In an elementary chemistry course, oxidation might be defined as a chemical reaction where oxygen is gained or hydrogen is lost. Reduction is the opposite process to oxidation, a reaction where oxygen is lost or hydrogen is gained.
E.g. CuO(s) + H2(g) Cu(s) + H2O(g)
The copper (II) oxide loses oxygen and is reduced while the hydrogen gains oxygen and is oxidised. The hydrogen is called the reducing agent (reductant) and is oxidised. The copper (II) oxide is the oxidising agent (oxidant) and is reduced.
Oxidation and reduction occur together and the term redox reaction means a reaction where oxidation and reduction take place. This elementary treatment is easy to understand but it does not include all possible reactions.
Oxidation is a process where electrons are lost.
Reduction is a process where electrons are gained.
Consider the reaction where magnesium burns in chlorine to produce magnesium chloride.
Mg(s) + Cl2(g) MgCl2(s)
There are two processes occurring:
Mg Mg2+ + 2e- and Cl2 + 2e- 2Cl-
Magnesium loses electrons and is oxidised; chlorine gains electrons and is reduced. These two equations are called half-equations.
The concept of electron transfer is useful in balancing redox equations.
The following simple rules can be used to obtain a balanced equation:
Consider the reaction between permanganate ions and iodide ions in acid solution.
|1.||MnO4- Mn2+ + 4H2O||2I- I2|
|2.||Balance the hydrogen atoms with H+.|
|MnO4- + 8H+ Mn2+ + 4H2O||2I- I2|
|Balance half-equations electrically by adding electrons.|
|MnO4- + 8H+ + 5e- Mn2+ + 4H2O||2I- I2 + 2e-|
|3.||Multiply each half-equation by the number of electrons appearing in the other half-equation, and add the equations to eliminate the electrons.|
|4.||2MnO4- + 16H+ + 10e- 2Mn2+ + 8H2O||10I- 5I2 + 10e-|
|Adding: 2MnO4- + 16H+ + 10I- 2Mn2+ + 5I2 + 8H2O|
The concept of oxidation state (or number) has been devised to give a guide to the extent of oxidation or reduction in a species. The concept is without direct chemical foundation, but is very useful, being appropriate to both ionic and covalently bonded species.
Oxidation states are assigned to atoms in molecules or ions to shew how much they are oxidised or reduced according to some simple rules:
|O||-2 (except in peroxides, -1 and combined with fluorine)|
|H||+1 (except in metal hydrides, -1)|
|Cl||-1 (except when combined with O and F)|
An element is oxidised when its oxidation state increases.
An element is reduced when its oxidation state decreases.
Part of this site was last updated on 21st January 2009.
This work is licensed under a Creative Commons Attribution-NonCommercial 2.0 England & Wales Licence.
Copyright © Article Gems 2006-09.