CH.20

Gain and Loss of Oxygen The earliest view of HYPERLINK "" 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 2H2 + O2 -> 2H2O the hydrogen is oxidized and the oxygen is reduced. The combination of nitrogen and oxygen which occurs at high temperatures follows the same pattern. N2 + O2 -> 2NO 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, CH4 + 2O2 -> CO2 + 2H2O 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. 2PbO2 -> 2PbO + O2 Gain and Loss of Hydrogen The original view of HYPERLINK "" oxidation and reduction is that of adding or removing hydrogen. 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. CH4 + 2O2 -> CO2 + 2H2O 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): CH3OH -> CH2O + H2 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. CO + H2 -> CH3OH The CO is reduced because it gains hydrogen, and the hydrogen is oxidized by its association with the oxygen.