The Vocabulary of Electron Theft

 

Oxidation and Reduction

Chemists use a new vocabulary to discuss reactions that involve electron theft. To understand that vocabulary, we’ll look at a relatively simple reaction:

\(2 ~ AgNO_3 + Cu \rightleftharpoons Cu(NO_3)_2 + 2 ~ Ag\)

As discussed in the opening page of this unit, this reaction involves the silver ions stealing electrons from the copper atom. We can think about these as separate occurrences:

The copper atom loses two electrons and becomes a +2 ion

\(Cu \rightarrow Cu^{+2} + 2~e^{-1}\)

Each silver ion takes in one electron and becomes a silver atom

\(Ag^{+1} + e^{-1} \rightarrow Ag\)

Let’s look at the silver first. The charge on the silver goes from +1 down to 0. This is called reduction, which is easy to remember since the charge was reduced. This reduction occurred because the silver took in an electron. Thus the official definition of reduction is the gain of one or more electrons.

The copper charge starts at 0 and rises to +2. This process is called oxidation. The source of this name is less obvious because it comes from a time when we didn’t even know that electrons existed. If a piece of copper (charge = 0) is left exposed to the air for a long time (think of an old statue) it will turn green. This is the color of copper oxide. Since the process involves the copper reacting with oxygen, chemists called the process oxidation. Later, we realized that the reaction involves the copper losing electrons and the definition of oxidation became the broader statement that it is today. Oxidation is the loss of one or more electrons.

Two mnemonics to remember the definitions:

LEO the lion says GER

• LEO: Loss of Electrons is Oxidation
• GER: Gain of Electrons is Reduction

OIL RIG

• OIL: Oxidation Is Loss (of electrons)
• RIG: Reduction Is Gain (of electrons)

Since no atom loses electrons unless they are stolen, all reactions involve BOTH reduction and oxidation. As a short-cut, chemists call these redox reactions.

Oxidizing and Reducing Agents

The other piece of vocabulary looks at cause and effect. Let’s continue using the reaction

\(2 ~ AgNO_3 + Cu \rightleftharpoons Cu(NO_3)_2 + 2 ~ Ag\)

The copper was oxidized, that is, the copper lost electrons. But why? No atom ever gives away electrons. The cause is simple. The silver ions took them. Since the silver ions caused the copper to be oxidized, we call the silver ions the oxidizing agent.

We can also view the reaction from the other direction. The silver ions were reduced (they gained electrons). The source of those electrons was the copper, so the copper is the reducing agent.

Simply put,

• The oxidizing agent causes something else to be oxidized (by taking electrons)
• The reducing agent causes something else to be reduced (by giving electrons)

Be sure to notice that the oxidizing agent is the thing that is reduced and the reducing agent is the thing that is oxidized.

Of course, this example was designed to be simple, but redox reactions can be much more complex.