Electrochemical Cells

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Presentation transcript:

Electrochemical Cells Application of Electrochemical Cells Lesson 12

Banana Watch

Application of Electrochemical Cells 1. Zn/C or LeClanche Cell Anode: Zn Anode Reaction: Zn → Zn2+ + 2e- Cathode: C Cathode Reaction: Mn4+ +1e- → Mn+3 Electrolyte: NH4Cl and MnO2 Inexpensive Not rechargeable Short life

Application of Electrochemical Cell 2. The Alkaline Cell Anode: Zn Anode Reaction: Zn → Zn2+ + 2e- Cathode: C Cathode Reaction: Mn4+ +1e- → Mn3+ Electrolyte: KOH and MnO2 More expensive Not rechargeable Longer life

Application of Electrochemical Cells 3. The Lead Acid Battery (Automobile) Anode: Pb Anode Reaction: Pb → Pb2+ + 2e- Cathode: PbO2 Cathode Reaction: PbO2 + HSO4- + 3H+ + 2e- → PbSO4 + 2H2 Electrolyte: H2SO4 Rechargeable Long life Large current

Application of Electrochemical Cells The Fuel Cell Overall Reaction: H2 + ½O2 → H2O + energy Expensive 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 Anode Fe(s) → Fe2+ + 2e- Cathode Water Drop 1/2 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+ + 2e- Cathode Water Drop 1/2 O2 + H2O + 2e- → 2OH- OH- Fe2+ Fe2+ OH- Fe Fe Fe e- e- Fe(OH)2(s) Fe(OH)2(s) Iron Surface Rust- low solubility

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 Fe Cathode or Reduction - - Zn Anode - - - - - - Attach a piece of Zn or Mg All lower than Fe. Fe Cathode or Reduction - - Zn Anode Zn → Zn2+ + 2e- Sacrificial 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- Scrap iron anode Fe(s) → Fe2+ + 2e- - Cathode No oxidation! 50 km Fe pipe How 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.