Electrochemical cell electric potential difference generated

So far we have considered only standard cell potentials, that is, the electric potential difference developed by a chemical reaction that is at equilibrium in an electrochemical cell at normal atmospheric pressure and a temperature of 25 C, and when the chemical species are present in standard concentrations. We can derive an expression for the electric potential difference generated under nonequilibrium and nonstandard conditions (Fcdi) follows. If we write Eq. (2.41) in terms of concentrations and remove the requirement of molar concentrations, we get... 

Exercise 6.9. Calculate the initial electric potential difference generated in an electrochemical cell at 298K by the redox reaction... 

To visualize how electrochemical cells generate electrical potential differences, consider a zinc electrode dipped into a solution of zinc sulfate. From the macroscopic perspective, nothing happens. At the molecular level, however, some of the zinc atoms of the electrode are oxidized to ions ... 

If the ammeter in Figure 6.1 were replaced by a voltmeter, we could measure the electric potential difference (in volts, indicated by V) between the two electrodes of an electrochemical cell. Experiments show that for any two metal electrodes (e.g., Cu and Ag), this potential difference depends on the relative concentrations of Cu (aq) and Ag" (aq) in the two solutions, as well as temperature, pressure, etc. However, if the temperature is kept at 25°C, the pressure is constant at 1 atm, and the concentrations of the two aqueous ions are kept equal (say at 1 M), then, provided not too much current is drawn, any two metal electrodes generate a steady potential difference the magnitude of which depends on the nature of the electrodes (e.g., 0.46 V when the electrodes are Cu and Ag). 

Exercise 6.6. An electrochemical cell has electrodes made of zinc and copper and operates under standard conditions. Which electrode is the anode and which the cathode Which way will electrons flow in the external (wire) portion of the circuit What is the maximum electric potential difference that this cell can generate Will Zn(s) spontaneously reduce Cu (aq), or will Cu(s) spontaneously reduce Zn (aq) ... 

Section 2.4 that when a chemical system is at equilibrium AG=0, Hence, when the species in an electrochemical cell are in equilibrium, the Gibbs free energy is a minimum and the cell generates zero electric potential difference (i.e., the battery has run down ). 

The electrochemical cell with zinc and copper electrodes had an overall potential difference that was positive (+1.10 volts), so the spontaneous chemical reactions produced an electric current. Such a cell is called a voltaic cell. In contrast, electrolytic cells use an externally generated electrical current to produce a chemical reaction that would not otherwise take place. 

An electrochemical cell generates a potential difference E. (The symbol E, commonly used in electrochemistry, refers to electromotive force, an archaic term for potential difference.) The electrical work done when n moles of electrons is passed by the cell can be found using Eq. (15-1), w = -nFE. It can be shown that the electrical work done by an electrochemical cell, at constant temperature and pressure, is equal to the change in Gibbs free energy of the cell components,... 

Recall from Chapter 16 that an object s potential energy is due to its position or composition. In electrochemistry, electrical potential energy is a measure of the amount of current that can be generated from a voltaic cell to do work. Electric charge can flow between two points only when a difference in electrical potential energy exists between the two points. In an electrochemical cell, these two points are the two electrodes. The potential difference of a voltaic cell is an indication of the energy that is available to move electrons from the anode to the cathode. 

Sah [1970] introduced the use of networks of electrical elements of infinitesimal size to describe charge carrier motion and generation/recombination in semiconductors. Barker [1975] noted that the Nemst-Planck-Poisson equation system for an unsupported binary electrolyte could be represented by a three-rail transmission line (Figure 2.2.8fl), in which a central conductor with a fixed capacitive reactance per unit length is connected by shunt capacitances to two resistive rails representing the individual ion conductivities. Electrical potentials measured between points on the central rail correspond to electrostatic potential differences between the corresponding points in the cell while potentials computed for the resistive rails correspond to differences in electrochemical potential. This idea was further developed by Brumleve and Buck [1978], and by Franceschetti [1994] who noted that nothing in principle prevents extension of the model to two or three dimensional systems. 

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