A previous article described oxidation and reduction reactions. Such reactions were originally given a definition that depended on a substance subject to the addition or withdrawal of oxygen.
While the definition of oxidation as the addition of oxygen was adequate for combustion reactions occurring in air, many similar reactions not involving oxygen as a reactant have since been investigated.
For example reactions involving sulfur, sodium, chlorine and others. Hence naming these as "oxidation" after oxygen does not focus on what these reactions have in common, nor does it provide a theoretical explanation of the processes involved. A better definition was required.
Electron Transfer in Redox Reactions
To explain the electronic transfer concept, the element magnesium will be used as example. Magnesium often appears dull and covered with a coating that needs to be scraped off. This layer is magnesium oxide, which results from the corrosion of the magnesium in air. Magnesium has reacted with atmospheric oxygen to form magnesium oxide:
2Mg + O2 --> 2MgO
Magnesium oxide is an ionic compound and consists of Mg2+ ions and O2- ions. Each magnesium atom, therefore, must have lost two electrons to form and Mg2+ ion. Each oxygen atom in the oxygen molecule must have gained two electrons to form an oxide of ion O2-. The above reaction can be represented by two half equations. The first shows the loss of two electrons from each magnesium atom:
Mg --> Mg2+ + 2e
The second equation shows the gain of two electrons by each atom in the oxygen molecule:
O2 + 4e --> 2O2-
Definition of Oxidation and Reduction
In the transfer of electrons from magnesium atoms to oxygen atoms, magnesium atoms also lose two valence electrons. In fact, this is the mechanism in many other reactions and this loss of electrons is a broader, more widely used definition of oxidation.
- Oxidation Is the Loss of electrons
- Reduction Is the Gain of electrons
Note, the definition of oxidation and reduction can be recalled using the memory aid (mnemonic) using the highlighted letters above, OIL RIG.
Writing Overall Redox Equations
When writing redox reactions, it is customary to first write the two half equations and them add them together to obtain an overall equation. The final equation does not show any electrons transferred, since the electrons lost in the oxidation reaction are gained in the reduction reaction. For example, consider the reaction of copper in silver solution.
Each copper atom that is oxidised loses two electrons
Cu --> Cu2+ + 2e
Each Ag+ ion that is reduced gains one electron
Ag+ + e --> Ag, or doubling terms (to achieve two electrons)
2Ag+ +2e --> 2Ag
Adding the above half equations, one gets the overall equation for the redox reaction:
Cu + 2Ag+ --> Cu2+ + 2Ag
In the overall redox reaction, the number of atoms of each element present in the products is equal to the number present in the reactants. Atoms are conserved in in all chemical reactions. Moreover, the total charge on the product side of the equation is equal to the total charge on the reactant side. Charge is conserved in all chemical reactions.
References:
- Heinemann Chemistry 1 - VCE Units 1 & 2. Nicole Lukins et al., Heinemann Harcourt Education, 4th Edition. 2006.
- Chemistry for Dummies. John T. Moore. Wiley Publishing, Inc. Hoboken, NJ. 2003.
Harry P. Schlanger - Hello, I started out as a physicist working for research organisations. Mostly in the area of heat transfer in solids and porous media. ...
