Chapter 18
18.1
18.2
18.3
18.4
18.5
18.11
18.12
18.13
Electrochemistry
Galvanic Cells
Shorthand Notation for Galvanic Cells
Cell Potentials and Free-Energy Changes
for Cell Reactions
Standard Reduction Potentials
Using Standard Reduction Potentials
Electrolysis and Electrolytic Cells
Commercial Applications of Electrolysis
Quantitative Aspects of Electrolysis
We are already familiar with batteries which convert chemical energy
into electrical energy. The process can be reversed to produce
chemical species using electrical energy, e.g. production of Al,
electrodeposition of Cu (electroplating)
In this chapter we learn about these and also the quantitative aspects
of electrochemistry.
We already know about redox reactions (Chem 1010 and Chapter 4)...
2 CuSO4 (aq) + Zn(s) ------>ZnSO4 (aq) + Cu(s)
As this spontaneous reaction proceeds, the Zn(s) dissolves, Cu
precipitates. This a redox reaction:
Net ionic equation: Cu2+ (aq) + Zno (s) ----> Cuo (s) + Zn2+ (aq)
This can be split into two half reactions:
Cu2+ (aq) + 2eCuo (s)
o
2+
Zn (s)
Zn (aq) +2e-
reduction (gain of electrons)
oxidation (loss of electrons)
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Other than producing new compounds, the above system does no
useful work. It is possible to harness the electron transfer, and
produce an electrical current by separating the two half reactions
into two half cells (Galvanic Cell)
18.1 Galvanic Cells
If the above reaction (Cu2+ / Zn) is set up in two separate containers
(cells) with a conducting bridge between them, a current will flow
through an external circuit (wire).
Now have a galvanic cell (battery) where electrons flow through the
external circuit. Sometimes called voltaic or electrolytic cells
The salt bridge is a device which allows overall electrical neutrality to
be maintained....here a simple system is a glass tube filled with a gelled
(doesn t run out of tube) aqueous solution of a non-reacting salt (NH4Cl,
KNO3, etc.)
Cell Diagram: the convention is that for a spontaneous reaction:
a ) the anode half cell where oxidation takes place is written on the
left,
Zn (s)
Zn 2+(aq) + 2eb) the cathode half cell where reduction takes place is written on the
right
Cu2+ (aq) + 2eCu (s)
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c) electrons travel through the external circuit from the anode
compartment to the cathode compartment
d) anions travel from the cathode to the anode compartment through
the salt bridge to maintain charge neutrality in the anode compartment
as cations are produced by dissolution of the anode.
e) Cations travel from the anode to the cathode compartment through
the salt bridge to maintain charge neutrality in the cathode
compartment as cations are removed by deposition on the cathode.
f) The anode has a negative charge (provides e-) and
the cathode has a positive charge (uses up e-).
18.2 Shorthand Notation for Galvanic Cells
Drawing a diagram for a galvanic cell, or describing it, is too timeconsuming to do on a regular basis. There is a shorthand notation that
has been developed to help. Consider our zinc-copper reaction of Figure
18.2
Zn (s) + Cu2+ (aq)
Zn2+ (aq) + Cu (s)
We can describe the resulting Galvanic Cell completely by
Zn (s) | Zn2+ (aq) || Cu2+ (aq) | Cu (s)
A single vertical line indicates a change in phase, (like that between a
solid electrode and the solution its immersed in).
A double vertical line indicates a salt bridge.
Since the salt bridge always separates the two half-cells, we see the halfreactions are well defined.
As we read from left to right, we see that the first half-cell has a halfreaction where solid zinc loses electrons to give zinc ions (an oxidation
half-reaction). The second half-cell has the half-reaction where copper
ions gain electrons to become solid copper (the reduction half-reaction).
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The leftmost chemical in the notation is the anode, while the
rightmost chemical is the cathode.
Additionally, the electrons always flow from left to right, which is the
way we read the shorthand.
Problem 18.2
Write the shorthand notation for a galvanic cell that uses the
reaction
Fe (s) + Sn2+ (aq)
Fe2+ (aq) + Sn (s)
Ans: Since the solid iron is oxidized it should be our anode on the left.
Since the solid tin results from the reduction of tin ions, it is our cathode
on the right.
Fe (s) | Fe2+ (aq) || Sn2+ (aq) | Sn (s)
Problem 18.3
Write a balanced equation for the overall cell reaction and give a
brief description of the galvanic cell represented by
Pb (s) | Pb2+ (aq) || Br2 (l) | Br- (aq) | Pt (s)
Ans: done in class
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