electrochmistry

Oxidation-Reduction (Redox) Reactions Involve a
Transfer of Electrons from One Species to Another
One form of energy that has tremendous practical signifi cance is electric energy.
A day without electricity from either the power company or batteries is unimaginable
in our technological society. The branch of chemistry that studies the interconversion
of electric energy and chemical energy is called electrochemistry.
Electrochemical processes are oxidation-reduction reactions (or redox reactions)
in which the energy released by a spontaneous reaction is converted to electricity
or in which electric energy is used to cause a nonspontaneous reaction to occur.
In redox reactions, electrons are transferred from one substance to another. The
reaction between magnesium metal and hydrochloric acid is an example of a redox
reaction:
Mg(s) 1 2HCl(aq) ¡MgCl2 1 H2(g)
In this reaction, the Mg metal loses two electrons, and the two H1 ions in the
HCl solution gain one electron each. Species that lose electrons in a redox reaction
are said to be oxidized, and those that gain electrons are said to be reduced. In the
reaction of magnesium metal with hydrochloric acid, for example, Mg metal is oxidized
and H1 ions are reduced. The Cl2 ions are spectator ions because they do not
participate directly in the redox reaction.
Oxidation-reduction reactions are very much a part of the world around us. They
range from the burning of fossil fuels to the action of household bleach. Additionally,
most metallic and nonmetallic elements are obtained from their ores by the process
of oxidation or reduction. Many important redox reactions take place in water (rusting,
for example), but not all redox reactions occur in aqueous solution. Some, such as
the formation of calcium oxide (CaO) from calcium and oxygen, occur when a solid
interacts with a gas:
2Ca(s) 1 O2(g) ¡2CaO(s)
CaO is an ionic compound made up of Ca21 and O22 ions. In this reaction, two Ca atoms
give up or transfer four electrons to two O atoms (in O2). For convenience, we can think
of this process as two separate steps, one involving the loss of four electrons by the two
Ca atoms and the other being the gain of four electrons by an O2 molecule:
2Ca¡2Ca21 1 4e2
O2 1 4e2¡2O22
Each of these steps is called a half-reaction, and each half-reaction explicitly shows
the electrons involved in the oxidation portion or the reduction portion of a redox
reaction. The sum of the oxidation half-reaction and the reduction half-reaction gives
the overall redox reaction.
An oxidation reaction is the half-reaction that involves a loss of electrons. Chemists
originally used oxidation to denote the combination of elements with oxygen.
Now, however, “oxidation” has a broader meaning that includes reactions not involving
oxygen. A reduction reaction is a half-reaction that involves a gain of electrons.
In the formation of calcium oxide, calcium is oxidized. It acts as a reducing agent
because it donates electrons to oxygen and causes oxygen to be reduced. Oxygen is
reduced and acts as an oxidizing agent because it accepts electrons from calcium,
causing calcium to be oxidized. An oxidation reaction cannot occur without a concomitant
reduction reaction, and vice versa. The number of electrons lost by a reducing
agent must equal the number of electrons gained by an oxidizing agent