Electrochemical Cell

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Experiment 10
Electrochemical Cell
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What are the goals of this experiment?
To build a Cu-Zn galvanic cell. To study
the effect of changing concentration of the
electrolytes on the voltage output of the
galvanic cell
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Electrochemistry
A study of chemical changes produced by
electric current and with the production of
electricity by chemical reactions.
All electrochemical reactions involve the
transfer of electrons and are therefore
oxidation-reduction reactions.
The reacting system is contained in a cell, and
an electric current enters or exits by
electrodes.
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Electrochemistry
The sites of oxidation and reduction are
separated physically so that oxidation occurs at
one location while reduction occurs at the
other.
Electrons flow from site of oxidation to the site
of reduction. Flow of electrons implies flow of
electric current. Current is usually denoted by
symbol I and unit amperes (amps).
Current I is related to voltage V through,
ohms Law. R is called the resistance.
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Electrochemistry
Cu-Zn galvanic cell
Metallic Cu (s) ----- Electrode Metallic Zinc (s)
----- Electrode CuSO4 (aq) ---------
Electrolyte ZnSO4 (aq) --------- Electrolyte
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What is oxidation?
Current definition
Loss of Electrons is Oxidation (LEO)
Na Na e-
Positive charge represents electron deficiency
ONE POSITIVE CHARGE MEANS DEFICIENT BY ONE
ELECTRON
Oxidation occurs at the Anode
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What is reduction?
Current definition
Gain of Electrons is Reduction (GER)
Cl e- Cl -
Negative charge represents electron richness
ONE NEGATIVE CHARGE MEANS RICH BY ONE ELECTRON
Reduction occurs at the cathode
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Sign conventions in a galvanic cell
A battery has a positive terminal and a negative
terminal
Cathode is assigned a positive () sign
Anode is assigned a negative (-) sign
Anodic Oxidation (AO) and Cathodic Reduction
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Will this setup sustain flow of electrons?
Reducing half, will have negative Charges.
Will have excess anions. SO42-
Oxidizing half, will have positive Charges. Will
have excess cations (Zn2) but deficient in
anions.
No, this set up does not sustain flow of
electrons due to charge build up.
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Will this setup sustain flow of electrons?
We therefore need to provide a pathway for the
anions to flow from the area where they are no
longer needed (where Cu2 is being converted to
neutral Cu) to the area where they are needed
(where neutral Zn is being converted to Zn2). In
other words, the solutions must be connected so
that ions can flow to keep the net charge in each
compartment zero
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How do we connect the two halves?
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How do we connect the two halves?
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Why do electrons migrate spontaneously from one
electrode to the other?
The electrons move through the wire because by
doing so they will move from a higher-energy to
a lower-energy.
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How to identify the oxidizing half-cell and
reducing half-cell?
When two half-cells are connected, the one with
the larger reduction potential (the one with
greater tendency to undergo reduction) acquires
electrons from the half-cell with lower
reduction potential, which is therefore forced
to undergo oxidation. The half-cell with the
higher standard reduction potential acts as the
cathode (Reduction) and the half-cell with the
lower standard reduction potential acts as the
anode (Oxidation).
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What is reduction potential?
It is the voltage that is generated when a
half-cell undergoes reduction.
Where can I obtain these values from?
From the table of standard reduction potentials
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Why is it called standard reduction potential?
Because these voltages were measured, when the
half-cells underwent reduction under a set of
standard conditions in combination with a
standard electrode.
What is a standard electrode?
It is an electrode, whose reduction/ oxidation
potential is known.
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standard reduction potential
A reference electrode has been arbitrarily chosen
and its standard reduction potential has been
assigned a value of exactly 0 V. This reference
electrode is called the standard hydrogen
electrode.
2 H(aq, 1.00 M) 2 e-
H2(g, 1atm) E?
0.00 V
Standard conditions Concentration 1.0
M Pressure 1.0 atm
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standard reduction potential
E?
Units
Volts (V)
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standard reduction potential
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An Example What if I were to build a Cu-Ag
galvanic cell?
If we were to build a galvanic cell, we should
keep in mind that, there are two half-cells. One
of them undergoes oxidation and the other
undergoes reduction.
For that we will need to know the values of
standard Reduction potential of each half cell.
Cu2(aq) 2e- Cu(s) E?
0.34 V
Ag (aq) e- Ag (S) E?
0.80 V
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An Example What if I were to build a Cu-Ag
galvanic cell?
Cu2(aq) 2e- Cu(s) E?
0.34 V (Oxidizing, Anode)
Ag (aq) e- Ag (S) E?
0.80 V (Reducing, Cathode)
So, Ag electrode is the positive() electrode and
Cu electrode is the negative (-) electrode.
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An Example How can I find the Standard reduction
potential of the Cu-Ag galvanic cell?
Cu2(aq) 2e- Cu2(aq) E?
0.34 V (Oxidizing, Anode)
Ag (aq) e- Ag (S) E?
0.80 V (Reducing, Cathode)
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An Example How can I find the potential of the
Cu-Ag galvanic cell under any concentration
condition?
Cu2(aq) 2e- Cu (s) E?
0.34 V (Oxidizing, Anode)
Ag (aq) e- Ag (S) E?
0.80 V (Reducing, Cathode)
Cu(s) Cu2(aq) 2e- 1
Ag (aq) e- Ag (S) 2
2 e- involved in the overall process.
Cu(s) 2Ag Cu2(aq) 2Ag
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Nernst Equation
Where Ecell Voltage measured for the reactant
and product concentrations summarized by the
reaction quotient Q
E?cell Voltage measured when reactant and
product concentrations are 1 M (or 1 atm for
gases) R
gas constant in units appropriate for the system,
8.314 JK-1mol-1 T absolute
temperature in K n number
of moles of electrons transferred in the
oxidation-reduction reaction F
96,485 coul/mol e- ln Q
natural logarithm of the reaction quotient for
the reaction.
Concentration of pure solids are to be omitted.!!
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An Example How can I find the potential of the
Cu-Ag galvanic cell under any concentration
condition?
2 e- involved in the overall process.
Cu(s) 2Ag Cu2(aq) 2Ag(s)
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An Example How can I find the potential of the
Cu-Ag galvanic cell under any concentration
condition?
2 e- involved in the overall process.
Cu(s) 2Ag Cu2(aq) 2Ag(s)
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An Example How can I find the potential of the
Cu-Ag galvanic cell under any concentration
condition?
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An Example How can I find the potential of the
Cu-Zn galvanic cell under any concentration
condition?
1 Cu2(aq) 1 Zn(s) ? 1 Cu(s) 1
Zn2(aq)
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An Example How can I find the potential of the
Cu-Zn galvanic cell under any concentration
condition?
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An Example How does the voltage output of the
Cu-Zn galvanic cell change with change in
concentration of Cu2(aq) and Zn2(aq)?
Zn2(aq) (M) Cu2(aq) (M) Ecell (V)
2.00 1.00 E?cell 8.89 ? 10-3
1.00 1.00 E?cell
1.00 2.00 E?cell 8.89 ? 10-3
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Standard free energy change (?G?) and Free
energy change ?G)
Free energy in this context refers to the energy
that is generated by the movement of electrons
for doing work.

• Stands for change
• G stands for Gibbs free energy

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Units of free energy change