Chemistry 12
Redox
Applications of Electrochemical Cells
1. The Lead-Acid Storage Battery (Automobile battery)
Anode (Pb Plate)
Cathode (PbO2 plate)
Pb
PbO2
Electrolyte H2SO4 (aq)
This cell is rechargeable (rxn can be reversed)
• Anode half-reaction (cell is discharging / operating)
Pb(s) + HSO4-(aq) Æ PbSO4 (s) + H+(aq) + 2e0
ON Pb _____creases
∴ this is an _________ half-rxn
+2
(recall LEOA)
• Cathode half-rxn (operating)
PbO2 (s) + HSO4-(aq) + 3H+(aq) + 2e- Æ PbSO4 (s) + 2H2O (l)
ON=____
GERC
ON=____
• Overall redox rxn (operating)
Pb(s) + PbO2(s) + 2H+(aq) + 2HSO4-(aq) Æ 2PbSO4 (s) + 2H2O(l) + electrical energy
Anode
Cathode
From H2SO4
electrolyte
White solid forms on plates
as battery discharges
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Chemistry 12
Redox
Note:
electrolyte
as the cell operates (as the rxn proceeds):
a float with gradations
1. anode and cathode dissolve, forming white solid (PbSO4) on both electrodes
plates.
2. [H+]↓ and [HSO4-]↓ and [H2O]↑
3. ∴ density of electrolyte decreases (since H2SO4 more dense than H2O)
∴ measure density using “hydrometer”
If float floats high….electrolyte is dense…..∴battery is full.
(now we just use a voltmeter)
• The carging reaction:
Electrical energy + 2PbSO4(s) + H2O (l) Æ Pb(s) + PbO2(s) +2H+(aq) + 2HSO4-(aq)
Supplied by
alternator or
generator
2. The Zinc-Carbon battery
Cathode
Carbon
Carbon
Steel Case
MnO2 (s)
Zinc Anode
Rod
NH4Cl & ZnCl2 paste
electrolyte
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Chemistry 12
Redox
• Cathode half-rxn:
2MnO2 (s) + 2NH4+(aq) +2e- Æ 2MnO(OH) (s) + 2NH3 (aq)
simplified:
(GERC)
Mn4+ + e- Æ Mn3+
• Anode half-rxn:
Zn (s) + 4NH3 (aq) Æ Zn(NH3)42+ + 2e-
(LEOA)
simplified: Zn(s) Æ Zn2+ + 2e-
This type of battery:
• Not rechargeable
• Doesn’t last very long
• Fairly cheap
3. The Alkaline Dry Cell
•
•
•
•
•
+
Operates under basic conditions
Delivers much greater current
Voltage remains constant
More expensive
Lasts longer
MnO2 (cathode)
KOH
(Alkaline electrolyte)
Powdered Zn anode
Cathode half-rxn (GERC)
Anode half-rxn (LEOA)
2MnO2 + H2O +2e- Æ Mn2O3 + 2OHZn(s) +2OH- Æ ZnO + H2O +H2O +2e-
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Chemistry 12
Redox
4. Fuel cells
•
•
•
•
•
•
Are continuously fed fuel
chemical energy in fuel Æ electrical energy
about twice as efficient as burning gas or diesel (generators)
no pollution – only produces water
can use H2 and O2 or hydrogen rich fuels (e.g. methane CH4) and O2.
Use: space capsules. Carry H2 & O2 in tanks. H2O product used as drinking water
+ H2 out
H2 in
Anode half-rxn:
2H2(g) + 4OH-(aq) Æ 4H2O(l) + 4e-
O2 in
O2 out
Anode
Cathode
Cathode half-rxn:
O2(g) + 2H2O(l) + 4e- Æ 4OH-(aq)
Overall rxn:
2H2(g) + O2(g) Æ 2H2O(l)
KOH (aq)
electrolyte
Carbon
electrode
impregnated
with catalysts
5. The Breathalyzer Test
The breath of an intoxicated person contains ethanol C2H5OH.
Acidified dichromate (Eo = 1.23 ) will oxidize alcohol!
C2H5OH + K2Cr2O7 + H2SO4 Æ CH3COOH + Cr2(SO4)3 + K2SO4 + H2O
ON=?
•
•
•
•
ON=?
ON=?
ON=?
Exhaled air mixes with standardized acidified dichromate
Put in a spectrophotometer (detects green light)
More alcohol produces more green Cr2(SO4)3 (green)
Machine can detect too much alcohol
From above reaction, name:
a) The OA:
b) The RA:
c) The product of oxidation:
d) The product of reduction:
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