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EXPT 14. Electrochemical Cells
[Key Contents]
- ion, electrolyte, electric conductivity
- electrochemical series, ionization potential, electron affinity
- oxidation-reduction, Daniell cell, standard reduction potential
- Nernst equation
- solubility product
[References]
Principles of Modern Chemistry, 6th Ed. (Oxtoby et al.)
Ch 17. Electrochemistry
Chemistry for Life, Chemistry for Better Life (Kim et al.)
Ch 9. Equilibrium Reactions
[Goal]
- to witness the electrical nature of matter
- to verify the electrochemical series
- to learn to construct a Daniell cell and measure cell potential
- to verify the concentration-dependence of the cell potential and
use it to determine solubility product
[Background]
To understand the behavior of electrons in molecules is to undertand
chemistry.
Electrons
are
directly
involved
in
chemical
bonding.
Electrons play a central role in oxidation-reduction and acid-base
chemistry.
Electron is a mysterious particle. It is 1840 times lighter than
proton; yet the magnitude of its negative charge is the same as that
of the positive charge of the proton. It is hard to say where it is; yet
it directs where the protons should be in a molecule in terms of the
distance
and
angle
between
molecules cannot even exist.
atomic
nuclei.
Without
electrons,
That electron is light and mobile is why we depend on electronics
to make radio, TV, computer, semiconductor, mobile phones, etc. Of
many devices that depend on electron, the electrochemical cell has
profound
historical
and
practical
implications.
Even
before
electromagentic induction was discovered by Faraday, electrochemical
cell had been discovered by Galvani and Volta. And electrochemical
cell had been used in electrolysis of water and in the discovery of
several metallic elements prominently by Humphrey Davy.
The driving force of an electrochemical cell is the difference in
affinity to electrons among different elements. Such a difference is
also responsible for other key physical and chemical properties of
various elements. If electrons are equally shared among all atoms in
a compound, we will not have the wonderful world around us.
We can make electrons flow in a circuit using elements of
different electron affinity. The difference creates an electric potential
and makes an electrochemical cell possible.
The cell potential depends on the concentration of the chemical
species involved in the following manner, called the Nernst equation.
Ecell is the cell potential and n is the number of electrons involved.
Ecell = Eocell - (0.0592/n) logQ
(values in volts, at 25 oC)
Thus one can determine the concentration ratio of the chemical
species involved from the potential measurement. In this experiment,
you will see how it works in determining the solubility product of a
sparingly soluble salt.
[Apparatus and Chemicals]
Expt 1. battery, cable, LED, sugar, salt, orange, distilled water
Expt 2. Cu, Zn, Pb plate, 1.0 M Zn(NO3)2 soln, 1.0 M Pb(NO3)2 soln,
1.0 M Cu(NO3)2 soln
Expt 3. Zn, Pb plate, salt bridge, beaker, voltmeter, cable, sand
paper, 0.1 M Zn(NO3)2 soln, Cu(NO3)2 soln (0.1 M, 0.01 M, 0.001 M),
0.010 M AgNO3 soln, 0.020 M (NO3)2 soln, KCl
Expt 4. Zn plate, silver wire, salt bridge, beaker, voltmeter, cable
0.010M AgNO3 soln, 0.020M Zn(NO3)2 soln, KCl
[Procedure]
Expt 1. Electrical Conductivity
1) Connect LED to the battery and check if the light goes on.
2) Connect distilled water in a beaker as part of the circuit and
see if the light is turned on.
3) Connect sugar in a beaker as part of the circuit and see if the
light is turned on.
4) Add distilled water to the sugar and mix. Stir the solution and
see if the light is turned on.
5) Connect salt in a beaker as part of the circuit and see if the
light is turned on.
6) Add distilled water to the salt and mix. Stir the solution and
see if the light is turned on.
7) Couple a mandarin orange to the circuit and see if the light is
turned on.
Expt 2. Electrochemical Series
1) Prepare three 1 cm x 0.5 cm plates of copper, zinc and lead
and polish the surface with sand paper.
2) Half-fill three mixing wells with 1.0 M Zn(NO3)2 soln. Immerse
the copper, zinc and lead plates about half-way in each of the wells
and note changes.
3) Repeat using 1.0 M Pb(NO3)2 soln and 1.0 M Cu(NO3)2 soln.
Expt 3. Electrochemical Cell
1) Cut 1 cm x 7 cm plates of zinc and copper. Polish the surface
with sand paper.
2) Fill a 100 mL beaker with about 80 mL of 1.0 M Zn(NO3)2 soln
and immerse the zinc plate about half-way. Fill another 100 mL
beaker with 1.0 M Cu(NO3)2 soln and immerse the copper plate.
Connect the two solutions with a salt bridge.
3) Complete the circuit with a DC voltmeter between the zinc
plate and the copper plate and record the measured voltage of the
cell.
4) Similarly, measure the standard cell voltage for the Zn-Pb
couple as well as the Cu-Pb couple.
5) Dilute the Cu(NO3)2 solution successively to 0.1 M, 0.01 M, and
0.001 M. Connect each solution with 0.1 M Zn(NO3)2 solution to
construct a Daniell cell. Measure the voltage and compare with the
expected value. Wash the electrode with a diluted solution before
voltage measurement.
Expt 4. Solubility Product of AgCl
1) Construct a Daniel cell using 50 mL 0.010 M AgNO3 solution
and 0.020 M Zn(NO3)2 solution and measure voltage.
Zn(s) | Zn2+(aq) || Ag+(aq) | Ag(s)
2) Measure the voltage and compare with the expected value.
3) Salt forms when you add solid KCl to silver nitrate solution.
Adjust the final concentration of potassium ion to 0.030 M and
measure the voltage. Stir the solution after adding KCl so that the
concentration is uniform.
[Data Analysis]
Expt 1. Electrical Conductivity
1) Describe electrical conductivity of water, sugar, salt, and their
solutions.
2) What is the conclusion you can draw about the difference in
electronegativity between sodium and chlorine? How about between
hydrogen, carbon, and oxygen? Justify your conclusion by referring to
the literature values of electronegativity.
3) Is it fair to say that ionic bond is an extreme case of a polar
covalent bond?
Expt 2. Electrochemical Series
Arrange Cu, Zn, and Pb in the order of increasing reduction
potential.
Expt 3. Electrochemical Cell
In each case, plot measured voltage against 0.0592 logQ and
determine n.
Expt 4. Solubility Product of AgCl
Determine Ksp of AgCl from measured voltage and compare with
the book value.
[Additional Material]
1. Daniell Cell, Voltaic Pile
Why is Zn and Cu commonly used?
2. Nernst equation
Let's use Nernst equation to calculate the cell voltage of the
following cell. The standard voltage is 1.10V.
Zn(s) | Zn2+(1.00 x 10-5 M) || Cu2+ (0.100M) | Cu(s)
Zn(s) + Cu2+(aq) ↔ Zn2+(aq) + Cu(s)
2+
2+
-4
Q =[Zn ]/[Cu ] = 1.00 x 10
Ecell = Eocell - (0.0592/n) log Q
= 1.10 - (0.0592/2) log (1.00 x 10-4)
= 1.10 - (-0.120)
= 1.22 (V)
n = 2