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Section instructor: Supratim Giri
Office: BMBT 416
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Email: [email protected]
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Concentration cell with porous membrane:
AnodeCathode
Ag AgCl Cl- (a1) AgAgClCl- (a2)
H+ (a1) H+ (a2)
Cl- (a1)Cl- (a2)
a1 > a2
e-
porous membrane
Ag AgCl Cl- (a1) AgAgClCl- (a2)
Systematic notation (concentration cell) Nernst equation:
Ecell = Eo
cell RTln a2/a1
(migration of ions under electromotive force)
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Liquid junction potential (in absence of porous membrane)
Anode Cathode
Ag AgCl Cl- (a1) AgAgClCl- (a2)
H+ (a1) H+ (a2)
Cl-
(a1)Cl- (a
2)
a1 > a2
e-
+
++
++
-
--
--
Liquid junction potential interferes with the cell emf! We need
a salt bridgeto eliminate that. Even a porous membrane produces a
small amount of LJP.
junction potential
Liquid junction potential is also known as diffusion
potential
migration of ionsunder electromotiveforce shown by blue
arrow
migration of ions underdiffusion process shownby red arrow
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Commercial cells: Leclanchs battery (primary cells)
(wet cell)
Zn (s) Zn2+ (aq), MnO2 (s), Mn2O3 (s) C (s)NH4Cl (aq)
An irreversible cellNon-rechargeable cell
Zn (s) Zn2+ (aq)
MnO2 (s) Mn2O3 (s)2e- 2OH-H2O
2e-+
+ + +
Eo red = - 0.76 V
Eo red = o.64 V
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Commercial dry cells: (primary cells)
(dry cells)
In modern era, instead of aqueous NH4Cl medium,NH4Cl paste is
used in dry cells.
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Alkaline batteries (primary)
Zn (s) Zn2+ (aq), MnO2 (s), Mn2O3 (s) Ni(s)KOH (aq)
Zn (s) Zn(OH)2 2e-+
KOH paste is used (dry cell)
Li-ion batteries are completely reversible, known as secondary
batteries
At anode:
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Commercial cells: Plants battery (secondary cells)
Pb (s) PbSO4(s) PbO2 (s)PbSO4 (s)H2SO4 (aq)
Pb PbO2
+-
PbSO4 PbSO4
separator membrane
Reversible cell reaction, so rechargeable batteries!
In commercial lead-acid batteries six such chambers are stacked
in parallel.Up to 400 A of current is produced in few seconds.
(aq. H2SO4)
H+
H+
H+
H+
H+
H+
HSO4-
HSO4-
HSO4-
HSO4-
HSO4-
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Cell reactions in lead acid batteries
Balance the cell reaction in sulfuric acid medium
Pb (s) + PbO2 (s) PbSO4 (s)
At anode:
At cathode:
Pb (s) PbSO4 (s)
PbSO4 (s)PbO2 (s)
Net reaction:
The net reaction is reversed when the cell is recharged!
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Hydrogen fuel cells
H2 (g) 2H+ (aq) 2e-
O2 (g) + 2H+ (aq) + 2e- H2O (l)
+ Eored = o.0 V
Eored = o.4 V
Overall cell reaction:
O2 (g) + H2 (g) H2O (l)
We can design a cell in which, hydrogencan be oxidized at anode
and oxygencan be reduced at cathode and wecan obtain a cell
voltage!
Such type of cell is known as hydrogenfuel cells that can power
an automobile
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Summary:Balancing redox reaction in ion-electron method via half
cells
Spontaneous electrochemical cell: Galvanic cell (e.g. Daniell
cell)
Prediction of spontaneity of an electrochemical reaction: G <
0, Ecell > 0
Nernst equation
Cell potential, half cell potentials, liquid junction potential
& std. reduction potentials
Application of std. electrode potentials
Determination of solubility productDetermination of mean ionic
activity coefficientsDetermination of pH
Properties of ideal and non-ideal solution
Commercial primary cells : wet cell, dry cell, alkaline
battery
Commercial secondary cells : lead acid battery
Hydrogen fuel cells
