CHAPTER 17
Electrochemistry – part 2
Electrolysis and Electrolytic
Cells


Anode: where oxidation takes place
◦ Anions are oxidized at this electrode
◦ labeled positive to reflect anions attraction to anode
Cathode: where reduction takes places
◦ Cations are reduced at this electrode
◦ Labeled negative to reflect the cations attraction to cathode
Electrolysis and Electrolytic Cells
•
Electrolysis: The process of using
an electric current to bring about
chemical change.
•
Process occurring in
galvanic cell and electrolytic cells
are the reverse of each other
•
In an electrolytic cell, two
inert electrodes are dipped into an
aqueous solution
Electrolysis and Electrolytic Cells
Electrolysis: The process of using an electric current to bring about
chemical change.
Quantitative Aspects of
Electrolysis
Charge(C) = Current(A) × Time(s)
Moles of
e
= Charge(C) ×
1 mol e
96,500 C
Faraday constant
Example

God can be plated out of a solution containing Au3+
according to the following half-reaction:
Au3+(aq) + 3e-  Au(s)
What mass of gold (in grams) will be plated by the follow of
5.5A of current for 25 minutes?
Example

Silver can be plated out of a solution containing Ag+
according to the following half-reaction:
Ag+(aq) + e-  Ag(s)
How much time (in minutes) would it takes to plate 12.0 g of
silver using a current of 3.0A?
Led-acid storage batteries




Consists of six cells wired in series.
Each cell contains a porous lead anode and a lead oxide
cathode, both immersed in sulfuric acid.
An electric current is drawn from the battery, both the anode
and cathode become coated with PbSO4(s)
Can be recharged by running electric current through it in
reverse direction
Batteries
Lead Storage Battery
Anode:
Cathode:
Overall:
Pb(s) + HSO4(aq)
PbO2(s) + 3H+(aq) + HSO4(aq) + 2e
Pb(s) + PbO2(s) + 2H+(aq) + 2HSO41(aq)
PbSO4(s) + H+(aq) + 2e
PbSO4(s) + 2H2O(l)
2PbSO4(s) + 2H2O(l)
Dry-Cell Batteries

Zinc acts as the anode and a graphite rod immersed in a
moist, slightly acidic pasted of MnO2 and NH4Cl acts a
cathode.
Anode:
Zn(s) + 2OH(aq)
ZnO(s) + H2O(l) + 2e
Cathode:
2MnO2(s) + H2O(l) + 2e
Mn2O3(s) + 2OH(aq)
Batteries
Dry-Cell Batteries
Leclanché cell
Anode:
Cathode:
Zn(s)
2MnO2(s) + 2NH4+(aq) + 2e
Zn2+(aq) + 2e
Mn2O3(s) + 2NH3(aq)+ H2O(l)
Batteries
Nickel-Cadmium (“ni-cad”) Batteries
Anode:
Cathode:
Cd(s) + 2OH(aq)
NiO(OH)(s) + H2O(l) + e
Cd(OH)2(s) + 2e
Ni(OH)2(s) + OH(aq)
Nickel-Metal Hydride (“NiMH”) Batteries
Anode:
Cathode:
Overall:
MHab(s) + OH(aq)
NiO(OH)(s) + H2O(l) + e
MHab(s) + NiO(OH)(s)
M(s) + H2O(l) + e
Ni(OH)2(s) + OH(aq)
M(s) + Ni(OH)2(s)
Batteries
Lithium and Lithium Ion Batteries
Lithium
Anode:
xLi(s)
Cathode: MnO2(s) + xLi+(soln) + xe
xLi+(soln) + xe
LixMnO2(s)
Lithium Ion
Anode:
Cathode:
LixC6(s)
Li1-xCoO2(s) + xLi+(soln) + xe
xLi+(soln) + 6C(s) + xe
LiCoO2(s)
Fuel cells
Hydrogen-Oxygen Fuel Cell

Like batteries, but the
reactants must be constanly
replenished.

Normal batteries los their
ability to generate voltage
with use because the reactants
become depleted as electric
current is drawn from the
battery.

In fuel cell, the reactant – the
fuel-constanly flow through
the battery, generating electric
current as they undergo redox
reaction.
Corrosion
Corrosion: The oxidative deterioration of a metal.

Moisture must be present for
rusting to occur

Additional electrolytes promote
more rusting
◦ Such as NaCl, on the surface of
iron because it enhances
current flow

The presence of acid promotes
rusting. (H+ ions are involved in
the reduction of oxygen, lower
pH enhances the cathodic
reaction and leads to faster
rusting.
Preventing Corrosion




Keep dry
Coat the iron with a substance that is impervious to water
◦ Painting
Placing a sacrificial electrode in electrical contact with the
iron.
For some metals, oxidation protects the metal (aluminum,
chromium, magnesium, titanium, zinc, and others).
Corrosion
Prevention of Corrosion
1.
Galvanization: The coating of iron with zinc.
When some of the iron is oxidized
(rust), the process is reversed
since zinc will reduce Fe2+ to
Fe:
Fe2+(aq) + 2e
Fe(s)
Zn2+(aq) + 2e
Zn(s)
E° = 0.45 V
E° = 0.76 V
Corrosion
Prevention of Corrosion
2.
Cathodic Protection: Instead of coating the entire surface of the first
metal with a second metal, the second metal is placed in electrical contact
with the first metal:
Anode:
Cathode:
Mg(s)
O2(g) + 4H+(aq) + 4e
Mg2+(aq) + 2e E° = 2.37 V
2H2O(l)
E° = 1.23 V
Attaching a magnesium stake to iron will corrode the magnesium instead
of the iron. Magnesium acts as a sacrificial anode.
Molten salt- mixture of
cations and anions


In general:
◦ The cation that is most easily reduced (the one with least
negative, or most positive, reduction-half cell potential) is
reduced first
◦ The anion is most easily oxidize ( the one has the least
negative, or most positive, oxidation half-cell potential) is
oxidized first
The cations of active metals-those that are not easily reduced,
such as Li+, K+, Na+, Mg2+, Ca2+, and Al3+
- Cannot be reduced from aqueous solution by electrolysis
because water is reduced at lower voltage.
Electrolysis and Electrolytic Cells
Electrolysis of Molten Sodium Chloride
Anode:
Cathode:
Overall:
2Cl(l)
2Na+(l) + 2e
2Na+(l) + 2Cl(l)
Cl2(g) + 2e
2Na(l)
2Na(l) + Cl2(g)