Sections 14.2, 17.1
What is a redox reaction?
The formal name for a redox reaction is "oxidation reduction reaction," and you can see that "redox" is just shorthand for the words reduction and oxidation. Thus, in a redox reaction, two things happen. You guessed it -- oxidation and reduction. These two have to happen together. You cannot have an oxidation reaction without a corresponding reduction reaction. Its a bit like the idea behind a blood transfusion or an organ transplant. You cannot have a recipient unless you have a donor, and it does not make any sense to be a donor unless there is a recipient.
You may have noticed that "oxidation" starts with the same prefix as oxygen, suggesting that oxygen may be somehow involved in this process. Indeed, one definition of oxidation states that:
Oxidation is the gaining of bonds to oxygen
Organic fuel substances (such as wood, coal or gas) are examples of compounds that can be oxidized. In the process of burning, the carbon in these substances becomes bonded with oxygen, while some of the oxygen used to "burn" the fuel bonds to the hydrogen atoms from the fuel. Thus, one of the definitions of reduction states that:
Reduction is the gaining of hydrogen
Therefore, combustion reactions are good examples of redox reactions where one molecule gains oxygen (is oxidized) and one molecule gains hydrogen (is reduced).
For example, lets look at what happens when gasoline in your car is burned as you drive around town (heptane is a common hydrocarbon component of gasoline):
Note how the carbon atoms in heptane are oxidized (because the carbon atoms in heptane become bonded with oxygen atoms), while the oxygen is reduced (becomes bonded with hydrogen atoms).
These definitions of oxidation and reduction are useful. However, more general definitions of oxidation and reduction involve the movement of electrons between the compounds involved in the redox reaction. In the most broad definition of redox reactions:
Oxidation is the loss of electrons
Because electrons are negatively charged, an increase in electrons means a decrease in overall charge (the compound becomes more negatively charged). On the other hand, an atom that is oxidized has given up some of those negatively charged electrons, which will increase its overall charge (the compound becomes more positively charged). Notice that these definitions do not involve oxygen and hydrogen. Thus, redox reactions can occur with compounds that do not contain oxygen or hydrogen atoms.
An everyday example of a redox reaction that we are all familiar with is the process of rusting. Rust is the flaky brown substance that forms on iron objects left exposed to the elements for too long, especially if the objects get wet. Rust doesnt just form on the iron object, the iron actually turns into rust (rust is actually a form of oxidized iron).
We all intuitively know the chemical reaction of rusting:
Lets examine the oxidation and reduction reactions that are involved in the redox reaction of rusting. First, iron, in the presence of moisture (H2O) will lose electrons, becoming a positively charged ion in water:
Oxidation reaction: Iron is oxidized (loses e-)
Fe(s) Fe2+(aq) + 2 e-
Those electrons are then used to reduce the oxygen dissolved in the water (remember that water will have some dissolved oxygen and also H+ and OH- ions):
Reduction reaction: Oxygen is reduced (gains e-)
O2 + 4e- + 4 H+ 2 H2O
Those Fe2+ ions react with the OH- ions in water to produce iron hydroxide, which will dry in several steps to produce rust:
Fe2+ + 2OH- Fe(OH)2 Fe2O3 (rust)
Notice that the redox reaction needs water, which explains why a moist environment speeds up the rusting process. Rusting can occur in dry climates, but it tends to happen much more slowly due to the relatively low humidity in the air.
Because it is difficult to remember the definitions of oxidation and reduction that involve the movement of electrons, a common mnemonic that can help you keep these rules in mind is:
This is an especially appropriate mnemonic for remembering redox reactions because the burning of oil (a hydrocarbon fuel) is a redox reaction!
Copyright 2002, John Wiley & Sons Publishers, Inc.