Physical Chemistry
Lecture 40
Types of Electrochemical Cells
Simple cell
Simple cells have
two electrodes that
contact a solution

No junction
Example: Daniell
cell

Half-reactions occur
at separate
electrodes
Zn | ZnSO4 (ai ), CuSO4 (ai ) | Cu ( s )
Salt bridge
Used to connect two
solutions around an
electrode


Junction present
between different
liquid phases
Maintains separate
electrolyte solutions
Example: Daniell
cell with a KCl gel
salt bridge
Zn | ZnSO4 (ai ) || CuSO4 (ai ) | Cu ( s )
Gas electrode
Special
consideration when
gas is involved in
one or both halfreactions
Usually use platinum
wire to carry charge
Example: hydrogen
electrode
+
H (aq ) | H 2 , Pt
Compound electrode
Multiple solid phases
present
Must be intimate
contact to ensure
electrical connectivity
Example:


Silver/silver chloride
Major standard electrode
– calomel (Hg2Cl2 in
contact with mercury)
 Since mercury is liquid,
the electrode geometry
takes this into account
−
Cl | AgCl | Ag
Amalgam electrodes
Some metals, particularly alkali metals, react violently
with water


Sodium
Potassium
Use amalgam (mixture with mercury) as an electrode
in these cases

Example:
Na + | Na ( Hg )
Activity of metal in electrode is different from the
activity of the pure metal

Must take into account when using these electrodes
+
Na | Na | Na ( Hg )
E
θ
RT
ln (a Na ( Hg ) )
= −
F
Concentration cells
Electromotive force
produced by
concentration gradients
Example: two gas cells
with a pressure
differential between
them
H 2 ( P1 ) = H 2 ( P2 )
E = −
RT  P2 
ln 
2 F  P1 
Concentration cells
Concentration cells may also be produced by
having two solutions of different molality
involved
Example: sodium chloride solutions without
transference
Na (Hg ) | NaCl (ai, m1 ) || NaCl (ai, m2 ) | Na ( Hg )
Cell reaction :
E = −
NaCl (m2 )
→
NaCl (m1 )
←
2 RT  m1 
ln  −
F
 m2 
2 RT  γ 1, ± 
ln

F
 γ 2, ± 
Transference
Cells with liquid
junctions
At the junction, the
concentration
changes to
compensate
Reduces the voltage
from the value
without transference
Na ( Hg ) = Na + (m1 ) + e −
t+ Na + (m1 ) = t+ Na + (m2 )
t− Cl − (m2 ) = t− Cl − (m1 )
Na + (m2 ) + e −
= Na ( Hg )
Overall :
t− NaCl (m2 ) = t− NaCl (m1 )
2 RT  γ 1, ± m1 
E = − t−
ln

F
 γ 2, ± m2 
Donnan effect
Polyelectrolyte – a large ionic
molecule
Solution containing
polyelectrolyte and a small
electrolyte across a
semipermeable membrane
from a solution of the small
electrolyte
It can be shown that the
presence of charge gives an
additional osmotic pressure
across the membrane
Donnan effect
Affects other properties as
well
[
∆Π = RT cP + z 2cP2 / 4cs
]
Summary
Cells provide means to transform chemical energy to
electrical work
Diagrams specify cells
Electrodes are built especially to be used in certain
ways
Concentration cells produce electromotive force
because of a concentration difference
Transference affects the cell when liquid junctions are
present
Membrane potentials affect other physical properties


Donnan effect
Nerve transmission