Oxidation-Reduction Reaction

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Oxidation-Reduction Reaction

A type of chemical reaction that is significant to our daily lives is oxidation-reduction reactions. 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 species is always equal to the number of electrons gain by another species. The combination of the two reactions is known as a redox reaction. Species that participate in redox reactions are described as either reducing or oxidizing agents. An oxidizing agent is a species that causes the oxidation of another species. The oxidizing agent accomplishes this by accepting electrons in a reaction. A reducing agent causes the reduction of another species by donating electrons to the reaction.

Many common redox reactions involve metal species in solution, for example: Zn(s) + Cu²⁺(aq) Zn²⁺(aq) + Cu(s) The metal, zinc (Zn), is converted (oxidized) to zinc ions (Zn²⁺) by the loss of two electrons, while the copper ion (Cu²⁺) is reduced to copper metal (Cu) by the gain of two electrons. The oxidizing agent in this reaction is the copper ions that accept two electrons from zinc, causing zinc to be oxidized. The reducing agent in the reaction is zinc that is oxidized by donating two electrons to the copper ions.

The above reaction can be separated into two separate reactions called half reactions, one for oxidation and one for reduction. Zn(s) Zn²⁺(aq) + 2e- (oxidation half reaction) Cu²⁺(aq) + 2e- Cu(s) (reduction half reaction). When the two reactions are combined into a single redox equation, the electrons canceled each other out. So while electrons are shown in the individual half reactions, the electron transfer is implied in the redox reaction.

In general, a strong oxidizing agent is a species that has an attraction for electrons and can oxidize another species. The standard voltage reduction of an oxidizing agent is a measure of the strength of the oxidizing agent. The more positive the species' standard reduction potential, the stronger the species is as an oxidizing agent.

In reactions where the reactants and products are not ionic, there is still a transfer of electrons between 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. Since an oxidizing agent accepts electrons from another 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 oxidation-reduction reactions include combustion reactions that convert energy stored in fuels into thermal energy, the corrosion of metals, and metabolism of food by our bodies. The burning of natural gas is an oxidation-reduction reaction that releases energy [CH₄(g) + 2O₂(g) CO₂(g) + 2H₂O(g) + energy]. During the consumption of food, the human body uses a sequence of redox reactions to 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. Batteries, which supply electrical energy, use spontaneous oxidation-reduction reactions to start a car or power a calculator.