Zn(s)
Figure Number: 20 01 C
2
2 HCl(aq)
ZnCl2(aq)
H2(g)
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2
Zn(s) 2 H (aq)
0
Figure Number: 20 01.01 UN
Zn (aq) H2(g)
1
2
0
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Cd(s) NiO2(s) 2H2O(l)
0
4 2
Figure Number: 20 01.03 UN E20.1
Cd(OH)2(s) Ni(OH)2( s)
1 2
2 2 1
2 2 1
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(a)
Figure Number: 20 02 C
(b)
(c)
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A SPONTANEOUS OXIDATION-REDUCTION REACTION
An atomic-level view of how electrons are transferred in a spontaneous redox reaction.
Atoms in
Zn strip
Cu2 ions
in solution
Zn2 ion
2 e
Cu atom
Zn(s) Cu2(aq)
A strip of zinc is placed in a
solution of copper (II) sulfate. A
Cu2 ion comes in contact with
the surface of the Zn strip and
gains two elecrons from a Zn
atom; the Cu2 is reduced, and
the Zn atom is oxidized.
Figure Number: 20 03 C
Zn2(aq) Cu(s)
As the reaction proceeds, the zinc
dissolves, the blue color due to
Cu2(aq) fades, and copper metal
(the dark material on the zinc
strip and on the bottom of the
beaker) is deposited.
Electrons are transferred from the
zinc to the Cu2 ion, forming Zn2
ions and Cu(s). The resulting
colorless Zn2 ion enters the
solution, and the Cu atom remains
deposited on the zinc strip.
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Figure Number: 20 04
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Switch
e
e
Voltmeter
Zn
anode
NO3
Na
NO3
NO3
Zn2
NO3
Zn(s)
Cu
cathode
NO3
Cu2
Zn2 (aq) 2 e Cu2 (aq) 2 e
Cu(s)
Movement of cations
Movement of anions
Figure Number: 20 05
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Electron flow
Anode
Porous barrier
or salt bridge
Cathode
Anions
Cations
Anode compartment
Oxidation occurs
Figure Number: 20 06
Cathode compartment
Reduction occurs
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Atoms in
Zn strip
Cu2 ions
in solution
Zn2 ion
2e
Cu atom
(a)
Figure Number: 20 07 a, b
(b)
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2e
Cu(s)
Zn(s)
Anode
Cathode
Porous barrier
or salt bridge
2e
2e
Figure Number: 20 08
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High
potential
energy
Flow of electrons
Anode
Cathode
Low
potential
energy
Figure Number: 20 09
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To external circuit
1 atm H2 (g)
Pt wire
Pt atom
H ion
H3Oion
H2 molecule
e
1 M H (aq)
Reduction
Oxidation
Pt electrode
e
(b)
(a)
Figure Number: 20 10 C
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Switch
e
e
Voltmeter
Zn
anode
NO3 Na H2(g)
NO3 Anode
compartment
Zn(s)
Figure Number: 20 11
2
NO3 Zn
NO3 Cathode
compartment
(standard
hydrogen
electrode)
H
Zn2 (aq) 2 e
2 H(aq) 2 e
H2(g)
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More
positive
Ered(V)
Cathode
(reduction)
Ered(cathode)
Ecell
Anode
(oxidation)
Ered(anode)
More
negative
Figure Number: 20 12
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More
positive
Ered(V)
0.34
0.76
Figure Number: 20 13
Cu2 2 e
Cu
Cathode
Ecell (0.34) (0.76)
1.10 V
Anode
Zn
Zn2 2 e
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Most positive values of Ered
F2 (g) 2 e
2 F(aq)
2 H(aq) 2 e
H2 (g)
Li(aq) e
Li(s)
Most negative values of Ered
Figure Number: 20 14
Increasing strength of reducing agent
Increasing strength of oxidizing agent
Strongest
oxidizing agent
Strongest
reducing
agent
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e
Ni
anode
e
Salt bridge
Ni
cathode
[Ni2] 1.00 103 M [Ni2] 1.00 M
[Ni2] 0.5 M
(b)
(a)
Figure Number: 20 16 a, b
[Ni2] 0.5 M
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Electrical potential
Time
Figure Number: 20 17
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1 second
Figure Number: 20 18
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1.5 V
3.0 V
1.5 V
Figure Number: 20 20
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H2SO4
electrolyte
Figure Number: 20 21
Lead grid filled
with spongy
lead (anode)
Lead grid
filled with
PbO2
(cathode)
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Gasket
Separator
Cathode (MnO2
Anode
plus graphite)
(Zn plus KOH)
Figure Number: 20 22
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1.23 V
O2, H2O
exhaust
H2
exhaust
O2 inlet
H2
inlet
Anode
Figure Number: 20 23
Cathode
Porous
membrane
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Air
Water
droplet
Rust deposit
(Fe2O3 xH2O)
O2
Fe2 (aq)
e
(Cathode)
O2 4 H 4 e
or
O2 2 H2O 4 e
Figure Number: 20 25
Iron
2 H2O Fe
(Anode)
Fe2 2 e
4 OH
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Water
droplet
O2
Zn2 (aq)
e
Iron
(cathode)
Zinc
(anode)
Zn
Figure Number: 20 26
Zn2 2 e O2 4 H 4 e
2 H2O
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Ground
level
Soil
electrolyte
Water pipe,
the cathode
Insulated
copper wire
30 cm
Soldered
connection
Magnesium
anode
Figure Number: 20 27
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e–
Voltage
e–
source
Cathode
Anode
Na+
Cl–
Na(l)
Cl2(g)
Molten
NaCl
e–
e–
e–
e–
2 Cl–
Figure Number: 20 28
Cl2(g) 2 e–
2 Na+ 2 e–
2 Na(l)
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(b)
(a)
Figure Number: 20 29 C
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e
Voltage
source
e
Aqueous
NiSO4
Steel
cathode
Nickel
anode
Nickel
plate
Figure Number: 20 30
2
Ni (aq)
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Current
(amperes)
and time
Figure Number: 20 31
Quantity
of charge
(coulombs)
Moles of
electrons
(faradays)
Moles of
substance
oxidized or
reduced
Grams of
substance
oxidized or
reduced
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TABLE 20.1
Standard Reduction Potentials in Water at 25°C
Potential (V)
Reduction Half-Reaction
+2.87
+1.51
+1.36
+1.33
+1.23
+1.06
+0.96
+0.80
+0.77
+0.68
+0.59
+0.54
+0.40
+0.34
0 [defined]
-0.28
-0.44
-0.76
-0.83
-1.66
-2.71
-3.05
F2(g) + 2 e - ¡ 2 F -(aq)
MnO4 -(aq) + 8 H +(aq) + 5 e - ¡ Mn2+(aq) + 4 H 2O(l)
Cl2(g) + 2 e - ¡ 2 Cl -(aq)
Cr2O7 2-(aq) + 14 H +(aq) + 6 e - ¡ 2 Cr 3+(aq) + 7 H 2O(l)
O2(g) + 4 H +(aq) + 4 e - ¡ 2 H 2O(l)
Br2(l) + 2 e - ¡ 2 Br -(aq)
NO3 -(aq) + 4 H +(aq) + 3 e - ¡ NO(g) + 2 H 2O(l)
Ag +(aq) + e - ¡ Ag(s)
Fe 3+(aq) + e - ¡ Fe 2+(aq)
O2(g) + 2 H +(aq) + 2 e - ¡ H 2O2(aq)
MnO4 -(aq) + 2 H 2O(l) + 3 e - ¡ MnO2(s) + 4 OH -(aq)
I 2(s) + 2 e - ¡ 2 I -(aq)
O2(g) + 2 H 2O(l) + 4 e - ¡ 4 OH -(aq)
Cu2+(aq) + 2 e - ¡ Cu(s)
2 H +(aq) + 2 e - ¡ H 2(g)
Ni 2+(aq) + 2 e - ¡ Ni(s)
Fe 2+(aq) + 2 e - ¡ Fe(s)
Zn2+(aq) + 2 e - ¡ Zn(s)
2 H 2O(l) + 2 e - ¡ H 2(g) + 2 OH -(aq)
Al3+(aq) + 3 e - ¡ Al(s)
Na+(aq) + e - ¡ Na(s)
Li +(aq) + e - ¡ Li(s)
Figure Number: 20 T01
CH20_T01.eps
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