Transcript 13. - iannonechem.com

Application of
Electrochemical Cells
Lesson 12
Banana Watch
Application of Electrochemical Cells
1.
Zn/C or LeClanche Cell
Anode:
Anode Reaction:
Zn
Zn
Cathode:
Cathode Reaction:
Electrolyte:
C
Mn4+ +1e- → Mn+3
NH4Cl and MnO2
Inexpensive
→ Zn2+
Not rechargeable
+
2e-
Short life
Application of Electrochemical Cell
2.
The Alkaline Cell
Anode:
Anode Reaction:
Zn
Zn
Cathode:
Cathode Reaction:
Electrolyte:
C
Mn4+ +1e- → Mn3+
KOH and MnO2
More expensive
Not rechargeable
→ Zn2+
+
2e-
Longer life
Application of Electrochemical Cells
3.
The Lead Acid Battery (Automobile)
Anode:
Anode Reaction:
Cathode:
Cathode Reaction:
Electrolyte:
Pb
Pb
→ Pb2+
+
2ePbO2
PbO2 + HSO4- + 3H+ + 2e- → PbSO4 + 2H2
H2SO4
Rechargeable
Long life
Large current
Application of Electrochemical Cells
4. The Fuel Cell
Overall Reaction:
Expensive
H2 + ½O2 → H2O + energy
Requires fuel
Environmentally friendly
Nickel Cadmium
Rechargable
Cordless Phones
Nickel Metal Hydride
Rechargable
Lithium
Rechargable
Cameras
Laptops
Corrosion of Iron
Corrosion is oxidation:
Fe(s) → Fe2+ + 2e-
Rust is initially Fe(OH)2 which dries to become Fe2O3.
There are three requirements for the corrosion of iron.
Iron
Water
Oxygen
Corrosion is spontaneous or an electrochemical cell.
Corrosion of Iron
The anode reaction is the oxidation of Fe
The cathode reaction is the reduction of O2 and H2O- outer circle of drop
Cations to cathode and anions to anode
Electrons flow from the anode to the cathode
Anode Fe(s) → Fe2+ + 2eCathode
1/ O + H O + 2e- → 2OH2 2
2
OHFe
Fe(OH)2(s)
Water Drop
Fe2+ Fe2+
eFe e-
OHFe(OH)2(s)
Rust- low solubility
Fe
Iron Surface
Methods of Preventing Corrosion
Protective Coatings
Paint
Grease
Electroplating
Plastic
Cathodic Protection
Remember corrosion is oxidation
Make iron the cathode of a cell, which is the site of reduction,
oxidation cannot occur.
Fe Nail in Water
All lower than Fe.
Attach a piece of Zn or Mg
- Fe-
Cathode or Reduction
Zn Anode
-
Zn → Zn2+ + 2eSacrificial anode
Electrons flow from anode to cathode to protect the Fe
Mg bracelets on a ship
Cathodic Protection
Remember corrosion is oxidation Make iron the cathode of a cell,
which is the site of reduction, oxidation cannot occur.
+
e-
Cathode No oxidation!
50 km Fe pipe
Scrap iron anode Fe(s) → Fe2+ + 2eHow do we protect it from corrosion?
Coat with plastic
Make it the cathode or negative side of an electrolytic cell
Make Fe the negative side of an electrolytic cell
Cathodic Protection- red and H2 bubbles
Unprotected nail- blue indicating Fe2+
Fe and Cu- not a good idea
Add a piece of Zn to the Fe
Cathodic Protection- red and H2 bubbles
Solution is NaCl, K3Fe(CN)6, and phenolphathalein. Corrosion of Fe will show as blue- reaction with K3Fe(CN)6. Cathodic protection will reduce water and show as bubbles and
pink.
BC Fast Ferry
The Aluminum hull is protected by an electrolytic cell
When it was first put to sea this system was not in operation.
The Paint peeled off requiring a new multimillion dollar paint job.