Oxidation-Reduction Reactions
Many of the chemical reactions that occur around us can be classified as
oxidation-reduction reactions (redox). The terms oxidation and reduction go
together because you cannot have one occurring without the other taking
place at the same time.
The terms oxidation and reduction have been around for a long time and
have had different meanings throughout this time. The original definitions
are listed below along with some examples of each.
oxidation - the combination of an element or a compound with oxygen to
form oxides.
C + O2 ----------------------> CO2
CH4 + 2O2 ------------------------> CO2 + 2H2O
reduction - the loss of oxygen from a compound.
2Fe2O3 + 3C -------------------------> 4Fe + 3CO2
The definitions were later extended to include the loss or gain of electrons.
The extended definitions are listed below.
oxidation - oxidation occurs when a substance loses electrons or gains
oxygen
reduction - reduction occurs when a substance gains electrons or loses
oxygen
With the extended definition oxidation cannot occur without reduction. If a
substance is gaining electrons, the electrons must be gained from another
substance, which is losing electrons. The following reaction illustrates the
loss and gain of electrons between two substances in a chemical reaction.
Mg + S ------------------------> Mg2+S2-
In this reaction magnesium loses its two valence electrons and forms the
magnesium ion. Sulfur gains the two electrons lost by the magnesium and
completes its valence shell to form the sulfide ion. The opposite charged
ions are attracted and form magnesium sulfide. Two electrons are
transferred between the two atoms so oxidation and reduction have both
occurred.
The substance that loses electrons is oxidized and it is also called the
reducing agent.
The substance that gains electrons is reduced and it is also called the
oxidizing agent.
Here is another example of a redox reaction. See if you can identify what is
being oxidized and reduced and identify the oxidizing agent and the
reducing agent.
2K + Cl2 ----------------------> 2KCl
Solution:
Two potassium (K) atoms are being oxidized because they go from a neutral
charge to a +1 charge, therefore each atom is losing 1 electron. The two
chlorine atoms in Cl2 are being reduced because they go from a neutral
charge to a -1 charge on each atom, therefore each atom is gaining 1
electron. A total of two electrons are lost by the potassium atoms and a total
of two electrons are gained by the chlorine atoms. The total number of
electrons lost by a substance must equal the total number of electrons gained
by a substance or else the reaction is not balanced. The potassium atoms are
the reducing agents because they provide the electrons for the chlorine to be
reduced and the chlorine atoms are the oxidizing agents because they accept
the electrons being lost by the potassium atoms.