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Electrochemistry is the study of the electronic and electrical aspects of chemical 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 mnemonic devices. Two of the most popular are "OIL RIG" (Oxidation Is Loss,
Reduction Is Gain) and "LEO says GER" (Lose Electrons: Oxidization, Gain Electrons: Reduction). The substance which loses
electrons is also known as the reducing agent, or reductant, and the substance which accepts the electrons is
called the oxidizing agent, or oxidant. The oxidizing angent is always being reduced in a reaction; the
reducing agent is always being oxidized.
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: flourine fires are often unquenchable, as fluorine is an even stronger
oxidant (it has a higher electronegativity) than oxygen).
Spontenaity
A spontaneous electrochemical
reaction can be used to generate an electrical current, in electrochemical
cells. This is the basis of all batteries and
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.
See also
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