Classical Idea of Redox Reactions – Oxidation and Reduction Reactions

The Classical View: Oxidation and Reduction

The story of redox reactions begins long before electrons were discovered. Early chemists noticed that many substances changed dramatically when they reacted with the air around them. The most obvious change was the addition of oxygen — a process they called oxidation.

Oxidation originally meant the addition of oxygen to a substance. This was a natural starting point because the atmosphere contains about 20% dioxygen ($O_2$), and many elements on Earth exist as oxides precisely because they have combined with oxygen over geological time.

Consider these straightforward examples:

In the first reaction, magnesium gains oxygen to become magnesium oxide. In the second, sulphur gains oxygen to become sulphur dioxide. Both are oxidation reactions by this original definition.

But chemists soon realised that the idea of oxidation could be extended. Look at the combustion of methane:

Here, carbon gains oxygen — that fits the original definition. But notice that hydrogen is also being removed from the methane molecule. This observation led to a broader interpretation: oxidation could also be defined as the removal of hydrogen from a substance.

Another example makes this even clearer:

In this reaction, hydrogen sulphide loses its hydrogen atoms to form elemental sulphur. The hydrogen is removed, and the sulphur is said to be oxidised.

Broadening the Definition: Electronegative and Electropositive Elements

As chemical knowledge grew, it became clear that reactions similar to those above could occur without oxygen at all. What mattered was the nature of the element being added or removed. Chemists began to think in terms of electronegative elements (elements that attract electrons strongly, like oxygen, fluorine, chlorine) and electropositive elements (elements that lose electrons easily, like metals).

Consider what happens when magnesium reacts with fluorine, chlorine, or sulphur:

In each case, magnesium combines with an electronegative element. These reactions are clearly analogous to the oxidation of magnesium by oxygen. So the definition was extended: oxidation is the addition of oxygen or any electronegative element to a substance, or the removal of hydrogen or any electropositive element from a substance.

This broader definition even covers reactions that don't involve oxygen at all. For example:

Here, potassium ferrocyanide loses potassium ions (an electropositive element) to become potassium ferricyanide. This removal of an electropositive element qualifies as oxidation.

The Classical View of Reduction

Just as oxidation had an original narrow meaning, so did reduction. Originally, reduction meant the removal of oxygen from a compound. The name itself comes from the process of extracting metals from their ores — the metal oxide is "reduced" to the pure metal.

The classic example is the decomposition of mercuric oxide upon heating:

Mercuric oxide loses oxygen to form mercury metal. This is reduction.

But just as with oxidation, the definition was broadened. Reduction came to include:

• Removal of oxygen from a substance
• Removal of an electronegative element from a substance
• Addition of hydrogen to a substance
• Addition of an electropositive element to a substance

Here are examples illustrating each of these:

Removal of an electronegative element (chlorine):

Ferric chloride loses chlorine atoms (an electronegative element) to become ferrous chloride. The iron is reduced.

Addition of hydrogen:

Ethene gains hydrogen to become ethane. This is a reduction reaction.

Addition of an electropositive element (mercury):

Mercuric chloride gains mercury (an electropositive element) to form mercurous chloride. The mercury is reduced.

The Birth of "Redox" …