Batteries
Self contained electrochemical cell
•! Primary batteries (not rechargeable)
•! Secondary batteries (rechargeable)
•! Research Needed to Improve Batteries:
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Dry Cell (Flashlight Battery)
Anode: Zn(s)
Zn(s) ! Zn2+(aq) + 2e-
Cathode: NH4Cl + MnO2 + graphite paste
2NH4+(aq) + 2MnO2(s) + 2e- ! Mn2O3(s) + 2NH3(aq) + H2O
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Battery Connection in Series
Total emf is the sum of the individual emf’s
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Dry Cell Battery: Alkaline Version
ALKALINE CELL BATTERY: NH4Cl is replaced by KOH.
Provides up to 50% more energy.
Zn is used as a powder mixed with the electrolyte.
Anode: Zn(s) + 2OH- ! Zn(OH)2(aq) + 2eCathode: 2H2O(s) + 2MnO2(s) + 2e- !2MnO(OH)(s) + 2OH-(aq)
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Lead/Acid Batteries
DURING DISCHARGE:
Anode:
Pb(s) + SO42-(aq) ! PbSO4(s) + 2e-
E°
Cathode:
PbO2(s) + SO42-(aq) + 4H+ + 2e- ! PbSO4(s) + 2H2O
Overall:
Pb(s) + PbO2(s) + 2H2SO4 ! 2PbSO4(s) + 2H2O
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12 V Automotive Lead-Acid Battery
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Rechargeable Nickel-Cadmium Batteries
Anode: Cd metal
Cd(s) + 2OH-(aq) ! Cd(OH)2(s) + 2eCathode: NiO2(s)
NiO2(s) + 2H2O + 2e- ! Ni(OH)2(s) + 2OH-(aq)
Overall: E° =
Cd(s) + NiO2(s) + 2H2O ! Cd(OH)2(s) + Ni(OH)2(s)
PROBLEMS:
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Batteries: Rechargeable
NiMH:
Metal Hydride
During Discharge:
Cathode: NiO(OH)(s)
NiO(OH)(s) + 2H2O + 2e! ! Ni(OH)2(s) + 2OH!
Anode: NiMH
MH + OH! !H2O + M + e!
NiMH= Nickel + metal alloy with dissolved H atoms
M = ZrNi2 or LaNi5 (intermetallic compounds)
E0cell = 1.2 V
Advantages: Light weight, Last longer
Uses:
Batteries in hybrid cars
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Batteries: Rechargeable
Lithium Ion Battery
During Discharge:
Anode: solid (in graphite) Li(s) ! Li+ + eCathode :
Li+ + CoO2 + e- ! LiCoO2
Overall:
Li(s) + CoO2 ! LiCoO2
E0cell = 3.7 V
Advantages: Light weight, high energy density
Uses:
Cell phones, laptops
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Hydrogen Oxygen FUEL CELL
Assets:
!!
!!
!!
!!
Drawbacks:
!!
!!
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Anode:
2H2(g) + 4OH-(aq) ! 4H2O(l) + 4e-
Cathode:
O2(g) + 2H2O(l) + 4e- ! 4OH-(aq)
Overall:
2H2(g) + O2(g) ! 2H2O(l)
E0cell = 1.23 V
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Sample Problem:
What is Ecell for a fuel cell running in air (PO2 = 0.2 atm), at
pH = 2, with PH2 = 1 atm?
O2(g) + 4 H+(aq) + 4e! ! 2 H2O(l)
2 H+(aq) + 2e!
! H2(g)
Eored = +1.229 V
Eored = 0
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Methanol Fuel Cells
Methanol can be made from CO2 + H2.
Use of CO2 sequestered from power plants
“Carbon neutral fuel”
Anode:
CH3OH (g) + H2O(g) ! CO2(g)+ 6H+ + 6e-
Cathode:
3/2 O2(g) + 6H+ + 6e- ! 3H2O(g)
Overall:
CH3OH (g) + 3/2 O2(g) ! CO2(g)+ 2H2O(l)
Assets:
Drawbacks:
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Corrosion:
Spontaneous electrochemical process
•! What is needed for corrosion to occur? (besides Fe)
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•! How do we prevent corrosion?
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Corrosion
E°
E°
Overall reaction in acid:
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How Can Corrosion Be Prevented?
Cathodic Protection
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Corrosion
CATHODIC PROTECTION OF IRON
Galvanized Steel:
Tin cans:
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Corrosion
CATHODIC PROTECTION OF IRON PIPES
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ELECTROLYSIS
Definition:
An electrolytic cell consists of two electrodes in:
Electrolysis forces the reaction to run in the reverse:
The anode:
The cathode:
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COMMERCIAL APPLICATIONS OF ELECTROLYSIS:
Production of metals
Na
Al
Purification of Metals
Cu
Electroplating
Ag
Au
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ELECTROLYSIS OF MOLTEN NaCl
What are the reactions at the electrodes?
E°
Cathode:
Anode:
___________________________
Overall:
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ELECTROLYSIS OF AQUEOUS NaCl
********What species are present in the system?
Which will be the CATHODE?
E°
Which will be the ANODE?
What are the reactions at the electrodes?
Cathode:
Anode: ______________________________
Overall:
What is left behind?
THIS IS THE CHLOR-ALKALAI PROCESS; 3-5% OF WORLD’S
ELECTRICITY USED TO DRIVE THIS REACTION
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ELECTROLYSIS OF AQUEOUS Na2SO4
DOES THIS PROCESS PRODUCE Na(s)?
********What species are present in the system?
Which will be the CATHODE?
E°
Which will be the ANODE?
What are the reactions at the electrodes?
Cathode:
Anode: ______________________________
Overall:
What is left behind?
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Electrometallurgy of Aluminum
Hall Process Electrolysis Cell is used to produce aluminum.
Problem: Al2O3 melts at 2000°C. Why is this a problem?
Solution: molten cryolite, Na3AlF6
What happens to a melting point when impurities are added?
Anode: C(s) + 2O2-(l) ! CO2(g) + 4eCathode: 3e- + Al3+(l) ! Al(l)
What happens to the graphite rods during the reaction?
To produce 1000 kg of Al, we need:
•!
kg of bauxite,
•!
kg of cryolite,
•!
kg of C anodes
•!
of energy.
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Electrorefining of Copper:
What are the reactions at the electrodes?
Anode: impure Cu ore; mixture of metals (Cu, Ni, Fe, Zn, Ag, Au, Pb...)
Cathode: thin sheet of pure Cu
As the electrolysis reaction proceeds,
what happens to Cu? Other metals?
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ELECTROLYSIS CALCULATIONS
1 mole of e- = charge of 1 Faraday
= 96,485 Coulombs
= charge on 1 mole of e1 Ampere = 1 coulomb/second
1 coulomb = 1 Amp-sec
Electromotive Force (EMF) = Cell potential
force that causes electrons to flow (voltage)
1 Watt = 1 Amp-Volt
1 Joule = 1 coul-Volt = 1 Amp-sec-Volt
= 1 Watt-sec
1 kW-hour = (1000 Watt)(3600 sec)
= 3.6 x 106 Watt-sec
= 3.6 x 106 Joules!
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ELECTROLYSIS CALCULATIONS
If Electrolysis gives 1.00 g of Cu (63.54 g/mol) from CuSO4 according
to the following reaction, How many Faradays (F) of charge are
required?
Cu2+ + 2e- ! Cu
How many Coulombs of charge is this?
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CALCULATIONS CONTINUED
If 1.00 g of Cu is obtained in 1 hour using 3030.1 C of charge, how
many amps of current are required?
If 2 amps were used, how long would it take to produce 1 g of Cu?
How can we calculate the maximum amount of work required to run
an electrolytic cell?!
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