OXIDATION AND REDUCTION
Oxygen makes up only about 20% of the air, yet is the essential component for so many reactions. Without it fuels would not burn, iron would not rust and we would be unable to obtain energy from our food molecules through respiration. Indeed animal life on the planet did not evolve until a certain concentration of oxygen had built up in the atmosphere over 600 million years ago. The term oxidation has been in use for a long time to describe these and other reactions where oxygen is added. Oxidation, though, is only half of the story, as it is always accompanied by the opposite process reduction, which was originally thought of in terms of loss of oxygen. Later, however the terms widened to include a much broader range of reactions. We now deﬁne these two processes, oxidation and reduction, as occurring whenever electrons are transferred from one reactant to another – and many of these reactions does not involve oxygen at all. For example photosynthesis, the process in which plants store chemical energy from light energy, involve oxidation and reduction reactions although oxygen itself is not used. Transferring electrons from one substance to another leads to a ﬂow of electrons, in other words to an electric current. Thus chemical reactions can be used to generate electricity – a simple battery works by using a reaction of oxidation and reduction in this way. As we will see, we can also reverse the process and use an electric current to drive a reaction to oxidation and reduction – this is the process known as electrolysis which has developed to become one of the most important industrial processes on which we depend. An understanding of oxidation and reduction is therefore at the heart of understanding a large branch of chemistry both in the laboratory and beyond.
Oxidation is the loss of electrons, reduction the gain of electrons.
The Nobel Prize in Chemistry 1992
The Royal Swedish Academy of Sciences awards the 1992 Nobel Prize in Chemistry to for his contributions to the theory of electron transfer reactions in chemical systems.
Rudolph A. Marcus
Oxidation of metals
Most metals are oxidized by the oxygen in air. This process is called corrosion. Electrons leap from the metal to the oxygen molecules. The negative oxygen ions which are thus formed penetrate into
the metal, causing the growth of an oxide surface. As the oxide layer grows the rate of electron transfer decreases. The corrosion stops and the metal is made passive. The oxidation process may continue, however, if the electrons succeed in entering the metal through cracks or impurities in the metal or if the oxide layer is dissolved.
OXIDATION OF MAGNESIUM
When magnesium is burned in air, it gives a bright white ﬂame and produces a white powder, magnesium oxide: 2 Mg(s) + O2(g) ! 2 MgO(s) --------------------------------------2Mg(s) ! 2 Mg
California Institute of Technology,Pasadena, California, USA
(s) + 4 e-
O2(g) + 4 e ! 2O2-(s)
2 Mg(s) + O2(g) ! 2MgO(s)
Oxidation-reduction reactions have many farreaching applications in our lives. Some of these applications are so common, that we take them for granted; others are not so obvious. The following are just a few examples of oxidationreduction reactions. • • • • • • • • Bleaching Agents Photosynthesis Metabolism Nitrogen Fixation Combustion The Dry Cell Battery Electrochemistry Photo-oxidation (Photogray Æ Glasses) Corrosion
The rusting of metals, the process involved in photography, the way living systems produce and utilize energy, and the operation of a car battery, are but a few examples of a very common and important type of chemical reaction. These chemical changes are all classiﬁed as "electron-transfer" or oxidation-reduction reactions. The term, oxidation , was derived from the observation that almost all elements reacted with oxygen to form compounds called, oxides. A typical example...
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