ELECTROCHEMISTRY
Chapter 21
Electric automobile
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2
TRANSFER REACTIONS
Atom transfer
HOAc + H2O ---> OAc- + H3O+
Electron transfer
Cu(s) + 2 Ag+(aq) ---> Cu2+(aq) + 2 Ag(s)
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Electron Transfer Reactions
• Electron transfer reactions are oxidationreduction or redox reactions.
• Redox reactions can result in the
generation of an electric current or be
caused by imposing an electric current.
• Therefore, this field of chemistry is often
called ELECTROCHEMISTRY.
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4
Review of Terminology for
Redox Reactions
• OXIDATION—loss of electron(s) by a
species; increase in oxidation number.
• REDUCTION—gain of electron(s);
decrease in oxidation number.
• OXIDIZING AGENT—electron acceptor;
species is reduced.
• REDUCING AGENT—electron donor;
species is oxidized.
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OXIDATION-REDUCTION
REACTIONS
Direct Redox Reaction
Oxidizing and reducing agents in direct
contact.
Cu(s) + 2 Ag+(aq) ---> Cu2+(aq) + 2 Ag(s)
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OXIDATION-REDUCTION
REACTIONS
Indirect Redox Reaction
A battery functions by transferring electrons
through an external wire from the reducing
agent to the oxidizing agent.
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Why Study Electrochemistry?
• Batteries
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Why Study Electrochemistry?
• Batteries
• Corrosion
A rusted car.
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Why Study Electrochemistry?
• Batteries
• Corrosion
• Industrial
production of
chemicals such as
Cl2, NaOH, F2 and
Al
A rusted car.
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Why Study Electrochemistry?
• Batteries
• Corrosion
• Industrial
production of
chemicals such as
Cl2, NaOH, F2 and
Al
• Biological redox
reactions
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The heme group
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Electrochemical Cells
• An apparatus that allows
a redox reaction to occur
by transferring electrons
through an external
connector.
• Product favored reaction
---> voltaic or galvanic cell
----> electric current
• Reactant favored reaction
---> electrolytic cell --->
electric current used to
cause chemical change.
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Batteries are voltaic
cells
CHEMICAL CHANGE -->
ELECTRIC CURRENT
Zn
Zn metal
metal
With time, Cu plates out
onto Zn metal strip, and
Zn strip “disappears.”
2+ ions
Cu
Cu2+
ions
•Zn is oxidized and is the reducing agent
Zn(s) ---> Zn2+(aq) + 2e•Cu2+ is reduced and is the oxidizing agent
Cu2+(aq) + 2e- ---> Cu(s)
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CHEMICAL CHANGE -->
ELECTRIC CURRENT
Zn
Zn metal
metal
2+ ions
Cu
Cu2+
ions
Oxidation: Zn(s) ---> Zn2+(aq) + 2eReduction: Cu2+(aq) + 2e- ---> Cu(s)
-------------------------------------------------------Cu2+(aq) + Zn(s) ---> Zn2+(aq) + Cu(s)
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CHEMICAL CHANGE -->
ELECTRIC CURRENT
Zn
Zn metal
metal
2+ ions
Cu
Cu2+
ions
Electrons are
transferred from Zn to
Cu2+, but there is no
useful electric current.
Oxidation: Zn(s) ---> Zn2+(aq) + 2eReduction: Cu2+(aq) + 2e- ---> Cu(s)
-------------------------------------------------------Cu2+(aq) + Zn(s) ---> Zn2+(aq) + Cu(s)
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15
CHEMICAL CHANGE -->
ELECTRIC CURRENT
•To obtain a useful
current, we separate the
oxidizing and reducing
agents so that electron
transfer occurs thru an
external wire.
•This is accomplished in a
GALVANIC or VOLTAIC
cell.
•A group of such cells is
called a battery.
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wire
e le c t ro ns
Zn
Zn 2+ ions
salt
bridge
Cu
Cu2+ ions
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wire
e le c t ro ns
Zn
Zn2+ ions
salt
bridge
Cu
Cu2+ ions
•Electrons travel thru external wire.
•Salt bridge allows anions and cations to
move between electrode compartments.
•This maintains electrical neutrality.
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Electrons move
from anode to
cathode in the wire.
Anions & cations
move thru the salt
bridge.
Electrochemical
Cell
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CELL POTENTIAL, E
Zn and Zn2+,
anode
Cu and Cu2+,
cathode
• Electrons are “driven” from anode to
cathode by an electromotive force or emf.
• 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.
Copyright © 1999 by Harcourt Brace & Company
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19
CELL
POTENTIAL, E
• For Zn/Cu cell, voltage is 1.10 V at 25  C and
when [Zn2+] and [Cu2+] = 1.0 M.
• This is the STANDARD CELL
POTENTIAL, Eo
• —a quantitative measure of the tendency of
reactants to proceed to products when all
are in their standard states at 25  C.
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20
Calculating Cell Voltage
• Balanced half-reactions can be added
together to get overall, balanced
equation.
2 I- ---> I2 + 2e2 H2O + 2e- ---> 2 OH- + H2
------------------------------------------------2 I- + 2 H2O --> I2 + 2 OH- + H2
• If we know Eo for each half-reaction, we
could get Eo for net reaction.
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21
CELL POTENTIALS,
o
E
• Can’t measure 1/2 reaction Eo directly.
Therefore, measure it relative to a
STANDARD HALF CELL, SHE.
2 H+(aq, 1 M) + 2e- --> H2(g, 1 atm)
Eo = 0.0 V
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22
Zn/Zn2+ half-cell hooked to a SHE.
Eo for the cell = +0.76 V
Volts
-
Zn
+
Salt Bridge
H2
Zn2+
Zn
Zn2+ + 2eOXIDATION
ANODE
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H+
2 H+ + 2eH2
REDUCTION
CATHODE
23
Zn/Zn2+ half-cell hooked to a SHE.
Eo for the cell = +0.76 V
Volts
-
Zn
+
Salt Bridge
H2
Zn2+
Zn
Zn2+ + 2eOXIDATION
ANODE
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H+
2 H+ + 2eH2
REDUCTION
CATHODE
24
Volts
Zn
-
+
Salt Bridge
H2
Zn2+
Zn
Zn2+ + 2eOXIDATION
ANODE
H+
2 H+ + 2eH2
REDUCTION
CATHODE
Overall reaction is reduction of H+ by Zn metal.
Zn(s) + 2 H+ (aq) --> Zn2+ + H2(g)
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Eo = +0.76 V
25
Volts
Zn
-
+
Salt Bridge
H2
Zn2+
Zn
Zn2+ + 2eOXIDATION
ANODE
H+
2 H+ + 2eH2
REDUCTION
CATHODE
Overall reaction is reduction of H+ by Zn metal.
Zn(s) + 2 H+ (aq) --> Zn2+ + H2(g)
Eo = +0.76 V
Therefore, Eo for Zn ---> Zn2+ (aq) + 2e- is
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Volts
Zn
-
+
Salt Bridge
H2
Zn2+
Zn
Zn2+ + 2eOXIDATION
ANODE
H+
2 H+ + 2eH2
REDUCTION
CATHODE
Overall reaction is reduction of H+ by Zn metal.
Zn(s) + 2 H+ (aq) --> Zn2+ + H2(g)
Eo = +0.76 V
Therefore, Eo for Zn ---> Zn2+ (aq) + 2e- is
+0.76 V.
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27
Volts
Zn
-
+
Salt Bridge
H2
Zn2+
Zn
Zn2+ + 2eOXIDATION
ANODE
H+
2 H+ + 2eH2
REDUCTION
CATHODE
Overall reaction is reduction of H+ by Zn metal.
Zn(s) + 2 H+ (aq) --> Zn2+ + H2(g)
Eo = +0.76 V
Therefore, Eo for Zn ---> Zn2+ (aq) + 2e- is
+0.76 V.
Zn is a (better) (poorer) reducing agent than H2.
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28
Volts
Zn
-
+
Salt Bridge
H2
Zn2+
Zn
Zn2+ + 2eOXIDATION
ANODE
H+
2 H+ + 2eH2
REDUCTION
CATHODE
Overall reaction is reduction of H+ by Zn metal.
Zn(s) + 2 H+ (aq) --> Zn2+ + H2(g)
Eo = +0.76 V
Therefore, Eo for Zn ---> Zn2+ (aq) + 2e- is
+0.76 V.
Zn is a better reducing agent than H2.
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29
2+
Cu/Cu
and H2
+
/H
Cell
Volts
Cu
+
Salt Bridge
H2
Cu2+
Cu2+ + 2eCu
REDUCTION
CATHODE
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H+
H2
2 H+ + 2eOXIDATION
ANODE
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Cu/Cu2+ and H2/H+ Cell
Eo = +0.34 V
Volts
Cu
+
Salt Bridge
H2
Cu2+
Cu2+ + 2eCu
REDUCTION
CATHODE
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H+
H2
2 H+ + 2eOXIDATION
ANODE
Cu/Cu2+ and H2/H+ Cell
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Volts
Cu
+
Salt Bridge
H2
Cu2+
Cu2+ + 2eCu
REDUCTION
CATHODE
H+
H2
2 H+ + 2eOXIDATION
ANODE
• Overall reaction is reduction of Cu2+ by H2 gas.
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Cu/Cu2+ and H2/H+ Cell
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Volts
Cu
+
Salt Bridge
H2
Cu2+
Cu2+ + 2eCu
REDUCTION
CATHODE
H+
H2
2 H+ + 2eOXIDATION
ANODE
• Overall reaction is reduction of Cu2+ by H2 gas.
• Cu2+ (aq) + H2(g) ---> Cu(s) + 2 H+(aq)
• Measured Eo = +0.34 V
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Cu/Cu2+ and H2/H+ Cell
33
Volts
Cu
+
Salt Bridge
H2
Cu2+
Cu2+ + 2eCu
REDUCTION
CATHODE
•
•
•
•
H+
H2
2 H+ + 2eOXIDATION
ANODE
Overall reaction is reduction of Cu2+ by H2 gas.
Cu2+ (aq) + H2(g) ---> Cu(s) + 2 H+(aq)
Measured Eo = +0.34 V
Therefore, Eo for Cu2+ + 2e- ---> Cu is
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Cu/Cu2+ and H2/H+ Cell
34
Volts
Cu
+
Salt Bridge
H2
Cu2+
Cu2+ + 2eCu
REDUCTION
CATHODE
•
•
•
•
H+
H2
2 H+ + 2eOXIDATION
ANODE
Overall reaction is reduction of Cu2+ by H2 gas.
Cu2+ (aq) + H2(g) ---> Cu(s) + 2 H+(aq)
Measured Eo = +0.34 V
Therefore, Eo for Cu2+ + 2e- ---> Cu is
• +0.34 V
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Cu/Cu2+ and H2/H+ Cell
Volts
Cu
+
Salt Bridge
H2
Cu2+
Cu2+ + 2eCu
REDUCTION
CATHODE
H+
H2
2 H+ + 2eOXIDATION
ANODE
• Eo for Cu2+ + 2e- ---> Cu is +0.34 V
• Now we can calculate Eo for the Zn/Cu cell.
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35
Zn/Cu Electrochemical Cell
wire
elect rons
Zn
Anode,
negative,
source of
electrons
Zn2+ ions
salt
bridge
Cu
Cu2+ ions
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
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36
Uses of Eo Values
These experiments show we can
a)
decide on relative ability of
elements to act as reducing agents (or
oxidizing agents)
b)
assign a voltage to a half-reaction
that reflects this ability.
wire
elect rons
Zn
Zn2+ ions
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salt
bridge
Cu
Cu2+ ions
37
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TABLE OF STANDARD
POTENTIALS
oxidizing
ability of ion
Eo (V)
Cu2+ + 2e-
Cu
+0.34
2 H+ + 2e-
H2
0.00
Zn2+ + 2e-
Zn
-0.76
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reducing ability
of element
39
Standard Redox Potentials, Eo
oxidizing
ability of ion
Eo (V)
Cu2+ + 2e-
Cu
+0.34
2 H+ + 2e-
H2
0.00
Zn2+ + 2e-
Zn
-0.76
reducing ability
of element
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• Any substance on the
right will reduce any
substance higher than it
on the left.
• Zn can reduce H+ and
Cu2+.
• H2 can reduce Cu2+ but
not Zn2+
• Cu cannot reduce H+ or
Zn2+.
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Using Standard Potentials, Eo
• See Table 21.1 or Screen 21.6.
• Which is the best oxidizing agent:
O2, H2O2, or Cl2? _________________
• Which is the best reducing agent:
Hg, Al, or Sn? ____________________
• In which direction does the following
reaction go?
Cu(s) + 2 Ag+(aq) ---> Cu2+(aq) + 2 Ag(s)
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o
E
for a Voltaic Cell
Volts
Cd
Cd2+
Cd --> Cd2+ + 2eor
Cd2+ + 2e- --> Cd
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Fe
Salt Bridge
Fe2+
Fe --> Fe2+ + 2eor
Fe2+ + 2e- --> Fe
41
o
E
for a Voltaic Cell
Volts
Cd
Fe
Salt Bridge
Cd2+
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Fe2+
From the table, you see
• Fe is a better reducing
agent than Cd
• Cd2+ is a better
oxidizing agent than
Fe2+
Overall reaction
Fe + Cd2+
---> Cd + Fe2+
Eo = +0.04 V
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43
o
E
and D
o
G
Eo is related to DGo, the free
energy change for the reaction.
o
DG
= -nF
o
E
where F = Faraday constant
= 9.6485 x 104 J/V•mol
and n is the number of moles of
electrons transferred
Michael Faraday
1791-1867
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44
Michael Faraday
1791-1867
Originated the terms anode,
cathode, anion, cation,
electrode.
Discoverer of
• electrolysis
• magnetic props. of matter
• electromagnetic induction
• benzene and other organic
chemicals
Was a popular lecturer.
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Eo and DGo
DGo = - n F Eo
For a product-favored reaction
Reactants ----> Products
DGo < 0 and so Eo > 0
Eo is positive
For a reactant-favored reaction
Reactants <---- Products
DGo > 0 and so Eo < 0
Eo is negative
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45