Chapter 20: Electrochemistry
20.1 Oxidation-Reduction Reactions
Oxidation-reduction reactions (or redox reactions) involve the transfer of electrons from one species to
another. In other words, the oxidation state of one or more substances in the reaction changes. The
transfer of electrons inherent in an oxidation-reduction process can be used to produce energy in the
form of electricity. Electrochemistry is the study of the relationships between chemical reactions and
electrical energy.
We identify a reaction as oxidation-reduction by comparing the oxidation numbers of atoms in the
reactants and products. If the oxidation numbers change, the reaction is an oxidation-reduction.
Zn(s) + 2H+(aq)  Zn2+(aq) + H2(g)
In this reaction zinc metal loses electrons to become a cation, and thus zinc has been oxidized. Hydrogen
gains electrons to become hydrogen gas. Hydrogen has been reduced.
20.2 Balancing Oxidation-Reduction Equations
When balancing equations previously, we have been concerned only with conservation of mass—the
amount of each element must be the same on both sides of the equation. In balancing oxidationreduction reactions, however, we must also be concerned with balancing the number of electrons
transferred. In the equation
Balancing the mass automatically balances the total charge on each side. In many reactions, though,
balancing the mass does not result in balancing charge. In the next equation the law of conservation of
mass appears to have been obeyed. However, note that the total charge on the left side is +3, while the
total charge on the right side is only +2.
The two processes, oxidation of manganese metal and reduction of chromium(III) ion, do not
correspond to the transfer of the same number of electrons. We must find a way to balance the charges
without disrupting the mass balance. In this case we can balance the equation by inspection. If we
multiply the manganese on each side by 3 and the chromium on each side by 2, we get
In the balanced equation the total charge (on both the left and right sides) is +6.
20.3 Voltaic Cells
The electrical energy produced by a spontaneous oxidation-reduction reaction can be "harnessed" using
a voltaic cell (also called a galvanic cell). In a voltaic cell the two half-reactions are made to occur in
separate compartments (half-cells).
The electrode at which oxidation occurs is called the anode. The electrode at which reduction takes
place is called the cathode.
20.4 Cell EMF
Electrons spontaneously flow from one species to another—and through the wire from the anode to the
cathode of a voltaic cell—because of a difference in potential energy, or a potential difference. The
potential difference between two electrodes is measured in volts. One volt is equal to one joule per
coulomb.
The potential difference that drives electrons through the wire in a voltaic cell is called
the electromotive force or emf. For a voltaic cell the emf is denoted Ecell and referred to as the cell
potential.