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Oxidation, Reduction, and Redox Reactions

Before we begin a discussion of the nuts and bolts of redox reactions, learn one simple mnemonic that will make your life much easier when it comes to redox chemistry:

OIL RIG

What this stands for is:

Oxidation Is Loss of electrons

Reduction Is Gain of electrons

Oxidation

Let's first look at a simple oxidation reaction. Remember, Oxidation Is Loss of electrons:

Fe2+ Fe3+ + e-

You can see that in the reaction, from left to right the Fe becomes more positive in charge, which you can also think of as becoming less negative. Since electrons carry a negative charge, then the Fe must be losing electrons if it is becoming less negative. Since the Fe is losing electrons, then this reaction is an oxidation.

Reduction

Now let's look at a reduction reaction. Remember, Reduction Is Gain of electrons:

Cu2+ + e- Cu+

Notice that for this reaction, from left to right the Cu becomes less positive in charge, or more negative. So if the Cu is becoming more negative, it must be gaining an electron, meaning that it is getting reduced (this is a reduction reaction).

Redox Reactions

It is important to note that the above oxidation and reduction reactions are only half-reactions. In reality, electrons are like protons in acid base reactions in that in solution they are not found floating around, for they immediately get taken up by some other molecule. What this means is that oxidation or reduction reactions do not take place alone, but are always coupled together. That is why they are called redox reactions (reduction-oxidation reactions). So if something in solution gets oxidized, then something else must be reduced (and vice versa).

Using our above reactions as an example, let's say we have equal amounts of both iron(II), iron(III), copper(I), and copper(II) existing in solution:

Fe2+ + Cu2+ Fe3+ + Cu+

If the redox reaction proceeds from left to right, then the Fe is becoming less negative and the Cu is becoming more negative, meaning that the electron on the Fe is being given to the Cu. Therefore, Fe is losing electrons and being oxidized (OIL) and Cu is gaining electrons and being reduced (RIG).

However, if the reaction proceeds from right to left, then the opposite is true, with Fe reduced and Cu oxidized. How do we know which way the reaction will go? To answer this, we must take a look at:

Oxidation-Reduction Energetics



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