Gain and Loss of Oxygen

The earliest view of oxidation and reduction is that of adding oxygen to form an oxide (oxidation) or removing oxygen (reduction). They always occur together. For example, in the burning of hydrogen

the hydrogen is oxidized and the oxygen is reduced. The combination of nitrogen and oxygen which occurs at high temperatures follows the same pattern.

This formation of nitric oxide oxidizes the nitrogen and reduces the oxygen. In some reactions, the oxidation is most prominent. For example in the burning of methane,

both carbon and hydrogen are oxidized (gain oxygen). The accompanying reduction of oxygen is perhaps easier to see when you describe reduction as the gaining of hydrogen.

On the other hand, the reaction of lead dioxide at high temperatures appears to be just reduction.

The reduction of the lead dioxide is clear, but the associated oxidation of oxygen is easier to see when you describe oxidation as the losing of electrons.

Gain and Loss of Hydrogen

The original view of oxidation and reduction is that of adding or removing oxygen . An alternative approach is to describe oxidation as the loss of hydrogen and reduction as the gaining of hydrogen. This has an advantage in describing the burning of methane.

With this approach it is clear that the carbon is oxidized (loses all four hydrogens) and that part of the oxygen is reduced (gains hydrogen). Another reaction where the hydrogen approach makes things clearer is the passing of methanol over a hot copper gauze to form formaldehyde and hydrogen gas (Hill and Kolb):

Both carbon-containing molecules have the same oxygen content, but the formation of the formaldehyde is seen to be oxidation because hydrogens are lost. The formation of H₂ is a reduction process as the two released hydrogens get together.

The formation of methanol from reacting carbon monoxide with hydrogen combines oxidation and reduction in the single molecular product.

The CO is reduced because it gains hydrogen, and the hydrogen is oxidized by its association with the oxygen.

Gain and Loss of Electrons

The original view of oxidation and reduction is that of adding or removing oxygen. An alternative view is to describe oxidation as the losing of electrons and reduction as the gaining of electrons. One example in which this approach is of value is in the high temperature reaction of lead dioxide .

In this reaction the lead atoms gain an electron (reduction) while the oxygen loses electrons (oxidation).

This electron view of oxidation and reduction helps you deal with the fact that "oxidation" can occur even when there is no oxygen! The definition of redox reactions is extended to include other reactions with nonmetals such as chlorine and bromine. For example, the reaction

Magnesium loses electrons and is therefore said to be "oxidized", whereas the chlorines gain electrons and are said to be reduced. Another way to judge that the chlorine has been reduced is the fact that the charge on the atoms is made more negative, or reduced. Treating that charge as an "oxidation number" is another way to characterize oxidation and reduction.

The view of oxidation and reduction as the loss and gain of electrons, respectively, is particularly appropriate for discussing reactions in electrochemical cells. For example, in the zinc-copper cell, the oxidation and reduction half-reactions are

Oxidation Number

The original view of oxidation and reduction is that of adding or removing oxygen. An alternative view which is useful in dealing with ions is to define an oxidation number which is equal to the net charge of the product of a reaction. Oxidation is then viewed as a reaction which increases the oxidation number and reduction as one which reduces the oxidation number. This view of oxidation and reduction helps you deal with the fact that "oxidation" can occur even when there is no oxygen! The definition of redox reactions is extended to include other reactions with nonmetals such as chlorine and bromine. For example, in the reaction

the Mg is seen to increase in oxidation number from 0 to 2 (oxidation) while the chlorine atoms experience a decrease in oxidation number from 0 to -1 (reduction).

This is essentially equivalent to viewing oxidation and reduction as losing or gaining electrons, but may be easier to remember.