates (solid lines) provides thermodynamic information, while raising the scan rate leads to a peak separation (broken lines) the analysis of which gives the rate of interfacial electron exchange and additional information on how this is coupled to chemical reactions. B. Catalysis leads to a continual flow of electrons that amphfles the response and correlates activity with driving force under steady-state conditions here the catalytic current reports on the reduction of an enzyme substrate (sohd hne). Chronoamperometry ahows deconvolution of the potenhal and hme domains here an oxidoreductase is reversibly inactivated by apphcation of the most positive potential, an example is NiFe]-hydrogenase, and inhibition by agent X is shown to be essentially instantaneous.

Iron(IlI) in some solutions produces a well formed cathodic (reduction) wave, in which the iron(III) undergoes a thermodynamically reversible reduction to iron(II). Since the potential of this wave is determined by the energy difference between iron(ll) and (III), it could be expected that a solution of iron(II) in the same media would produce a well formed anodic (oxidation) wave at exactly the same potential. In both cases the potential of the wave would be determined by the same energy difference. In practice the two waves are separated by a very small difference in potential, (59/n mV where n is the number of electrons transferred, in this case 1). The current in one wave flows in the opposite direction to the other. If both iron(ll) and (III) are present in the same solution a single wave should develop with part cathodic and part anodic. The current direction will reverse part way up the wave. If the reversibility of the reaction is lost, through some addition to the solution, the cathodic and anodic waves will separate and move apart. 

When a metal, M, is immersed in a solution containing its ions, M, several reactions may occur. The metal atoms may lose electrons (oxidation reaction) to become metaUic ions, or the metal ions in solution may gain electrons (reduction reaction) to become soHd metal atoms. The equihbrium conditions across the metal-solution interface controls which reaction, if any, will take place. When the metal is immersed in the electrolyte, electrons wiU be transferred across the interface until the electrochemical potentials or chemical potentials (Gibbs ffee-energies) on both sides of the interface are balanced, that is, Absolution electrode Until thermodynamic equihbrium is reached. The charge transfer rate at the electrode-electrolyte interface depends on the electric field across the interface and on the chemical potential gradient. At equihbrium, the net current is zero and the rates of the oxidation and reduction reactions become equal. The potential when the electrode is at equilibrium is known as the reversible half-ceU potential or equihbrium potential, Ceq. The net equivalent current that flows across the interface per unit surface area when there is no external current source is known as the exchange current density, f. 

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