ELECTROCHEMISTRY AND RED-OX REACTIONS
One category of chemical reactions described briefly in Section 1.9.2 is oxidation-reduction reactions, or “red-ox” reactions for short. Oxidation- reduction (“red-ox”) reactions are reactions in which one substance is oxidized while another is simultaneously reduced. The processes of oxidation and reduction can be defined as follows. Oxidation is the loss of electrons, while reduction is the gain of electrons. However, diagnostically speaking, a substance is oxidized or reduced, respectively, if any one of the following conditions is met:
- • Loss of electrons
- • Gain of oxygen atoms
- • Loss of hydrogen atoms
- • Gain of electrons
- • Loss of oxygen atoms
- • Gain of hydrogen atoms
Consider the arrangement shown in Figure 2.2. This arrangement is known as an electrochemical cell, and the underlying electrochemical reaction involves simultaneous oxidation and reduction reactions. Electrochemical cells are the basis of operation of all batteries. If the reactions and current flow are spontaneous, as in any battery, the cell is called a voltaic or galvanic cell. If, instead, electrical energy must be supplied, the cell is called an electrolytic cell, and the process is known as electrolysis.
An electrochemical cell is comprised of two half-cells. In one, called the anode, oxidation occurs; in the other, called the cathode, reduction occurs. Each reaction is called a half-reaction. The sum of the two halfreactions is the overall or net reaction. The rusting of iron or the corrosion of any metal involves an oxidation-reduction process, with a net electromotive potential.
Some useful physical measurements and their units in electrochemical cells include the volt (V), the electron volt (eV), the coulomb (C), and the ampere (A). Since 1 volt = 1 joule per coulomb, and 1 ampere = 1 coulomb per second, then (current in amperes) x (time in seconds) = total charge transferred in coulombs.
Figure 2.2. A copper-zinc electrochemical galvanic cell
In addition, (potential in volts) x (charge in coulombs) = total energy produced or used in joules. See equations 2.43 through 2.49 for a summary of useful quantitative relationships for electrochemical cells.