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Types of redox reactions: Redox reactions are among the most common and most important chemical reactions in everyday life. The great majority of those reactions can be classified on the basis of how rapidly they occur. Combustion is an example of a redox reaction that occurs so rapidly that noticeable heat and light are produced. Corrosion, decay, and variousbiological processes are examples of oxidation that occurs so slowly that noticeable heat and light are not produced.
Combustion. Combustion means burning. Any time a material burns, an oxidation-reduction reaction occurs. The two equations below show what happens when coal (which is nearly pure carbon) and gasoline (C 8 H 18 ) burn. You can see that the fuel is oxidized in each case:
C + O 2 →CO 2
2 C 8 H 18 + 25 O 2 → 16 CO 2 + 18 H 2 O
In reactions such as these, oxidation occurs very rapidly and energy is released. That energy is put to use to heat homes and buildings; to drive automobiles, trucks, ships, airplanes, and trains; to operate industrial processes; and for numerous other purposes.
Rust. Most metals react with oxygen to form compounds known as oxides. Rust is the namegiven to the oxide of iron and, sometimes, the oxides of other metals. The process by which rusting occurs is also known as corrosion. Corrosion is very much like combustion, except that it occurs much more slowly. The equation below shows perhaps the most common form of corrosion, the rusting of iron.
4 Fe + 3 O 2 → 2 Fe 2 O 3
Decay. The compounds that make up living organisms, such as plantsand animals, are very complex. They consist primarily of carbon, oxygen, and hydrogen. A simple way to represent such compounds is to use the letters x, y, and z to show that many atoms of carbon, hydrogen, and oxygen are present in the compounds.
When a plant or animal dies, the organic compounds of which it is composed begin to react with oxygen. The reaction is similar to the combustion ofgasoline shown above, but it occurs much more slowly. The process is known as decay, and it is another example of a common oxidation-reduction reaction. The equation below represents the decay (oxidation) of a compound that might be found in a dead plant:
C x H y O z + O 2 → CO 2 + H 2 O
Biological processes. Many of the changes that take place within living organisms are also redox reactions. Forexample, the digestion of food is an oxidation process. Food molecules react with oxygen in the body to form carbon dioxide and water. Energy is also released in the process. The carbon dioxide and water are eliminated from the body as waste products, but the energy is used to make possible all the chemical reactions that keep an organism alive and help it to grow.



As with any type of chemical reaction, combustion takes place when chemical bonds are broken and new bonds are formed. It so happens that combustion is a particularly dramatic type of oxidation-reduction reaction: whereas we cannot watch iron rust, combustion is a noticeable event. Even more dramatic is combustion that takes place at a rate so rapidthat it results in an explosion.
As one might expect from what has already been said about oxidation-reduction, the oxygen is reduced while the carbon is oxidized.


At the same time, oxidation-reduction reactions are responsible for the spoiling of food, the culprit here being the oxidation portion of the reaction. To prevent spoilage, manufacturers of fooditems often add preservatives, which act as reducing agents.
Oxidation may also be linked with the effects of aging in humans, as well as with other conditions such as cancer, hardening of the arteries, and rheumatoid arthritis. It appears that oxygen molecules and other oxidizing agents, always hungry for electrons, extract these from the membranes in human cells. Over time, this can cause a...
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