Electrochemistry





Electrochemistry is the study of the electronic and electrical aspects of reactions.

The elements involved in an electrochemical reaction are characterized by
the number of electrons each has. The oxidation state of an ion is the
number of electrons it has accepted or donated compared to its neutral state
(which is defined as having an oxidation state of 0). If an atom or ion
donates an electron in a reaction its oxidation state is increased, if an
element accepts an electron its oxidation state is decreased.

For example when sodium reacts with chlorine, sodium donates one electron
and gains an oxidation state of +1. Chlorine accepts the electron and gains
an oxidation state of -1. The sign of the oxidation state
(positive/negative) actually corresponds to the value of each ion's
electronic charge. The attraction of the differently charged sodium and
chlorine ions is the reason they then ionicly bond to each other.

The loss of electrons of a substance is called oxidation, and the gain of
electrons is reduction. This can be easily remembered through the use of the
mnemonic, OIL RIG: Oxidation Is Loss Reduction Is Gain. The substance which
loses electrons is also known as the reducing agent, or reductant, and the
substance which accepts the electrons is called the oxidising agent, or
oxidant.

A reaction in which both oxidation and reduction is occurring is called a
redox reaction. These are very common; as one substance loses electrons the
other substance accepts them. Redox reactions are the basis for ionic
bonding.

Oxidation requires an oxidant. Oxygen is an oxidant, but not the only one.
Despite the name, an oxidation reaction does not necessarily need to involve
oxygen. In fact, even fire can be fed by an oxidant other than oxygen:
Fluorine fires are often unquenchable, as fluorine is an even stronger
oxidant (it has a higher electronegativity) than oxygen.

Common oxidizing agents and their products:
      Agent                    Product
 O2 oxygen       O2-, H2O, or CO2

 F2 fluorine     F-

 Cl2 chlorine    Cl-

 Br2 bromine     Br-

 I2 iodine       I-

 HNO3 nitric acidNO nitric oxide, NO2 nitric dioxide

 Cr2O72-         Cr2+

 MnO4-           Mn2+

Common reducing agents and their products:
    Agent            Product
 H2 Hydrogen H+, H2O

 metals      metal ions
 C           CO2 carbon dioxide

 hydrocarbonsCO2 carbon dioxide, H2O

A spontaneous electrochemical reaction can be used to generate an electrical
current, in electrochemical cells. This is the basis of all batteries or
fuel cells. For example, gaseous oxygen (O2) and hydrogen (H2) can be
combined in a fuel cell to form water and energy (a combination of heat and
current, typically).

Conversely, non-spontaneous electrochemical reactions can be driven forward
by the application of a current at sufficient voltage. The electrolysis of
water into gaseous oxygen and hydrogen is a typical example.