We have known in the above sections that, converting chemical
energy into heat energy by burning of fuel can be utilized in different
ways. Now we will learn how to convert chemical
energy directly to electrical energy and how to do chemical reaction
using electrical energy. Luigi Galvani and Alessandria Volta become able first
to convert chemical
energy into electrical energy. Galvani in the year 1780 and Volta in the
year 1800 have understood by separate experiment that, electricity can be
produced from the redo reaction which occurs spontaneously. In fact we have got
battery from their invention. Thus Galvanic
cell (which is known as Voltaic cell) is a kind of electrochemical
cell by which electrical energy can be produced from chemical
energy. On the other hand, chemical reaction can be done in
electrochemical cell using electrical energy. It is called electrolysis. The cell
where electrolysis is done
called electrolytic cell. Electrochemical cell is composed of different small
parts as electrode, salt bridge, and electrolytic solution. Different topic of
electrochemical cell is discussed below.
Electrical conductor and electrode
Conductor: The
materials in which electricity can pass, are called electrical conductor and
the materials in which electricity cannot pass, are called electrical
insulator. Metal, carbon, graphite, molted salt and acid, solutions of alkali
and salt etc. work as electrical conductor. Depending upon the mechanism of
electrical conductivity conductors are divided into two classes such as, (1)
electronic and (2) electrolytic conductor. The conductor which passes
electricity through electron flow is called electronic conductor such as, all
metal and graphite. If electricity pass through ions of the conductor that is
called electrolytic conductor such as, molten salt and acid, solutions of alkali
and salt etc.
Electrode:
Electrode
is a metallic and nonmetallic electric conductor. These are called electronic
conductor. Electrode connect the electronic conductor and solution (which convey
ions) of electrochemical cell. Two electrodes are needed to construct a electrochemical
cell. One is anode and another is cathode.
In anode: (1) Oxidation reaction occurred. (2) Electrons of anion
of solution transfer to metallic rod (Anode).
In cathode: (1) Reduction reaction occurred. (2) Actions of
solution accept electrons of metallic rod (Cathode).
In electrolytic cells all metallic rod or graphite rod are used as
anode and cathode. In this cell the same metallic rod or different metallic
rods can be used as anode or cathode. Metallic rod only works as electron
conveyor and do not take part in any reaction. The metallic rod which is connected
to the positive end of the battery, used in electrolytic cell, work as anode
and the rod which is connected to the negative end of the battery, used in
electrolytic cell, work as cathode. The method of construction of anode and cathode
in Galvanic cell is different from the electrolytic cell. Electrode is
constructed by placing a metallic rod into the electrolytic solution of that
metal. In Galvanic cell different metallic rod is used as anode and cathode.
(Anode and cathode can also be constructed by using the same metallic rod in
two electrolytes of different density, you will learn about this in the next
class). Anode and cathode of Galvanic cell is determined by the re-activity of
metal. The more reactive metal acts as anode and the less reactive metal acts
as cathode between the metal used as electrode.
Metal/Metal ion Electrode
There are different types of electrodes. Among them metal/metal
ion electrode is the best. If metallic rod is dipped in the solution of that
metal then it is called metal/metal ion electrode such as- Copper rod is dipped
in the solution of copper sulfate, then it is called copper/copper(II) or Cu │Cu2+(aq) electrode.
Similarly, Ag │Ag+(aq),
Zn │Zn2+(aq)
metal/metal ion electrodes are remarkable examples.
Electrode reaction
We have known about the metal│metal ion
electrode above. We can write the electrode Ag│Ag+(aq) as-
Ag(s)
= Ag+ (aq) + e-
Metal│metal
ion reactions always reversible. That is in electrode reaction metal Ag(s) converts
to Ag+(aq) ion donating electron and dissolve in solution. On the
other hand, if one electron can give to the Ag+(aq) ion, than Ag+(aq)
ion will convert to metallic Ag(s). Thus electrode reaction is redox reaction.
That means in any electrode reaction there will be donating or accepting of
electron. But we know that oxidation-reduction occur simultaneously. If
one electrode donate electron (oxidation) than, does another electrode not
required to accept the electron? Actual matter is that. There are two
electrodes for electrochemical reactions- anode and cathode. In electrochemical
reactions the electrode which donate electron to electrolytic substance is
called cathode, again the electrode which accept electron from electrolytic
substance is called anode. Electrode reaction can be spontaneous. Otherwise,
electrode reaction is completed by supplying electricity to the electrode.
Anode reaction: M → M+ + e-
Cathode reaction: X + e- → X-
End
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