18.3
Electrolysis
Driving a non-spontaneous
Oxidation-Reduction Reaction
Dr. Fred Omega Garces
Chemistry 201
Miramar College
1
Electrolysis
January 13
Voltaic Vs. Electrolytic Cells
Voltaic Cell
Energy is released from spontaneous
redox reaction
Electrolytic Cell
Energy is absorbed to drive
nonspontaneous redox reaction
System does work on load
(surroundings)
Surrounding (power supply) do work
on system (cell)
Oxidation Reaction
X g X+ + eReduction Reaction
e- + Y+ g Y
Overall (Cell) Reaction
X + Y+ g X+ + Y, ΔG = 0
2
Oxidation Reaction
A- g A + eReduction Reaction
e- + B+ g B
Overall (Cell) Reaction
A- + B+ g A + B, Δ G> 0
Electrolysis
General Characteristics of voltaic
and electrolytic cells. A voltaic
cell generates energy from a
spontaneous reaction (ΔG < 0),
whereas an electrolytic cell
requires energy to drive a nonspontaneous reaction (ΔG > 0). In
both types of cell two electrodes
dip into electrolyte solution, and
an external circuit provides the
means for electrons to flow.
Oxidation takes place at the
anode, and reduction takes place
at the cathode, but the relative
electrode changes are opposite in
the two cells. The anode in the
electrolytic cell is now the
positive (+) electrode and the
cathode in the electrolytic cell is
now the negative (-) electrode.
January 13
Electrolysis
What is the reaction which occurs in an electrolysis process ?
Voltaic/Galvanic Cell - Produce electricity
Electrolysis - Causes nonspontaneous reaction to occur by
using external energy source.
Electrolytic Cell
• Two electrodes (inert or non inert)
• External potential (power)
Source of electricity, battery (e- pump)
g Drive a non spontaneous reaction
e-
3
g
+
-
e- g
pulls electron
push electron
Draws electron
to positive
(Oxidation)
Anode (+)
A g A- + e-
Pumps electron
to reaction
(Reduction)
Cathode (-)
e- + B+ g B
Electrolysis
January 13
Electrolysis: Molten Salt
Question: Electrolysis NaCl
What is the direction of e- flow ?
Which is the Anode/ Cathode
Which is the Oxidation / Reduction ?
Which is the Positive / Negative terminal ?
What are the Half rxns.
With NaCl
Electrode
Pt(inert)
NaCl
Ions ...
Pt
Na+
Cl-
Anode Rxn
(inert)
2Cl- g Cl2 + 2e-
Cathode Rxn
E°
(inert)
Na+ +e- g Na(s)
-1.36 V
-2.71 V
To drive rxn, Battery requires 4.07 V
Complications due to H2O undergoing electrolysis process.
This reaction must be considered.
4
Electrolysis
January 13
Water Complications in Electrolysis
In an electrolysis, the most easily oxidized and
most easily reduced reaction occurs.
When water is present in an electrolysis reaction,
then water (H2O) can be oxidized or reduced
according to the reaction shown.
Electrode
Ions ... Anode Rxn
Pt (inert)
H2 O
H2 O
Net Rxn Occurring:
2 H2O g
5
Cathode Rxn
H2O(l)+ 2e- g H2(g)+ 2OH-(aq)
2 H2O(l) g 4e- + 4H+(g) + O2(g)
2 H2(g)+ O2 (g)
Electrolysis
E°
-0.83 V
-1.23 V
E° = - 2.06 V
January 13
Electrolysis of NaCl
Question:
i) What is the direction of e- flow ?
ii) Which is the Anode/ Cathode
iii) Which is the Oxidation / Reduction ?
iv Which is the Positive / Negative terminal ?
v) What are the Half rxns.
Reaction which occurs is the reaction requiring
least E° potential
Specie
Ions ...
Pt
NaCl
Overall Rxn:
Cathode Rxn
(inert)
(inert)
Na+
Cl-
H2 O
Anode Rxn
H2 O
Anode:
H2O
2Cl-
g Cl2 +
2e-
H2O(l) g 4e- + 4H+(aq)
Na+ + e- g Na(s)
H2O(l)+ 2e- g H2(g)+ 2OH-(aq)
+ O2(g)
2 H2O(l) g 4e- + 4H+(aq) + O2(g)
Cathode:
H2O(l)+ 2e- g H2(g)+ 2OH-(aq)
E°
-2.71 V
-1.36 V
- 0.83 V
-1.23 V
- 1.23 V
- 0.83 V
E°Cell = -2.06 V
Electrolysis of water is the reaction occurring.
6
Electrolysis
January 13
Electrolysis of MgCl2
What is the reaction in an electrolysis
reaction of MgCl2.
Reaction which occurs is the reaction
requiring least E° potential
Specie
Pt
MgCl2
H2 O
V
Overall Rxn:
Ions ...
Anode Rxn
Cathode Rxn
(inert)
(inert)
Mg2+ +2e- g Mg(s)
Mg2+
ClH2 O
2Cl- g Cl2 + 2e-
H2 O
2 H2O(l) g 4e- + 4H+(g) + O2(g)
E°
H2O(l)+ 2e- g H2(g)+ 2OH-(aq)
Anode: 2 H2O(l) g 4e- + 4H+(aq) + O2(g)
Cathode: H2O(l)+ 2e- g H2(g)+ 2OH-(aq)
-2.37 V
-1.36 V
-0.83
-1.23 V
- 1.23 V
-0.83 V
E°Cell = -2.06 V
∴ MgCl2 undergoes no chemical change. Electrolysis of water is the reaction occurring.
7
Electrolysis
January 13
Electrolysis with
non-inert Electrode
What is the reaction in an electrolysis
reaction of NaCl with iron nails
Specie
Ions ...
Fe
Anode Rxn
Cathode Rxn
E°
Fe g Fe+2 + 2e-
NaCl
Na+
Cl-
H2 O
Overall Rxn:
Na+ + e- g Na(s)
-2.71 V
2Cl- g Cl2 + 2e-
H2 O
H2 O
+0.44 V
-1.36 V
H2O(l)+ 2e- g H2(g)+ 2OH-(aq)
2 H2O(l) g 4e- + 4H+(g) + O2(g)
Anode: Fe g Fe+2 + 2eCathode:
-0.83 V
-1.23 V
0.44 V
H2O(l)+ 2e- g H2(g)+ 2OH-(aq)
-0.83 V
E°Cell = -.39 V
∴Iron oxidize at the anode and water is reduced at the cathode.
A potential of 0.39 V is used to drive this reaction.
8
Electrolysis
January 13
Electrolysis of Copper
A net reaction of zero, yet a process does take place.
A concentration cell based on the Cu/
Cu2+ half-reaction. A, Even though the
half-reactions involve the same components,
the cell operates because the half-cell
concentrations are different. B, The cell
operates spontaneously until the half-cell
concentrations are equal. Note the change
in electrodes (exaggerated here for clarity)
and the equal color of solutions.
9
Electrolysis
January 13
Electro-Plating
Example 20.97 BLB: Calculate the minimum applied voltage required to cause
the following electrolysis reaction to occur, assuming that the anode is platinum
and the cathode is nickel: Ni2+(aq)+ 2Br-(aq) g Ni(s) + Br2(l)
Specie
Pt
Ni+2
H2 O
10
Ions ...
Anode Rxn
(Inert)
Cathode Rxn
(Inert)
E°
Ni2+
BrH2 O
H2 O
Electrolysis
January 13
Electro-Plating
Example 20.97 BLB: Calculate the minimum applied voltage required to cause
the following electrolysis reaction to occur, assuming that the anode is platinum
and the cathode is nickel: Ni2+(aq)+ 2Br-(aq) g Ni(s) + Br2(l)
Specie
Pt
Ni+2
H2 O
11
Ions ...
Ni2+
BrH2 O
H2 O
Anode Rxn
(Inert)
2Br- g Br2 + 2e-
Cathode Rxn
(Inert)
Ni+2 +2 e- g Ni(s)
H2O(l)+ 2e- g H2(g)+ 2OH-(aq)
2 H2O(l) g 4e- + 4H+(g) + O2(g)
Electrolysis
E°
-0.28 V
-1.065 V
-0.83 V
-1.23 V
January 13
Electro-Plating
Example 20.97 BLB: Calculate the minimum applied voltage required to cause the
following electrolysis reaction to occur, assuming that the anode is platinum and the
cathode is nickel: Ni2+(aq)+ 2Br-(aq) g Ni(s) + Br2(l)
Specie
Pt
Ni+2
H2 O
Overall Rxn:
Ions ...
Ni2+
BrH2 O
H2 O
Anode Rxn
(Inert)
2Br- g Br2 + 2e-
Cathode Rxn
(Inert)
Ni+2 +2 e- g Ni(s)
H2O(l)+ 2e- g H2(g)+ 2OH-(aq)
2 H2O(l) g 4e- + 4H+(g) + O2(g)
Cathode:
E°
-0.28 V
-1.065 V
-0.83 V
-1.23 V
Ni+2 +2 e- g Ni(s)
-0.28 V
Anode: 2Br- g Br2 + 2e-
1.06 V
E°Cell = -1.35 V
∴A larger voltage than the E°Cell = 1.35 V is
required to overcome cell resistance (overvoltage)
if rxn is to occur at reasonable rate.
12
Electrolysis
January 13
Overvoltage
At anode (Oxidation)
2H2O g
E°
4H+ + O2 + 4e-
- 1.23 V
Actually the voltage necessary to drive this reaction is
greater than the theoretical value.
Overvoltage - Voltage necessary to overcome activation energy
Reason: i) Slow e- transfer at electrode surface
ii) Process (Kinetics) of bond breaking.
The actual overvoltage depends on the type of electrode and can only be
determine experimentally.
For water an additional 0.4V is necessary for oxidation, therefore,
instead of -1.23 V for water reduction, -1.6 V is necessary.
13
Electrolysis
January 13
Electrolysis Reactions order
A Solution contains the ions H+, Cu2+, Ca2+ and Ni2+, each at a
concentration of 1.0 M.
Determine the sequence of ions to be reduce at the Cathode ?
Reduction at Cathode
14
E°
1st)
2H+ g H2
0.00 V
2nd)
Cu+2 g Cu
0.34 V
3rd)
Ca2+ g Ca
-2.87 V
4th)
Ni2+ g Ni
0.25 V
Electrolysis
January 13
Electrolysis Reactions
A Solution contains the ions H+, Cu2+, Ca2+ and Ni2+, each at a
concentration of 1.0 M.
Determine the sequence of ions to be reduce at the Cathode ?
Reduction at Cathode
E°
1st)
Cu+2 g Cu
0.34 V
2nd)
Ni2+ g Ni
0.25 V
3rd)
2H+ g H2
0.00 V
4th)
Ca2+ g Ca
-2.87 V *
* This reaction cannot be reduce in aqueous solution since H2O will be
reduced first H2O reduction:
H2O(l)+ 2e- g H2(g) + 2OH-(aq)
15
Electrolysis
-0.83 V
January 13
Electrolysis Stoichiometry
The amount of metal plated can be calculated based on stoichiometry.
1 Ampere = Coulomb per time (C/s)
1 Faraday = 96485 C / mol e-
Q 100: Silver is electroplated. i) What mass of silver plates if a current of 6.8A flows through
the cell in 72 min. ii) What electrode will the silver plate-out.
Reaction: Ag+(aq) + e- g Ag(s) (Reduction occurs at the cathode for electrolysis)
Given:
Goal:
6.8C = 1s
96485 C = 1 mol e-
Mass Ag (g)
72 min = 432 sec
107.87 g Ag = 1 mol Ag
72 min •
60 sec
1 min
•
6.8 C
1 sec
•
1 mol e- 1 mol Ag 107.87 g Ag
•
•
=
96485 C 1 mol e1 mol Ag
Mass Ag = 32.84 g of Silver
16
Electrolysis
January 13
Electrolysis Stoichiometry
The amount of metal plated can be calculated based on stoichiometry.
1 Ampere = Coulomb per time (C/s)
1 Faraday = 96485 C / mol e-
Q 99: Copper can be electroplated at the cathode of an electrolysis cell by the half-reaction:
Reaction: Cu+2(aq) + 2e- g Cu (s) (Reduction occurs at the cathode for electrolysis)
How much time would it take to plate 325 mg of copper with 5.6 A?
Given:
5.6 C = 1s
Goal:
Time for electrolysis
96485 C = 1 mol e325 mg = .325 g
63.55 g Cu = 1 mol Cu
0.325 g Cu •
1 mol Cu 2 mol e- 96485 C 1 sec
•
•
•
=
63.55 g 1 mol Cu 1 mol e- 5.6 C
Time electrolysis = 176 sec = 180 sec
17
Electrolysis
January 13
Voltaic Vs. Electrolytic Cells
Voltaic: Generate E° (+)
Conductivity
Electrolytic: Consumes E° (-)
Conductivity
Anode (-) Oxidation
Cathode (+) Reduction
Anode (+) Oxidation
Cathode (-) Reduction
e- flow (pump by battery)
migration of ions (in cell)
Lowest E° reaction occurs
e- flow (external circuit)
migration of ions (in cell)
complete circuit (bridge)
-
Oxidation Anode
M g M+ + eNeg - Terminal
18
- +
+
Reduction Cathode
M+ + e- g M
Pos - Terminal
Reduction Cathode
M+ + e- g M
Neg - Terminal
Electrolysis
Oxidation Anode
M g M+ + ePos - Terminal
January 13
Summary
General characteristics
of voltaic and
electrolytic cells. A
voltaic cell generates
energy from a
spontaneous reaction
(ΔG < 0), whereas an
electrolytic cell requires
energy to drive a
nonspontaneous reaction
(Δ G>0). In both types
of cell, two external
circuits provides the
means or electrons to
flow. Oxidation takes
place all the anode, and
reduction takes place at
the cathode, but the
relative electrode
changes are opposite in
the two cells.
19
Voltaic Cell
Energy is released from spontaneous redox
reaction
Electrolytic Cell
Energy is absorbed to drive
nonspontaneous redox reaction
System does work on load
(surroundings)
Surrounding (power supply) do work on
system (cell)
Oxidation Reaction
X g X+ + eReduction Reaction
e- + Y+ g Y
Overall (Cell) Reaction
X + Y+ g X+ + Y, ΔG > 0
Electrolysis
Oxidation Reaction
A- g A + eReduction Reaction
e- + B+ g B
Overall (Cell) Reaction
A- + B+ g A + B, Δ G> 0
January 13