CELL
CELL POTENTIAL,
POTENTIAL, EE
1
2
CELL
CELL POTENTIAL,
POTENTIAL, EE
Zn and Zn2+,
anode
• For Zn/Cu
Zn/Cu cell, voltage is 1.10 V at 25 ˚C and
when [Zn2+] and [Cu2+] = 1.0 M.
• This is the STANDARD CELL
Cu and Cu2+,
cathode
• Electrons are “driven” from anode to
cathode by an electromotive force or
• —a quantitative measure of the tendency of
reactants to proceed to products when all
are in their standard states at 25 ˚C.
• For Zn/Cu
Zn/Cu cell, this is indicated by a voltage
of 1.10 V at 25 ˚C and when [Zn 2+] and [Cu2+]
= 1.0 M.
4
CELL
CELL POTENTIALS,
POTENTIALS, EEoo
Can’t measure 1/2 reaction E o directly.
Therefore, measure it relative to a
STANDARD HYDROGEN CELL, SHE.
22 H
aq,
aq, 11 M)
atm
H++((aq,
M) ++ 2e2e- <---->
<----> H
H22(g,
(g, 11 atm)
atm))
Eo
= 0.0 V
Calculating
Calculating Cell
Cell Voltage
Voltage
• Balanced half-reactions can be added
together to get overall, balanced
equation.
2+((aq)
Zn(s)
(s) --->
aq)
Zn
aq) ++ 2eZn(s)
---> Zn
Zn2+
2e2+((aq)
Cu
aq)
aq) ++ 2eCu2+
2e- --->
---> Cu(s)
Cu(s)
--------------------------------------------------------------------------------------2+((aq)
2+((aq)
Cu
aq)
(s) --->
aq)
aq) ++ Zn(s)
Zn
aq) ++ Cu(s)
Cu2+
Zn(s)
---> Zn
Zn2+
Cu(s)
POTENTIAL, Eo
emf.
3
If we know Eo for each half-reaction, we
could get Eo for net reaction.
5
2+ half-cell
Zn/Zn
Zn/Zn2+
half-cell hooked
hooked to
to aa SHE.
SHE.
EEoo for
for the
the cell
cell == +0.76
+0.76 VV
Negative
electrode
Zn
Volts
-
+
Positive
electrode
Salt Bridge
6
Volts
Zn
-
+
Salt Bridge
H2
Zn2+
Zn
Zn2+ + 2eOXIDATION
ANODE
H+
2 H+ + 2eH2
REDUCTION
CATHODE
H2
Supplier
of
electrons
H+
Zn2+
Zn2+
Zn --> 2+ + 2eZn
Zn + 2eOxidation
OXIDATION
Anode
ANODE
H+
Acceptor
of
electrons
2
+ 2e- --> H2
2 H+ + 2eH
Reduction 2
REDUCTION
Cathode
CATHODE
Page 1
Overall reaction is reduction of H + by Zn metal.
Zn(s)
Zn(s) + 2 H+ (aq
(aq)) --> Zn2+ + H2(g)
Eo = +0.76 V
Therefore, Eo for Zn ---> Zn2+ (aq
(aq)) + 2e- is +0.76 V
Zn is a (better) (poorer) reducing agent than H 2.
7
2+ and
Cu/Cu
Cu/Cu2+
and H
H22/H
/H++ Cell
Cell
2+ and
Cu/Cu
Cu/Cu2+
and H
H22/H
/H++ Cell
Cell
8
Zn/Cu
/Cu Electrochemical
Zn
Zn/Cu
Electrochemical Cell
Cell
wire
Volts
Eo = +0.34 V
Cu
+
electrons
Salt Bridge
Positive
Cu
+
-
Negative
Cu2+ + 2eCu
REDUCTION
CATHODE
Cu2+
Cu2+
+ 2e- --> Cu
Cu2+ + 2eCu
Reduction
REDUCTION
Cathode
CATHODE
Supplier
of
electrons
H+
Overall reaction is reduction of
by H2 gas.
Cu2+ (aq
(aq)) + H2(g) ---> Cu(s) + 2 H +(aq)
aq)
Measured Eo = +0.34 V
Therefore, Eo for Cu2+ + 2e- ---> Cu is
H --> 2
+ 2eH22 Oxidation
2 H+ + 2eOXIDATION
Anode
ANODE
+0.34 V
10
electrons
Zn2+ ions
salt
bridge
TABLE
TABLE OF
OF STANDARD
STANDARD
REDUCTION
REDUCTION POTENTIALS
POTENTIALS
oxidizing
ability of ion
wire
Zn
H2
2 H+ + 2eOXIDATION
ANODE
Cu 2+
H+
Uses of E
Eoo Values
• Organize halfreactions by
relative ability to
act as oxidizing
agents
• Table 21.1
• Use this to predict
cell potentials and
direction of redox
reactions.
Anode,
negative,
source of
electrons
H+
Cu2+
Salt Bridge
H2
Acceptor
of
electrons
Zn
H2
Volts
Cu
Cu2+ ions
Cu2+ + 2e2 H+ + 2eZn2+ + 2e-
Eo (V)
Cu
H2
Zn
+0.34
0.00
-0.76
reducing ability
of element
Page 2
9
salt
bridge
Zn2+ ions
+
Cu
Cu2+ ions
Cathode,
positive,
sink for
electrons
Zn(s)
Eo = +0.76 V
Zn(s) ---> Zn 2+(aq)
aq) + 2eCu2+(aq)
Eo = +0.34 V
aq) + 2e- ---> Cu(s)
--------------------------------------------------------------Cu2+(aq)
aq) + Zn(s)
Zn(s) ---> Zn 2+(aq)
aq) + Cu(s)
Eo (calc’d
(calc’d)) = +1.10 V
11
12
Standard
Standard Redox
Redox Potentials,
Potentials, EEoo
oxidizing
ability of ion
Eo (V)
Cu2+ + 2e-
Cu
+0.34
2 H+ + 2e-
H2
0.00
Zn
-0.76
Zn2+ + 2e-
reducing ability
of element
Any substance on the right
will reduce any substance
higher than it on the left.
• Zn can reduce H+ and
Cu2+.
• H2 can reduce Cu 2+ but
not Zn2+
• Cu cannot reduce H + or
Zn2+.
Using Standard Potentials, Eo
13
Table 21.1
EEoo for
for aa Voltaic
Voltaic Cell
Cell
EEoo for
for aa Voltaic
Voltaic Cell
Cell
Volts
• Which is the best oxidizing agent:
Cd
O2, H2O2, or Cl2? _________________
Fe
Salt Bridge
Volts
• Which is the best reducing agent:
Cd
Hg, Al, or Sn? ____________________
Cd2+
• In which direction does the following
Cd --> Cd2+ + 2eor
Cd2+ + 2e- --> Cd
reaction go?
Cu(s) + 2 Ag +(aq) ---> Cu 2+(aq) + 2 Ag(s)
E˚ and G?
Fe --> Fe2+ + 2eor
Fe2+ + 2e- --> Fe
YES!
• Balanced half-reactions can be added
together to get overall, balanced
equation.
Fe
Salt Bridge
Fe2+
16
Calculating
Calculating Cell
Cell Voltage
Voltage
14
Cd2+
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
Michael
Michael Faraday
Faraday
1791-1867
1791-1867
Originated the terms anode,
cathode, anion, cation,
cation,
electrode.
Discoverer of
• electrolysis
• magnetic props. of matter
• electromagnetic induction
• benzene and other organic
chemicals
Was a popular lecturer.
22 II-- --->
---> II22 ++ 2e2e22 HH22O
O ++ 2e2e- --->
---> 22 OH
OH- ++ HH22
------------------------------------------------------------------------------------------------22 II- ++ 22 HH22O
O -->
-->II22 ++ 22 OH
OH- ++ HH22
If we know Eo for each half-reaction, we
could get Eo for net reaction.
Page 3
15
18
19
E
Eoo and
and G
G oo
Eo is related to Go, the free
energy change for the reaction.
G o = - n F Eo
where F = Faraday constant
= 9.6485 x 10 4 J/V•mol
J/V•mol
and n is the number of moles of
Michael Faraday
electrons transferred
1791-1867
E
Eoo and
and G
G oo
Go = - n F E o
For a product-favored reaction
Reactants ----> Products
Go < 0 and so Eo > 0
Eo is positive
For a reactant-favored reaction
Reactants <---- Products
Go > 0 and so Eo < 0
Eo is negative
Page 4
20