Galvanic cell

Galvanic cell

Potential difference between a piece of metal and the solution in which it is immersed. We can, measure the difference between the potentials of two electrodes that dip into the same solution, or more usefully, are in two different solutions. In the latter case, each electrode-solution pair constitutes an oxidation-reduction half cell, and we are measuring the sum of the two half-cell potentials

.

This arrangement is called a galvanic cell. A typical cell might consist of two pieces of metal, one zinc and the other copper; each immersed each in a solution containing a dissolved salt of the corresponding metal. The two solutions are separated by a porous barrier that prevents them from rapidly mixing but allows ions to diffuse through.
If we connect the zinc and copper by means of a metallic conductor, the excess electrons that remain when Zn²⁺ ions emerge from the zinc in the left cell would be able to flow through the external circuit and into the right electrode, where they could be delivered to the Cu²⁺ ions which become "discharged", that is, converted into Cu atoms at the surface of the copper electrode. The net reaction is the oxidation of zinc by copper(II) ions:

Zn(s) + Cu²⁺ → Zn²⁺ + Cu(s)

but this time, the oxidation and reduction steps (half reactions) take place in separate locations:

Oxidation State:   Zn(s) → Zn²⁺ + 2e^{–    (left} electrode)

Reduction State:  Cu²⁺ + 2e^–→ Cu(s)   (right electrode)

The reaction can be started and stopped by connecting or disconnecting the two electrodes. If we place a variable resistance in the circuit, we can even control the rate of the net cell reaction by simply turning a knob. By connecting a battery or other source of current to the two electrodes, we can force the reaction to proceed in its non-spontaneous or reverse direction.
By placing an ammeter in the external circuit, we can measure the amount of electric charge that passes through the electrodes, and thus the number of moles of reactants that get transformed into products in the cell reaction.
Electric charge q is measured in coulombs. The amount of charge carried by one mole of electrons is known as the faraday, which we denote by F. Careful experiments have determined that 1 F = 96467 c. For most purposes, you can simply use 96,500 coulombs as the value of the faraday.
When we measure electric current, we are measuring the rate at which electric charge is transported through the circuit. A current of one ampere corresponds to the flow of one coulomb per second.

The need for this can be understood by considering what would happen if the two solutions were physically separated. Positive charge (in the form of Zn²⁺) is added to the electrolyte in the left compartment, and removed (as Cu²⁺) from the right side, causing the solution in contact with the zinc to acquire a net positive charge, while a net negative charge would build up in the solution on the copper side of the cell. These violations of electro neutrality would make it more difficult (require more work) to introduce additional Zn²⁺ ions into the positively-charged electrolyte or for electrons to flow into the right compartment where they are needed to reduce the Cu²⁺ ions, thus effectively stopping the reaction after only a chemically insignificant amount has taken place.
In order to sustain the cell reaction, the charge carried by the electrons through the external circuit must be accompanied by a compensating transport of ions between the two cells. This means that we must provide a path for ions to move directly from one cell to the other. This ionic transport involves not only the electro active species Cu²⁺ and Zn²⁺, but also the counter ions, which in above example are nitrate, NO₃^–.
Thus the positive charge resulting from the release of Zn²⁺ in the left compartment could be compensated by the drift of these ions into the right side, or equally well by diffusion of nitrate ions from the right-hand cell to the left.
More detailed studies reveal that both processes occur, and that the relative amounts of charge carried through the solution by positive and negative ions depends on their relative mobility’s, which express the velocity with which the ions are able to make their way through the solution. Since negative ions tend to be larger than positive ions, the latter tend to have higher mobility’s and carry the larger fraction of charge. In the diagram below, sodium and sulfate ions serve the same function. (Neither of these ions is able to undergo oxidation or reduction in the presence of water.)