Oxidation-Reduction Reaction
Oxidation-reduction reactions are significant to many geochemical reactions (e.g., the production of natural gas). In addition, oxidation-reduction reactions are critical in many carbon-based biological processes.
The term oxidation was originally used to describe reactions in which an element combines with oxygen. In contrast, reduction meant the removal of oxygen. By the turn of this century, it became apparent that oxidation always seemed to involve the loss of electrons and did not always involve oxygen. In general, oxidation-reduction reactions involve the exchange of electrons between two species.
An oxidation reaction is defined as the loss of electrons, while a reduction reaction is defined as the gain of electrons. The two reactions always occur together and in chemically equivalent quantities. Thus, the number of electrons lost by one chemical species (a variation of an element or chemical compound) is always equal to the number of electrons gain by another chemical species. The combination of the two reactions is known as a redox reaction. Chemical species that participate in redox reactions are described as either reducing or oxidizing agents. An oxidizing agent is a chemical species that causes the oxidation of another chemical species. The oxidizing agent accomplishes this by accepting electrons in a reaction. A reducing agent causes the reduction of another chemical species by donating electrons to the reaction.
In general, a strong oxidizing agent is a species that has an attraction for electrons and can oxidize another chemical species. The standard voltage reduction of an oxidizing agent is a measure of the strength of the oxidizing agent. The more positive the chemical species' standard reduction potential, the stronger the chemical species is as an oxidizing agent.
In reactions where the reactants and products are not ionic, there is still a transfer of electrons between chemical species. Chemists have devised a way to keep track of electrons during chemical reactions where the charge on the atoms is not readily apparent. Charges on atoms within compounds are assigned oxidation states (or oxidation numbers). An oxidation number is defined by a set of rules that describes how to divide up electrons shared within compounds. Oxidation is defined as an increase in oxidation state, while reduction is defined as a decrease in oxidation state. Because an oxidizing agent accepts electrons from another chemical species, a component atom of the oxidizing agent will decrease in oxidation number during the redox reaction.
There are many examples of oxidation-reduction reactions in the world. Important processes that involve oxidationreduction reactions include combustion reactions that convert energy stored in fuels into thermal energy, the corrosion of metals, and metabolic reactions.
Oxidation-reduction reactions occur in both physical and biological settings (where carbon-containing compounds such as carbohydrates are oxidized). The burning of natural gas is an oxidation-reduction reaction that releases energy [CH4(g) + 2O2(g) → CO2(g) + 2H2O(g) + energy]. In many organisms, including humans, redox reactions burn carbohydrates that provide energy [C6H12O6(aq) + 6O2(g) → 6CO2(g) + 6H2O(l)]. In both examples, the carbon-containing compound is oxidized, and the oxygen is reduced.
Chemical Bonds and Physical Properties; Chemical Elements; Chemistry
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