Reduction and Oxidation in Redox Reactions
I. Oxidation Numbers
Every atom, ion or polyatomic ion has a formal oxidation number associated with it. This value compares the number of protons in an atom (positive charge) and the number of electrons assigned to that atom (negative charge). In many cases, the oxidation number reflects the actual charge on the atom, but there are many cases where it does not. Think of oxidation numbers as a bookkeeping exercise simply to keep track of where electrons go. There are rules to help you determine the oxidation number (also called oxidation state). See Oxidation Number∞.
Reduction means what it says: the oxidation number is reduced in reduction. This is accomplished by adding electrons. The electrons, being negative, reduce the overall oxidation number of the atom receiving the electrons.
Oxidation is the reverse process: the oxidation number of an atom is increased during oxidation. This is done by removing electrons. The electrons, being negative, make the atom that lost them more positive. I remember this phrase: LEO the lion says GER. LEO = Loss of Electrons is Oxidation GER = Gain of Electrons is Reduction Or you can use the mnemonic OIL RIG (Oxidation Is Losing electrons, Reduction Is Gaining electrons) Another way is to simply remember that reduction is to reduce the oxidation number. Therefore, oxidation must increase the value.
IV. Reduction-Oxidation Reactions
There are many chemical reactions in which one substance gets reduced in oxidation number (reduction) while another participating substance gets increased in oxidation number (oxidation). Such a reaction is called called a REDOX reaction. The RED, of course, comes from REDuction and OX from OXidation. However, it is pronounced re-dox and not red-ox. Here is a simple example of a redox reaction:
2Ag+ + Cu --> 2Ag + Cu2+
Redox equations need to be balanced but, except for the most simple ones, it cannot be done by inspection (also called trial and error). It typically takes quite a bit of work, especially when compared to how long it takes when the proper technique is used.
There is a technique used to balance redox reactions. It is called "balancing by half-reactions". The basic plan will be to split the full equation into two simpler parts (called half-reactions∞), balance them following several standard steps, then recombine the balanced half-reactions into the final answer.
V. Some Definitions
Oxidizing Agent - that substance which oxidizes somebody else. It is reduced in the process.
Reducing Agent - that substance which reduces somebody else. It is oxidized in the process.
It helps to remember these definitions by the opposite nature of what happens: the oxidizing agent gets reduced and the reducing agent gets oxidized