Alessandro Volta
(18 February 1745 – 5 March 1827)
The the Inventor of first Electrical Battery
Simple Voltaic Cell
The Simplest arrangement for producing an electric current is that known as the voltaic cell, so named after Volta its discoverer.
(First Model of Voltaic Cell)
This consist of a vessel containing some very dilute sulfuric acid(H2SO4) into which is dipped a plate of zinc(Zn) and one of Copper(Cu). With such an arrangement there is at once setup a potential difference between the two plates, and if they are joined by a wire an electric current will at once flow from the positive zinc to negative copper.
The voltage of an electrochemical cell involves an oxidation
reaction and a reduction reaction. In the case of a copper-zinc cell
the two half reactions are written as reductions, and their standard
cell potentials are:
Cu2+(aq) + 2e- ---> Cu(s) E° reduction = +0.34 V
Zn2+(aq) + 2e- ---> Zn(s) E° reduction = -0.76 V
Copper will be the reduced species in the copper-zinc cell due
it's larger standard reduction potential. The voltage then for a Zn,
Zn2+||Cu2+, Cu cell in which the copper ion and
zinc ion concentrations are equal is found as follows:
Zn(s) + Cu2+(aq) --->
Cu(s) + Zn2+(aq)
equation 1
Cu2+(aq) + 2e- ---> Cu(s)
E° reduction = +0.34 V
Zn(s) ---> Zn2+(aq) + 2e-
E° oxidation = -0.76 V
Ecell = Ereduction - Eoxidation
+1.10V = +0.34 V - (-0.76 V)
The voltage of any electrochemical cell is a function of the molar
concentrations of the compounds involved in the cell, as described by
the Nernst equation:
aA(s) + bB+(aq) --->
cC(s) + dD+(aq)
Ecell = E° - (0.06/n) log
{[D+]d/[B+]b}
Using equation 1 as the balanced chemical equation for the Zn,
Zn2+||Cu2+, Cu cell. Equation 3 becomes:
Ecell = E° - (0.03) log
[Zn2+]/[Cu2+]
As the Cu2+ concentration decreases the voltage of the
cell changes. The Cu2+ concentration can change due to a
dilution, or due to the formation of a complex ion,
Cu(NH3)42+, upon addition of
NH3.
The action
of the cell is explained in terms of the motion of the charged ions. At
the zinc rod, the zinc atoms get ionized and pass into solution as Zn++
ions. This leaves the zinc rod with two electrons more, making it
negative. At the same time, two hydrogen ions (2H+) are discharged at
the copper rod, by taking these two electrons. This makes the copper
rod positive. As long as excess electrons are available on the zinc
electrode, this process goes on and a current flows continuously in
external circuit. This simple cell is thus seen as a device which
converts chemical energy into electrical energy. Due to opposite charges
on the two plates, a potential difference is set up between copper and
zinc, copper being at a higher potential than zinc. The difference of
potential between the two electrodes is 1.08V.
