Untitled - Kelly Walsh High School

56 CHEMISTRY FOR THE UTTERLY CONFUSED
4-4 Oxidation-Reduction
Oxidation-reduction reactions, commonly called redox reactions, are an
extremely important category of reaction. Redox reactions include combustion,
corrosion, respiration, photosynthesis, and the reactions occurring in batteries.
Redox is a term that stands for reduction and oxidation. Reduction is the gain
of electrons and oxidation is the loss of electrons. In these reactions, the number
of electrons gained must be identical to the number of electrons lost. For
example, suppose a piece of zinc metal is placed in a solution containing the
Cu 2 cation. Very quickly, a reddish solid forms on the surface of the zinc metal.
That substance is copper metal. At the molecular level, the zinc metal is losing
electrons to form the Zn 2 cation and the Cu 2 ion is gaining electrons to form
copper metal. We can represent these two processes as:
Zn(s) l Zn2 (aq) 2 e (oxidation)
Cu2 (aq) 2 e l Cu(s) (reduction)
The electrons that are being lost by the zinc metal are the same electrons that
are being gained by the copper(II) ion. The zinc metal is being oxidized and the
copper(II) ion is being reduced.
Something must cause the oxidation (taking the electrons) and that substance
is the oxidizing agent (the reactant undergoing reduction). In the
example above, the oxidizing agent is the Cu 2 ion. The reactant undergoing
oxidation is the reducing agent because it is furnishing the electrons that are
being used in the reduction half-reaction. Zinc metal is the reducing agent
above. The two half reactions, oxidation and reduction, can be added together
to give you the overall redox reaction. When doing this, the electrons must
cancel—that is, there must be the same number of electrons lost as electrons
gained:
Zn(s) Cu2 (aq) 2 e l Zn2 (aq) 2 e Cu(s) or
Zn(s) Cu2 (aq) l Zn2 (aq) Cu(s)
In these redox reactions, there is a simultaneous loss and gain of electrons. In
the oxidation reaction part of the reaction (oxidation half-reaction), electrons
are being lost, but in the reduction half-reaction, those very same electrons are
being gained. Therefore, in redox reactions there is an exchange of electrons, as
reactants become products. This electron exchange may be direct, as when copper
metal plates out on a piece of zinc or it may be indirect, as in an electrochemical
cell (battery).