The electrochemical
cell where, electrode reaction occur simultaneously, that means no external
energy is required for reaction and chemical
energy is converted to electrical energy is called Galvanic cell. Daniel
cell is a Galvanic cell. In Daniel cell Cu│Cu2+(aq)
metal/metal ion electrode is used as cathode and Zn│Zn2+(aq)
metal/metal ion electrode is used as anode. In (fig-Galvanic cell) a Daniel cell
is demonstrated. Copper rod is dipped in copper sulfate solution as cathode in
a container; in another container zinc rod is dipped in zinc sulfate solution
as anode. A U shaped tube full of inert electrolyte (KCl) solution is dipped
into the two solutions to make contact with the solutions of the containers. Now,
if the two electrodes are connected with wire the following oxidation-reduction
reaction will happen spontaneously.
Anode reaction: Zn(s) → Zn2+ (aq)
+ 2e-
Cathode reaction: Cu2+ (aq) + 2e-
→ Cu(s)
Zn(s)
+ Cu2+ (aq) → Zn2+ (aq) + Cu(s)
That
is Zn anode oxidized by donating electron, dissolves as Zn2+(aq)
ion in solution. On the other hand Cu2+(aq) ion of
solution will settle on the cathode as metallic Cu (s), accepting
electrons from cathode. In fact the produced electrons of anode reach to cathode
by wire and make electronic equilibrium. Thus, if two electrodes are connected with
wire an electron flow will be created from anode to cathode. Electronic flow
means current flow. Thus we have understood, if an electric bulb is connected
with the wire of Daniel cell then the bulb will be enlightened. Now think how
long the electronic flow will continue. Besides this, what will be the condition
of mass of the Zn and Cu rod? Find out thinking yourself and write down in your
exercise book. |
Galvanic cell
|
Let us now think the activity and necessity of salt bridge. We saw
that, Zn2+(aq) ion produced in anode and go to solution,
on the other hand Cu2+(aq) ion of solution settle on
cathode as Cu metal. Thus in anode container there is abundance of Zn2+(aq)
and in cathode container there is deficit of Cu2+(aq)
ion. We know that, any ion (negative or positive) cannot exist freely. That is
a positive ion cannot be produced without the presence of a negative ion and
vice-e-versa. So equivalent amount of anion (sulphate ion) will be required for
the Zn2+(aq) ion, produced in anode, on the other hand as
a result of settling of Cu2+(aq) ion as Cu on the cathode
equivalent amount of negative ion (sulfate ion) will be free in solution. In
fact, if the equilibrium does not exist between the ions of two containers, the
reaction will not occur. So, if salt bridge is added salt bridge containing
positive (K+) and negative (Cl-) ion will remove the
imbalance of positive and negative ions in anode and cathode container.
Formation of Dry cells and Mechanism of Electron Transfer
 |
| Dry cell |
Generally small jar or pot made of Zn is used as anode in dry
cell. The jar is filled with manganese-die-oxide (MnO2) and
electrolytic solute. Making paste of ammonium chloride (NH4Cl) and
zinc chloride (ZnCl2) with water used as electrolyte. Starch is
added to condense the solution. Then filling the zinc jar with paste, cathode
rod is inserted in the middle position. Carbon rod coated with manganese-die-oxide
is used as cathode. If an intersection of dry cell is done, we will see the
carbon rod at the center of the cell and upon that a coat of manganese dioxide and
then condensed paste of starch, ammonium chloride and zinc chloride and at the
outer sphere a layer of zinc plate. We know electricity is produced by electron
flow, and electron flow can be produced by accepting –donating of electron
(oxidation-reduction). Let us look at the mechanism of production of electron
at anode of dry cell and receiving of electron at cathode.
Anode reaction: Zn(s) → Zn2+(aq) +
2e-
Cathode reaction: 2NH4+(aq) +
2MnO2(s) + 2e- → 2NH3(aq) + Mn2O3(s)
+ H2O(l)
Zn(s)
+ 2NH4 + (aq) + 2MnO2(s) → Zn2+
(aq) + 2NH3(aq) + Mn2O3(s)+ H2O(l)
In anode zinc rod is oxidized and produced two electron and zinc
ion. The zinc ion will mix with paste. On the other hand, manganese–die-oxide
in cathode reduced by receiving two electrons produced in anode. Ammonium ion
only helps to complete the reduction process of manganese-die-oxide. Carbon rod
conveys the produced electrons from anode to cathode. We know that the creation
of electric flow is the flow of current. So where current is required, if we
connect the dry cell, above reactions will take place and we will get current.
1.5 volt electric potential is found from dry cell.
Effect of battery on health and environment
We use different types of battery such as- dry cell, mercury
battery, lead-storage battery and lithium ion battery. These batteries are
composed of different metal and metal ions. We have seen above that, Zn rod and
MnO2 are used to make dry
cell. Zn and mercuric oxide (Hg2O)
are used in mercury cell. Again lead storage battery, which we use commonly to
operate microphone is mainly composed of lead (Pb) and lead oxide (PbO). Cobalt
oxide is used in lithium battery. The above mentioned metals are heavy metals.
In respect to chemical properties, such heavy metal and metal compounds used in
battery are toxic and carcinogenic to human body. So if we expose this battery
in open air this toxic metal and metal compounds mix with soil and water and
next they mix with soil and move to plants and crops. Similarly these toxic
chemicals enter into the body of aquatic animals and plants. In this way,
chemical used in battery destroy the balance of metallic substance of water and
soil and enter into our food chain. We may be affected by different disease
along with cancer if we take the food grown in the soil and water polluted by
battery waste. So you would not leave the battery waste anyhow to the open air
at no cost. Collecting the battery waste, recover the used metal and metallic
compounds through proper chemical treatment and may be used to make new
batteries in cyclic order. This is why, protection of environment that is
health protection and financial save both will be possible.
Chemical reaction by using electricity
We have seen that, in Galvanic cell such as Daniel cell, dry
cell battery anode and cathode reaction occurred spontaneously and electrical
energy is produced from chemical
energy.
But many reactions which do not occur spontaneously that can be occurred in electrochemical
cells by supplying
electricity from external source. It can be said easily, as the cell connected
bulb is lightened due to the production of current in Galvanic cells, on the
other hand in such types of cells the source of electrical energy is to connect
with cell instead of bulb. The cells where, electrode reactions are occurred by
using electrical energy are called the electrolytic cells. In electrolytic
cells electric energy converts to chemical
energy. Electroplating in metal, purification of metal, production of new
chemical compound is possible by electrolysis.
End
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