Exercise.
For practice, balance the following reactions by both the oxidation-number and half-reaction methods:






Although half-reactions have been introduced here merely as a means of obtaining a balanced overall reaction, they can have physical meaning of their own. If the iron oxidation and manganese reduction can be carried out in separate containers, and if these containers can be given suitable electrical connections, then we can make use of the energy released by this reaction as the electrons flow through a wire from the iron container to the manganese. This is the principle of the electrolytic cell or battery. An ordinary flashlight battery uses manganese reduction, and oxidation of the zinc battery casing. We will return to electrolytic cells and energy production in Chapter 17.