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Home » Chemistry Homework Help » Physical Chemistry » Electrochemical Cell
Electrochemical Cell
The electrical currents and reactions at the electrodes of an electrochemical cell depend exponentially on the overpotential.

The electrochemical cell provides a marvelous device for measuring the mechanical energy that can be obtained from, and thus the free energy change that accompanies, a reaction proceeding at a state of balance. At a state of balance forward and reverse reactions do occur, but they do so to equal extents. A very reasonable, if easily overlooked, question is: to what extent do each of these occur when a particular cell is at a state of balance? At one electrode a reaction supplies electrons to the electrode and at the other electrode a reaction removes electrons from the electrode. At equilibrium both reactions occur. The zero net current results from the equal electron removing anodic and electron supplying cathodic reactions.

Nothing in what was done it suggests the magnitude of these two opposing currents. These magnitudes depend on the rates with which the two opposing electrode reactions occur. Here we sample the topic called electrode kinetics.

In dealing with the rates of the opposing anodic and cathodic reactions, we are, of course, not restricted to the special reversible situation in which the rates of these reactions are equal. The rates can be made unequal by raising or lowering the applied voltage. Then either the anodic or the cathodic reaction occurs more rapidly than under reversible conditions. The study of the rates of electrode reactions begins with an analysis of the effect of the applied potential on the net current.

So far we have focused on the electrical potential difference which, when applied to the electrodes of an electrochemical cell, puts everything involved on the cell reaction at a state of balance. Now we want to deal with the role of the individual chemical species and to do so even when this state of balance is not in effect.

The potential of a molecule or an ion in the electrochemical system is defined as the energy required to create that species at the site it occupies.

For ordinary chemical reactions, this potential can be taken to be the partial molal energy. Now it is better to referred a to as the chemical kinetics, a term of ten used for the partial molal energy that is species can deliver to the mechanical surroundings, or in terms of the energy that would be required to create the species.

In electrochemical systems, the potential of any charged particle includes a term for the electrical potential. This is the potential that is treated in electrostatics when a test charge is imagined to  be in the neighborhood of another charge or charge system.

At equilibrium in an electrochemical cell, the total potential of any species is the same at all parts of the system. For example, in a cell, in which the reaction 2H+ + 2e- ⇌ H2 occurs, the potential of H+ on a platinum electrode is equal to that of the H+ ions in the solution. If the potential of the electrode is changed, as by changing the electrical potential applied to the cell, the potential of H+ on the electrode will be different from that of H+ ions in the solution. The tendency will be for the electrode reaction to proceed more in one direction than in the other, as the H+ ions move from a higher to a lower potential.

The current produced by the potential difference between an electrode and the solution in which it is immersed and the current produced by this potential difference can be measured by means of the electrode arrangement of the potential difference between the solution of the electrode under study and the working electrode is given by the potentiometer reading in the auxiliary circuit. The net current, which depends on the rate of reaction at the electrodes of the principal cell, is shown by the ammeter reading. The potential between the solution of the electrode under study and the working electrode is given by the potentiometer reading in the auxiliary circuit. When the principal circuit is adjusted to its reversible, equilibrium state, the auxiliary circuit will indicate some electrode potential difference ∆ Ørev. If the voltage applied to the principal circuit is changed so that a net current flows, the electrode potential shown by the auxiliary circuit will change to some value indicated as It is convenient to introduce the term overpotential n, defined for an electrode as: ∆ Ø. It is convenient to introduce the term overpotential η, defined for an electrode as:

Ή = ∆ Ø - ∆ Ørev

The net current that flows in the principle circuit can be interpreted as:

I = ic - ia Where subscripts c and a stand for cathodic and anodic, respectively.

Furthermore, the overpotential is said to be positive if it is such as to produce a positive I, that is, if it drives the cathodic current and suppresses the anodic current.

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