3. Electrolysis of aqueous sodium chloride solution.
Some possible anode reactions:
1681
3. Electrolysis of aqueous sodium chloride solution.
Some possible anode reactions:
2 Cl-(aq)
Cl2(g) + 2e-
(1)
1682
3. Electrolysis of aqueous sodium chloride solution.
Some possible anode reactions:
2 Cl-(aq)
Cl2(g) + 2e(1)
2 H2O(l)
4 H+(aq) + O2(g) + 4e- (2)
1683
3. Electrolysis of aqueous sodium chloride solution.
Some possible anode reactions:
2 Cl-(aq)
Cl2(g) + 2e(1)
2 H2O(l)
4 H+(aq) + O2(g) + 4e- (2)
Some possible cathode reactions:
1684
3. Electrolysis of aqueous sodium chloride solution.
Some possible anode reactions:
2 Cl-(aq)
Cl2(g) + 2e(1)
2 H2O(l)
4 H+(aq) + O2(g) + 4e- (2)
Some possible cathode reactions:
Na+(aq) + eNa(s)
(3)
1685
3. Electrolysis of aqueous sodium chloride solution.
Some possible anode reactions:
2 Cl-(aq)
Cl2(g) + 2e2 H2O(l)
4 H+(aq) + O2(g) + 4eSome possible cathode reactions:
Na+(aq) + eNa(s)
2 H2O(l) + 2e2 OH-(aq) + H2(g)
(1)
(2)
(3)
(4)
1686
3. Electrolysis of aqueous sodium chloride solution.
Some possible anode reactions:
2 Cl-(aq)
Cl2(g) + 2e2 H2O(l)
4 H+(aq) + O2(g) + 4eSome possible cathode reactions:
Na+(aq) + eNa(s)
2 H2O(l) + 2e2 OH-(aq) + H2(g)
2 H+(aq) + 2 eH2(g)
(1)
(2)
(3)
(4)
(5)
1687
3. Electrolysis of aqueous sodium chloride solution.
Some possible anode reactions:
2 Cl-(aq)
Cl2(g) + 2e(1)
2 H2O(l)
4 H+(aq) + O2(g) + 4e- (2)
Some possible cathode reactions:
Na+(aq) + eNa(s)
(3)
2 H2O(l) + 2e2 OH-(aq) + H2(g) (4)
2 H+(aq) + 2 eH2(g)
(5)
Experimentally it is found that Cl2(g) is liberated at
the anode and H2 is liberated at the cathode.
1688
3. Electrolysis of aqueous sodium chloride solution.
Some possible anode reactions:
2 Cl-(aq)
Cl2(g) + 2e(1)
2 H2O(l)
4 H+(aq) + O2(g) + 4e- (2)
Some possible cathode reactions:
Na+(aq) + eNa(s)
(3)
2 H2O(l) + 2e2 OH-(aq) + H2(g) (4)
2 H+(aq) + 2 eH2(g)
(5)
Experimentally it is found that Cl2(g) is liberated at
the anode and H2 is liberated at the cathode.
1689
It is not immediately clear what the cathode
reaction is.
1690
It is not immediately clear what the cathode
reaction is. Can we exclude the following situation:
Na(s) is formed which then reacts with water
1691
It is not immediately clear what the cathode
reaction is. Can we exclude the following situation:
Na(s) is formed which then reacts with water
2Na(s) + 2H2O(l)
2Na+(aq) + 2 OH-(aq) + H2(g)
1692
It is not immediately clear what the cathode
reaction is. Can we exclude the following situation:
Na(s) is formed which then reacts with water
2Na(s) + 2H2O(l)
2Na+(aq) + 2 OH-(aq) + H2(g)
It turns out (from experiment) that the ease of
reduction decreases in the order:
reaction (5) > reaction (4) > reaction (3)
1693
It is not immediately clear what the cathode
reaction is. Can we exclude the following situation:
Na(s) is formed which then reacts with water
2Na(s) + 2H2O(l)
2Na+(aq) + 2 OH-(aq) + H2(g)
It turns out (from experiment) that the ease of
reduction decreases in the order:
reaction (5) > reaction (4) > reaction (3)
The pH of an aqueous solution of NaCl is 7, so that
the concentration of H+(aq) is 1.0 x 10-7 M, which is
too low to make reaction (5) reasonable.
1694
It is not immediately clear what the cathode
reaction is. Can we exclude the following situation:
Na(s) is formed which then reacts with water
2Na(s) + 2H2O(l)
2Na+(aq) + 2 OH-(aq) + H2(g)
It turns out (from experiment) that the ease of
reduction decreases in the order:
reaction (5) > reaction (4) > reaction (3)
The pH of an aqueous solution of NaCl is 7, so that
the concentration of H+(aq) is 1.0 x 10-7 M, which is
too low to make reaction (5) reasonable. Hence,
reaction (4) is the cathode reaction.
1695
2 Cl-(aq)
Cl2(g) + 2e-
(1)
1696
2 Cl-(aq)
2 H2O(l) + 2e-
Cl2(g) + 2e-
(1)
2 OH-(aq) + H2(g) (4)
1697
2 Cl-(aq)
2 H2O(l) + 2eOverall reaction:
2 H2O(l) + 2 Cl-(aq)
Cl2(g) + 2e-
(1)
2 OH-(aq) + H2(g) (4)
Cl2(g) + 2 OH-(aq) + H2(g)
1698
2 Cl-(aq)
2 H2O(l) + 2eOverall reaction:
2 H2O(l) + 2 Cl-(aq)
Cl2(g) + 2e-
(1)
2 OH-(aq) + H2(g) (4)
Cl2(g) + 2 OH-(aq) + H2(g)
This electrolysis would produce three very useful
chemicals: chlorine gas, hydrogen gas, and sodium
hydroxide.
1699
2 Cl-(aq)
2 H2O(l) + 2eOverall reaction:
2 H2O(l) + 2 Cl-(aq)
Cl2(g) + 2e-
(1)
2 OH-(aq) + H2(g) (4)
Cl2(g) + 2 OH-(aq) + H2(g)
This electrolysis would produce three very useful
chemicals: chlorine gas, hydrogen gas, and sodium
hydroxide. Note that sodium chloride is cheap, but
electrical energy is not, unless one is situated near a
hydro-power source of electrical energy.
1700
Quantitative Aspects of Electrolysis
The quantitative aspects of electrolysis were largely
developed by Michael Faraday.
1701
Quantitative Aspects of Electrolysis
The quantitative aspects of electrolysis were largely
developed by Michael Faraday.
He observed that the mass of product formed or
reactant consumed at an electrode is proportional
to both the amount of electricity transferred at the
electrode and the molar mass of the substance in
question.
1702
In the electrolysis of molten NaCl,
Na+(l) + eNa(l)
1703
In the electrolysis of molten NaCl,
Na+(l) + eNa(l)
one Na atom is produced when one Na+ accepts
one electron from the electrode.
1704
In the electrolysis of molten NaCl,
Na+(l) + eNa(l)
one Na atom is produced when one Na+ accepts
one electron from the electrode.
To react one mole of Na+ ions we must supply
Avogadro’s number (6.02 x 1023) of electrons to the
cathode.
1705
In the electrolysis of molten NaCl,
Na+(l) + eNa(l)
one Na atom is produced when one Na+ accepts
one electron from the electrode.
To react one mole of Na+ ions we must supply
Avogadro’s number (6.02 x 1023) of electrons to the
cathode.
At the anode, the formation of one mole of Cl2(g)
results in the transfer of two moles of electrons
from the Cl- to the anode: 2 Cl-(aq)
Cl2(g) + 2e1706
Faraday: The faraday is the electrical charge carried
by one mole of electrons.
1707
Faraday: The faraday is the electrical charge carried
by one mole of electrons.
Symbol: F
1708
Faraday: The faraday is the electrical charge carried
by one mole of electrons.
Symbol: F
Unit of charge is the Coulomb, symbol C.
1709
Faraday: The faraday is the electrical charge carried
by one mole of electrons.
Symbol: F
Unit of charge is the Coulomb, symbol C.
Unit of electrical current is the Ampere, symbol A.
1710
Faraday: The faraday is the electrical charge carried
by one mole of electrons.
Symbol: F
Unit of charge is the Coulomb, symbol C.
Unit of electrical current is the Ampere, symbol A.
A coulomb is the quantity of electrical charge
passing a point in a second when the current
flowing is 1 ampere.
1711
Faraday: The faraday is the electrical charge carried
by one mole of electrons.
Symbol: F
Unit of charge is the Coulomb, symbol C.
Unit of electrical current is the Ampere, symbol A.
A coulomb is the quantity of electrical charge
passing a point in a second when the current
flowing is 1 ampere.
Q = It
1712
Faraday: The faraday is the electrical charge carried
by one mole of electrons.
Symbol: F
Unit of charge is the Coulomb, symbol C.
Unit of electrical current is the Ampere, symbol A.
A coulomb is the quantity of electrical charge
passing a point in a second when the current
flowing is 1 ampere.
Q = It
where the charge Q is in coulombs, the time t is in
seconds, and the current I is in amperes.
1713
Faraday: The faraday is the electrical charge carried
by one mole of electrons.
Symbol: F
Unit of charge is the Coulomb, symbol C.
Unit of electrical current is the Ampere, symbol A.
A coulomb is the quantity of electrical charge
passing a point in a second when the current
flowing is 1 ampere.
Q = It
where the charge Q is in coulombs, the time t is in
seconds, and the current I is in amperes.
Unit wise:
1C=1As
1714
Now
1 F = 96485 C mol-1
That is, 1 mole e- carries a charge of 96485 C of
charge.
1 mol e- = 96485 C
1715
Example: A current of 0.452 A is passed for 1.50
hours through an electrolytic cell containing molten
MgCl2. Write the electrode reactions, the overall
reaction, and calculate the amount of products
formed at the electrodes.
1716
Example: A current of 0.452 A is passed for 1.50
hours through an electrolytic cell containing molten
MgCl2. Write the electrode reactions, the overall
reaction, and calculate the amount of products
formed at the electrodes.
The only ions present in molten MgCl2 are Mg2+ and
Cl-.
1717
Example: A current of 0.452 A is passed for 1.50
hours through an electrolytic cell containing molten
MgCl2. Write the electrode reactions, the overall
reaction, and calculate the amount of products
formed at the electrodes.
The only ions present in molten MgCl2 are Mg2+ and
Cl-.
Cathode reaction: Mg2+(l) + 2eMg(l)
1718
Example: A current of 0.452 A is passed for 1.50
hours through an electrolytic cell containing molten
MgCl2. Write the electrode reactions, the overall
reaction, and calculate the amount of products
formed at the electrodes.
The only ions present in molten MgCl2 are Mg2+ and
Cl-.
Cathode reaction: Mg2+(l) + 2eMg(l)
Anode reaction: 2 Cl-(l)
Cl2(g) + 2e-
1719
Example: A current of 0.452 A is passed for 1.50
hours through an electrolytic cell containing molten
MgCl2. Write the electrode reactions, the overall
reaction, and calculate the amount of products
formed at the electrodes.
The only ions present in molten MgCl2 are Mg2+ and
Cl-.
Cathode reaction: Mg2+(l) + 2eMg(l)
Anode reaction: 2 Cl-(l)
Cl2(g) + 2eOverall reaction: Mg2+(l) + 2 Cl-(l)
Mg(l) + Cl2(g)
1720
The number of coulombs of electricity is obtained
from
1721
The number of coulombs of electricity is obtained
from
Q = It
1722
The number of coulombs of electricity is obtained
from
Q = It
3600s x 1C
(0.452
A)(1.50
hours)
x
=
1 hour 1 As
1723
The number of coulombs of electricity is obtained
from
Q = It
3600s x 1C
(0.452
A)(1.50
hours)
x
=
1
hour
1
As
= 2.44 x 103 C
1724
The number of coulombs of electricity is obtained
from
Q = It
3600s x 1C
(0.452
A)(1.50
hours)
x
=
1
hour
1
As
= 2.44 x 103 C
mass of Mg formed
1
mol
e
1mol Mg 24.3g Mg
= (2.44x 103 C)
96485 C 2mol e - 1mol Mg
1725
The number of coulombs of electricity is obtained
from
Q = It
3600s x 1C
(0.452
A)(1.50
hours)
x
=
1
hour
1
As
= 2.44 x 103 C
mass of Mg formed
1
mol
e
1mol Mg 24.3g Mg
= (2.44x 103 C)
96485 C 2mol e - 1mol Mg
= 0.307 g Mg
1726
mass of Cl2 formed
1727
mass of Cl2 formed
- 1mol Cl 70.9 g Cl
1
mol
e
2
2
= (2.44x 103 C)
96485 C 2mol e - 1mol Cl2
1728
mass of Cl2 formed
- 1mol Cl 70.9 g Cl
1
mol
e
2
2
= (2.44x 103 C)
96485 C 2mol e - 1mol Cl2
= 0.896 g Cl2
1729
Example: What current would have to be passed for
2.250 x 103 s to produce 0.500 g Mg in the
electrolysis of molten MgCl2?
1730
Example: What current would have to be passed for
2.250 x 103 s to produce 0.500 g Mg in the
electrolysis of molten MgCl2?
First calculate the number of coulombs needed.
1731
Example: What current would have to be passed for
2.250 x 103 s to produce 0.500 g Mg in the
electrolysis of molten MgCl2?
First calculate the number of coulombs needed.
1
mol
Mg
2
mol
e
96485
C
Q = 0.500 g Mg
24.3g Mg 1mol Mg 1 mol e-
1732
Example: What current would have to be passed for
2.250 x 103 s to produce 0.500 g Mg in the
electrolysis of molten MgCl2?
First calculate the number of coulombs needed.
1
mol
Mg
2
mol
e
96485
C
Q = 0.500 g Mg
24.3
g
Mg
1
mol
Mg
1
mol
e
= 3.97 x 103 C
1733
Example: What current would have to be passed for
2.250 x 103 s to produce 0.500 g Mg in the
electrolysis of molten MgCl2?
First calculate the number of coulombs needed.
1
mol
Mg
2
mol
e
96485
C
Q = 0.500 g Mg
24.3
g
Mg
1
mol
Mg
1
mol
e
= 3.97 x 103 C
Now from Q = I t, we have
1734
Example: What current would have to be passed for
2.250 x 103 s to produce 0.500 g Mg in the
electrolysis of molten MgCl2?
First calculate the number of coulombs needed.
1
mol
Mg
2
mol
e
96485
C
Q = 0.500 g Mg
24.3
g
Mg
1
mol
Mg
1
mol
e
= 3.97 x 103 C
Now from Q = I t, we have
3
3.97
x
10
C 1 As
Q
I
=
t
2.250 x 103 s 1C
1735
Example: What current would have to be passed for
2.250 x 103 s to produce 0.500 g Mg in the
electrolysis of molten MgCl2?
First calculate the number of coulombs needed.
1
mol
Mg
2
mol
e
96485
C
Q = 0.500 g Mg
24.3
g
Mg
1
mol
Mg
1
mol
e
= 3.97 x 103 C
Now from Q = I t, we have
3
3.97
x
10
C 1 As
Q
I
=
t
2.250 x 103 s 1C
= 1.76 A
1736
Galvanic Cells
Galvanic Cell (Voltaic Cell): A cell in which energy
released from a spontaneous chemical reaction is
used to generate electricity.
1737
1738
A + B
B+ + A-
1739
A + B
B+ + A-
1740