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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. Here is a lesson on the rules.
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 use this mnemonic to help me remember which is which: LEO the lion says GER.LEO = Loss of Electrons is Oxidation
GER = Gain of Electrons is Reduction
Another well-known mnemonic is this: OIL RIGOIL = Oxidation Is Loss (of Electrons)
RIG = Reduction Is Gain (of 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:Ag+ + Cu ---> Ag + Cu2+
I have deliberately not balanced it and I have also written it in net ionic form. I have found that kids studying redox get confused by net ionic form and how to change a full equation into net ionic form.
Redox equations need to be balanced but, except for the most simple ones, it cannot be done by inspection (also called trial and error). I take that back, complex ones can be done by 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. This method is also called the ion-electron method.
This is another technique called the "oxidation number method." I plan to ignore it.
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 me to remember these definitions by the opposite nature of what happens. By that, I mean the oxidizing agent gets reduced and the reducing agent gets oxidized.
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