Any reaction where electrons are transferred between reactants is called an reduction-oxidation reaction or redox reaction. These are reactions where one substance wants an electron so badly that it takes it away from another substance. Whether or not it succeeds depends on who it meets. For example, consider the reaction
FeCl2(aq) + CeCl4(aq) → FeCl3(aq) + CeCl3(aq)
What happened here? If we remove the spectator ions and write the net ionic equation we find:
Fe2+(aq) + Ce4+(aq) → Fe3+(aq) + Ce3+(aq)
Ce4+ took an electron from Fe2+! This is an oxidation/reduction reaction. In this example, Fe2+ is oxidized and Ce4+ is reduced. The charge of Fe went from +2 to +3, that is, it lost an electron. This process is called oxidation.
Oxidation: The loss of an electron by a substance.
Likewise, the charge of Ce went from +4 to +3, that is, it gained an electron. This process is called reduction.
Reduction: The gain of an electron by a substance.
Oxidation/reduction reactions are important because we can exploit them as a way of generating electrical current. For example, we know that Ce4+ will pull an electron away from Fe2+ when we mix the two in solution. The trick to making a battery is to find a way to make Ce4+ pull an electron from Fe2+ when they are not mixed together in a single solution.
In order to do this we set up two 1/2 reactions in separate beakers and connect them with a salt bridge. The salt bridge electrically connects the two beakers, but prevents Fe2+ and Ce4+ from mixing.
You need two 1/2 reactions to make a reaction, so, we add them together.
|Fe2+ → Fe3+ + e-|
|Ce4+ + e- → Ce3+|
|Fe2+ + Ce4+ →Fe3+ + Ce3+|
Notice that electrons on both sides of the half-reactions must cancel each other out when added together.
What about other atoms and molecules. How do you know if one chemical substance is strong enough to take an electron from another? We simply refer to a list known as the activity series.
Chemisty, The Central Science, 10th Ed.
4.19, 4.21, 4.23, 4.25, 4.27, 4.39, 4.41, 4.43, 4.45, 4.47, 4.49, 4.51, 4.53, 4.55, 4.57