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This page discusses the various definitions of oxidation and reduction (redox) in terms of the transfer of oxygen, hydrogen, and electrons. It also explains the terms oxidizing agent and reducing agent.
The terms oxidation and reduction can be defined in terms of the adding or removing oxygen to a compound. while this is not the most robust definition, as discussed below, it is the easiest to remember.
Definition 1a: Oxidation and Reduction with respect to Oxygen Transfer
For example, in the extraction of iron from its ore:
Because both reduction and oxidation are occurring simultaneously, this is known as a redox reaction.
An oxidizing agent is substance which oxidizes something else. In the above example, the iron(III) oxide is the oxidizing agent. A reducing agent reduces something else. In the equation, the carbon monoxide is the reducing agent.
Definition 1b: Oxidizing and Reducing Agents
These are old definitions which are no longer used, except occasionally in organic chemistry.
Definition 2a: Oxidation and Reduction with respect to Hydrogen Transfer
Notice that these are exactly the opposite of the oxygen definitions (#1).
For example, ethanol can be oxidized to ethanal:
An oxidizing agent is required to remove the hydrogen from the ethanol. A commonly used oxidizing agent is potassium dichromate(VI) solution acidified with dilute sulfuric acid. Ethanal can also be reduced back to ethanol by adding hydrogen. A possible reducing agent is sodium tetrahydridoborate, NaBH4. Again the equation is too complicated to consider at this point.
Definition 2b: Oxidizing and Reducing Agents
A more precise definition of oxidizing and reducing agents (from Definition 1b)
Definition 3: Oxidation and Reduction with respect to Electron Transfer
Remembering these definitions is essential, and easily done using this convenient acronym:
The equation below shows an obvious example of oxygen transfer in a simple redox reaction:
\[ CuO + Mg \rightarrow Cu + MgO\]
Copper(II) oxide and magnesium oxide are both ionic compounds. If the above is written as an ionic equation, it becomes apparent that the oxide ions are spectator ions. Omitting them gives:
In the above reaction, magnesium reduces the copper(II) ion by transferring electrons to the ion and neutralizing its charge. Therefore, magnesium is a reducing agent. Another way of putting this is that the copper(II) ion is removing electrons from the magnesium to create a magnesium ion. The copper(II) ion is acting as an oxidizing agent.
Confusion can result from trying to learn both the definitions of oxidation and reduction in terms of electron transfer and the definitions of oxidizing and reducing agents in the same terms.
Understanding is safer than thoughtless learning!
Jim Clark (Chemguide.co.uk)
Jim Clark (Chemguide.co.uk)