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The
faraday and electrochemical calculations
By
Francine Taylor-Campbell, Contributor
YOU
SHOULD BE ABLE TO:
*
Define the Faraday constant.
*Perform
calculations involving the mole
POINTS
TO NOTE:
The
Faraday (F) is the quantity of electricity
needed to remove one mole of electrons
from the cathode, during electrolysis
or to deposit one mole of electrons
on the anode during electrolysis.
The
Faraday Constant is the amount of
electric charge carried by one mole
of electrons, that is 96,500C
During
electrolysis electrons are taken from
the cathode by positive ions called
cations.
Eg.
2H+(aq)+ 2e = H2(g) Cu2+(aq)+ 2e =
Cu(s)
The
ions are, said to be, discharged.
Electrons
are deposited on the anode (+ve) by
negative ions called anions.
Eg.
2Cl-(aq)= Cl2(g) +2e 4OH- = 2H20 +
02 + 4e
Electrons
may also leave from the anode if the
anode dissolves.
Eg.
Cu = Cu2+ + 2e-
One
Faraday = 96,500 Coulombs i.e. 1F
= 96,500 C
The
coulomb is the unit of electrical
charge and is 1ampere flowing for
1 second(s).
i.e.
coulombs = amps x sec, quantity of
electricity = current x time (Q =
I x t)
Eg.
When 2 amps flow for 1 minute, the
quantity of electricity flowing (Q)
Q
= 2*60 = 120C
NOTE
The
Faraday may also be regarded as the
charge on 1mole of electrons.
Thus
F = Le L = Avogadro's number e = the
charge on one electron
Further
Examples.
What
mass of copper would be deposited
during electrolysis by 0.5F?
Cu2+
+ 2e = Cu
2F
64g
thus
0.5F = 16g Cu
Eg.
What mass of lead, would be produced
by a current of 5A, passed through
molten lead bromide for 1hr?
C=
A x s C= 5 x 60 x 60 = 18,000C
Now
Pb2+(l) + 2e = Pb(l)
2F
207g
193,000C
= 207g Pb
Thus,
18,000C = (207/ 193,000) x 18,000
= 19.2g Pb
Eg.
What volumes of (a) H2 (b) O2 would
be liberated at R.T.P when 0.1 F is
passed through dilute sulphuric acid?
4H+
+ 4e- = 2H2 4OH- = 2H2O + O2 + 4e-
Calculate
the volume of H2 for example 2H+ +
2e = H2
2F
1mole = 24 dm3 at R.T.P
Thus
0.1F = (24/2) x 0.1 dm3 = 1.2 dm3
H2 at
R.T.P
And volume of O2 = 0.6 dm3 at R.T.P
( from above equations).
*
Francine Taylor-Campbell teaches at
Munro College in St. Elizabeth and
is an assistant examiner with CXC.
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