|Electrochemistry involves oxidation-reduction reactions
can be brought about by electricity or used to produce electricity.
|Oxidation and reduction, which will be considered here as
and gain of electrons, occur in many chemical systems. The rusting of
the photosynthesis that takes place in the leaves of green plants, and
the conversion of foods to energy in the body are all examples of
changes that involve the transfer of electrons from one chemical
to another. When such reactions can be made to cause electrons to flow
through a wire or when a flow of electrons makes a redox reaction
the processes are referred to as electrochemical changes. The
of these changes is called electrochemistry.
|The applications of electrochemistry are widespread.
which produce electrical energy by means of chemical reactions are in
anything portable and electronic. In the laboratory, electrical
enable us to monitor chemical reactions of all sorts, even those in
as tiny as a living cell. In industry, many important chemical -
liquid bleach (sodium hypochlorite) and lye (sodium hydroxide) are
by electrochemical means. If it weren't for electrochemistry the
structural metals of aluminum and magnesium would only be laboratory
and most people would see them only in museums.
|Oxidation-reduction reactions or redox reactions involve
of electron density from one atom to another. Two example are the
of sodium with chlorine and the reaction of hydrogen with oxygen.
|2 Nao + Cl2 ------> 2 NaCl
2 H2 + O2 --------> 2 H2O
|At first glance these reaction appear to be very different
each other. In NaCl ions have been formed, and that has certainly
the transfer of electrons. The electron is transferred completely from
a sodium to a chlorine atom as the Na+ and Cl- ions
are created. But how about the reaction that produces
Here we have the formation of a molecule held together by covalent
- bonds in which electrons are shared. How does this reaction involve a
transfer of electrons? To answer this question, we have to look closely
at the bonds in both the reactants and products.
|Hydrogen and oxygen molecules are non-polar. This is
atoms in an H2 or O2 molecule are the same, so
electronegativity difference between them is zero. In a non- polar
the electron pair in the bond is shared equally, and neither atom
a partial charge. Stated another way, each atom in an H2 or
O2 molecule is electrically neutral. Now let's look at the
water. The electronegativities of hydrogen and oxygen are quite
oxygen being more electronegative than hydrogen. this means that the
bonds are quite polar, with the hydrogen carrying a substantial
partial charge and the oxygen carrying a substantial negative partial
|Now we can look at what happens to the electrons around an
during the reaction. A hydrogen atom begins with a zero positive charge
and finishes with a partial positive charge in H2O.
Similarly, oxygen begins with a zero partial charge in O2 and
finishes with a partial negative charge in H2O. Thus, during
the reaction there is a shift of electron density from a hydrogen atom
to an oxygen atom, and it is in this sense that the reaction of H2
is similar to the reaction of Na with Cl2.
|Many chemical reactions involve (or at least appear to
a shift of electron density by one atom to another. Collectively, such
reactions are called oxidation-reduction reactions, or simply
reaction. The term oxidation refers to the loss of electrons by
one reactant, and reduction refers to the gain of electrons by
For example, the reaction between sodium and chlorine involves a loss
electrons by sodium (oxidation of sodium) and a gain of electrons of
(reduction of chlorine).
|Nao ------> Na+ + e- (oxidation)
Cl2 + 2 e- -------> 2 Cl- (reduction)
we say that the sodium is oxidized and the chlorine is
|Oxidation and reduction always occur together. No
substance is ever
oxidized unless something else is reduced. Otherwise, electrons would
as a product of the reaction, and this is never observed. During a
reaction, then, some substance must accept the electrons that
substance loses. this electron-accepting substance is called the oxidizing
agent because it helps something else to be oxidized. The substance
that supplies the electrons is called the reducing agent because
it helps something else be reduced. Sodium is a reducing agent, for
when it supplies electrons to chlorine. In the process, sodium is
Chlorine is an oxidizing agent when it accepts electrons from the
and when that happens, chlorine is reduced to chloride ion. One way to
|The substance that is oxidized is the
|The substance that is reduced is the
|Redox reactions are very common. Whenever you use a
battery, a redox
reaction occurs. The metabolism of foods, which supplies our bodies
energy, also occurs by a series of redox reactions that use oxygen to
carbohydrates and fats to carbon dioxide and water. Ordinary household
bleach works by oxidizing substances that stain fabrics, making them
to remove from the fabric or rendering them colourless.
Liquid bleach contains hypochlorite OCl-, as the oxidizing agent.