Transcript 21.2 Half-Cells and Cell Potentials

ELECTROCHEMICAL CELLS
Chapter 20 : D8 C20
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21.2 Half-Cells and Cell Potentials >
Electrical Potential
Electrical Potential
What causes the electrical
potential of an
electrochemical cell?
• The electrical potential of a voltaic
cell is a measure of the cell’s ability
to produce an electric current.
• Electrical potential is usually measured in
volts (V).
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Electrochemical Reactions
Electrons are transferred from Al to Cu2+, but
there is no useful electric current. Energy
released as HEAT.
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If Al and Cu are separated work is
done by the electrons. Voltmeter is
used to measure the energy.
What
determines
the
measured
value?
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Terms Used for Voltaic Cells
Figure 20.3
The
Cu|Cu2+
and
Ag|Ag+
Cell
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21.2 Half-Cells and Cell Potentials >
Electrical Potential
The potential of an isolated halfcell cannot be measured.
• You cannot measure the electrical
potential of a zinc half-cell or of a
copper half-cell separately.
• When these two half-cells are
connected to form a voltaic cell,
however, the difference in potential
can be measured.
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CELL POTENTIAL, E
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1.10 V
Zn and Zn2+,
anode
Cu and Cu2+,
cathode
1.0 M
1.0 M
• For Zn/Cu cell, this is indicated by a
voltage of 1.10 V at 25 ˚C and when
[Zn2+] and [Cu2+] = 1.0 M.
CELL POTENTIAL, E
a quantitative measure of the tendency of
reactants to proceed to products when all are
in their standard states at 25 ˚C.
STANDARD CELL POTENTIAL, Eo
is when [Zn2+] and [Cu2+] = 1.0 M.
For Zn/Cu cell, potential is +1.10 V
at 25 ˚C
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21.2 Half-Cells and Cell Potentials >
Electrical Potential
The standard hydrogen electrode
is used with other electrodes so the
reduction potentials of the other cells
can be measured.
• The standard reduction potential
of the hydrogen electrode has
been assigned a value of 0.00 V.
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CELL POTENTIALS, Eo
Can’t measure 1/2 reaction Eo
directly. Therefore, measure it
relative to a STANDARD
HYDROGEN CELL.
Eo = 0.0 V
SHE
2 H+(aq, 1 M) + 2e- <----> H2(g, 1 atm)
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21.2 Half-Cells and Cell Potentials >
Electrical Potential
The difference between the reduction
potentials of the two half-cells is called
the cell potential.
reduction potential
– –
of half-cell in which
reduction occurs
cell potential =
or
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reduction potential
of half-cell in which
oxidation occurs
Ecell = Ered – Eoxid
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Zn/Zn2+ half-cell hooked to a SHE.
Eo for the cell = +0.76 V
Negative
electrode
Supplier
of
electrons
Zn --> Zn2+ + 2eOxidation
Anode
Positive
electrode
Acceptor
of
electrons
2 H+ + 2e- --> H2
Reduction
Cathode
Reduction of H+ by Zn
No current
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Overall: reduction of H+
by Zn metal.
Zn(s) + 2H+(aq) --> Zn2+ + H2(g) Eo =+0.76 V
Therefore, oxidation potential
Eo for Zn ---> Zn2+ (aq) + 2e- is +0.76 V
Cu/Cu2+ and H2/H+ Cell
Eo = +0.34 V
Positive
Acceptor
of
electrons
Cu2+ + 2e- --> Cu
Reduction
Cathode
Negative
Supplier
of
electrons
H2 --> 2 H+ + 2eOxidation
Anode
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Cu/Cu2+
and
H2/H+
Cell
Overall reaction
is reduction of
Cu2+ by H2 gas.
Cu2+ (aq) + H2(g) ---> Cu(s) + 2 H+(aq)
Measured Eo = +0.34 V
Therefore, reduction potential
Eo for Cu2+ + 2e- ---> Cu is +0.34 V
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Calculating Cell Voltage
• Balanced half-reactions can be added
together to get overall, balanced
equation.
Zn(s) ---> Zn2+(aq) + 2eCu2+(aq) + 2e- ---> Cu(s)
-------------------------------------------Cu2+(aq) + Zn(s) ---> Zn2+(aq) + Cu(s)
If we know Eo for each half-reaction, we
could get Eo for net reaction.
Zn/Cu Electrochemical Cell
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+
Anode,
negative,
source of
electrons
Cathode,
positive,
sink for
electrons
Zn(s) ---> Zn2+(aq) + 2eEo = +0.76 V
Cu2+(aq) + 2e- ---> Cu(s)
Eo = +0.34 V
--------------------------------------------------------------Cu2+(aq) + Zn(s) ---> Zn2+(aq) + Cu(s)
Eo (calc’d) = +1.10 V
21.2 Half-Cells and Cell Potentials >
Electrical Potential
The tendency of a given half-reaction
to occur as a reduction is called the
reduction potential.
• The half-cell in which reduction
occurs has a greater reduction
potential than the half-cell in which
oxidation occurs.
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Using Standard Potentials, Eo
Table 20.1
• Which is the best oxidizing agent (reduced):
O2, H2O2, or Cl2? _________________
• Which is the best reducing agent (oxidized):
Hg, Al, or Sn? ____________________
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21.2 Half-Cells and Cell Potentials >
Electrical Potential
The standard cell potential (E0cell) is the
measured cell potential when the ion
concentrations in the half-cells are 1M,
any gases are at a pressure of 101 kPa,
and the temperature is 25°C.
0
E cell
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=
0
E red
–
0
E oxid
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21.2 Half-Cells and Cell Potentials >
Calculating Standard
Cell Potentials
Calculating Standard Cell Potentials
How can you determine if a redox
reaction is spontaneous?
Positive Cell Potentials
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21.2 Half-Cells and Cell Potentials >
Calculating Standard
Cell Potentials
If the cell potential for a given
redox reaction is positive, then
the reaction is spontaneous as
written.
If the cell potential is negative,
then the reaction is
non-spontaneous.
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Uses of Eo Values
• Organize halfreactions by
relative ability to
act as oxidizing
agents (reduced)
• Table 20.1
• Use this to predict
cell potentials and
direction of redox
reactions.
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REDUCTION
POTENTIALS
21.2 Half-Cells and Cell Potentials >
Determine the cell reaction for a voltaic
cell composed of the following half-cells.
Fe3+(aq) + 3e– → Fe(s) E0cell = –0.036 V
Al3+(aq) + 3e– → Al(s) E0cell = –1.66 V
Oxidation:
Al(s) → Al3+(aq) + 3e–
(at anode)
Reduction:
Fe3+(aq) + 3e– → Fe(s)
(at cathode)
Al(s) + Fe3+(aq) → Al3+(aq) + Fe(s)
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21.2 Half-Cells and Cell Potentials >
Sample Problem 21.1
Determining Reaction Spontaneity
Zn(s) + 2Ag+(aq) → Zn2+(aq) + 2Ag(s)
Show that the following redox reaction
between zinc metal and silver ions is
spontaneous.
If E0cell is positive, the reaction is spontaneous.
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21.2 Half-Cells and Cell Potentials >
Sample Problem 21.1
• First identify the half-reactions.
Oxidation: Zn(s) → Zn2+(aq) + 2e–
Reduction: Ag+(aq) + e– → Ag(s)
• Write both half-cells as reductions with
their standard reduction potentials.
Zn2+(aq)
+
2e–
→ Zn(s)
Ag+(aq) + e– → Ag(s)
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0
E
Zn
2+
= –0.76 V
E0Ag+ = +0.80 V
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21.2 Half-Cells and Cell Potentials >
Sample Problem 21.1
• Calculate the standard cell potential.
0
E
0
cell = E
red – E
0
0
oxid = E
+
Ag
–E
0
Zn
2+
= +0.80 V – (–0.76 V) = +1.56 V
0
E cell
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> 0, so the reaction is spontaneous.
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More About
Calculating Cell Voltage
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Assume I- ion can reduce water.
2 H2O + 2e- ---> H2 + 2 OHCathode: Red
2 I- ---> I2 + 2eAnode: Ox
------------------------------------------------2 I- + 2 H2O --> I2 + 2 OH- + H2
Assuming reaction occurs as written,
E˚net = E˚cathode - E˚anode or = E˚red + E˚ox
= (-0.828 V) - (+0.535 V) = -1.363 V
Minus E˚ means rxn. occurs in opposite
direction
Using Standard Potentials, Eo
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Table 20.1
• In which direction do the following
reactions go?
Cu(s) + 2 Ag+(aq) ---> Cu2+(aq) + 2 Ag(s)
What is Eonet for the overall reaction?
Standard Redox Potentials, Eo
E˚net = “distance” from “top” half-reaction
(cathode) to “bottom” half-reaction (anode)
E˚net = E˚cathode - E˚anode
Eonet for Cu/Ag+ reaction = +0.46 V
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Eo
for a Voltaic Cell
Cd --> Cd2+ + 2eor
Cd2+ + 2e- --> Cd
Fe --> Fe2+ + 2eor
Fe2+ + 2e- --> Fe
All ingredients are present. Which way does
reaction proceed?
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21.2 Half-Cells and Cell Potentials >
In a voltaic cell in which one half-reaction is
Fe3+ + 3e– → Fe, which of the following
would occur as oxidation?
A. 2H+ + 2e– → H2
B. Al3+
+
3e–
→ Al
B.
C. Br2 + 2e– → 2Br–
D. Fe3+ + e– → Fe2+
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21.2 Half-Cells and Cell Potentials >
Key Concepts
The electrical potential of a cell results
from a competition for electrons between
two half-cells.
You can determine the standard reduction
potential of a half-cell by using a standard
hydrogen electrode and the equation for
standard cell potential.
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21.2 Half-Cells and Cell Potentials >
Key Concepts &
Key Equation
If the cell potential for a given redox
reaction is positive, then the reaction is
spontaneous as written. If the cell
potential is negative, then the reaction is
nonspontaneous.
E0cell = E0red – E0oxid
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21.2 Half-Cells and Cell Potentials >
Glossary Terms
• electrical potential: the ability of a voltaic cell
to produce an electric current
• reduction potential: a measure of the
tendency of a given half-reaction to occur as a
reduction (gain of electrons) in an
electrochemical cell
• cell potential: the difference between the
reduction potentials of two half-cells
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21.2 Half-Cells and Cell Potentials >
Glossary Terms
• standard cell potential (E0cell): the measured
cell potential when the ion concentrations in
the half-cells are 1.00M at 1 atm of pressure
and 25°C
• standard hydrogen electrode: an arbitrary
reference electrode (half-cell) used with
another electrode (half-cell) to measure the
standard reduction potential of that cell; the
standard reduction potential of the hydrogen
electrode is assigned a value of 0.00 V
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RED = REDUCED
e-
BLACK = OXIDIZED