Electrochemistry
Electrochemistry is a science that deals with the
transformation of chemical energy into electrical energy and
vice versa.
Electrolytic conductors contain mobile ions, which carry
an electrical charge. When an electrical potential is applied,
these ions will move in the direction appropriate to their charge.
As in electronic conductance, the electrical current is a
movement of electrical charge but this time the charge is carried
by ions of significant mass and electrolytic conductance is
accompanied by mass transfer.
This property of electrolytic conduction plays a major role in
some of the processes and phenomena important to metallurgist
corrosion
and
oxidation
of
metals,
electroplating,
electropolishing, electrolytic extraction and refining of metals.
Electrolyte:
Chemical substances are classified as electrolytes and
nonelectrolytes based on their behavior in an aqueous solution
or on fusion. Substances that are good conductors of electricity
in their molten state or in their aqueous solution are called
electrolytes (such as NaCl, CuSo4, acids and bases). On the
other hand, a substance whose aqueous solution or melt do not
conduct electricity is called a nonelectrolyte such as (glucose
and starch). Electrolytes in their aqueous solution or in molten
state dissociate into ions, these ions are responsible for the
conduction of current through them. However, all the
electrolytes are not ionized to the same extent. Depending upon
the extent of their ionization, these are classified to strong or
weaker electrolytes.
Strong electrolyte ionize almost completely into their ions when
dissolved in water. For example (NaCl, HCl, H2So4, KoH,
NaOH, CuSo4, and soon).
Weak electrolytes ionize to a small extent when dissolved in
water as HCOOH, NH4OH.
Electrolysis :
If two electronic conductors called "electrodes" are
placed in contact with an electrolyte consisting of ions X+ and
Y- , and an electrical potential is applied to the electrodes – for
example by means of a battery- then the positively charged ions
X+ will be attracted by the negatively charged electrode, and the
negative ions Y- by the positive charged electrode. The
positively charged ions X+ are called cations and the negatively
charged ions Y- anions. The electrode to which the cations are
attracted is called the cathode, and the electrode to which the
anions are attracted is called the anode as in fig(1). This process
known as electrolysis occurs whether the electrolyte is an
aqueous solution or a molten whether the ions are complex or
simple.
Fig. 1
The cations when they reach the cathode show atendency to pick
up electrons forming an electrically neutral species X which,
"discharges" at the cathode.
The anions lose electrons and discharges as the electrically
neutral
species
Y
on
reaching
the
anode.
We can define a cathode as an electrode at which electrons
tend to be consumed by the discharging species and an anode as
an electrode at which electrons tend to be produced as a result of
the electrode reaction. In this process electrons flow from the
anode to the cathode via the battery in the opposite direction to
the convential current I.
M. Faraday studied the electrolytic decomposition of aqueous
electrolytes the following laws relating the quantity of electricity
passed (the product of the current and the time during which it
was passed) to the amount substance discharged at an electrode:
Faraday's first Law of Electrolysis: " The amount of any
substance discharged or dissolved at an electrode is proportional
to the quantity of electricity passed.
Faraday's Second Law of Electrolysis: "If the same quantity of
electricity is passed through a variety of electrolytes, the
amounts of different substances discharged or dissolved at the
electrods is proportional to the chemical equivalent weights of
those substances.
"Conduction in Electrolytes"
Oh'm Law is obeyed by electrolytic conductors, where:
the potential difference applied a cross the conductor.
the current flowing.
the resistance of the conductor.
Increasing temperature increases the rate of diffusion of
the ions in the electrolyte, and therefore decreases the resistance
in contrast to the increase in resistance of clectronic conductors
with temperature.