Chap 26D - Electrochem

Describe differences between Alkaline–Manganese Battery and dry cell Main advantage: ~50% improvement in properties compared to Leclanche cell

• Further development of Leclanche cell using Zn and MnO2 as the electrode 

• Main differences:

  • Use of KOH as the electrolyte which allows further reduction of the MnO2 at the cathode to Mn(II)

  • Use of metallic zinc powder suspended in alkali as anode rather than a sheet and the cell arrangement is the opposite of the Leclanche cell

  • MnO2(s) (OS Mn: +4) + H2O(l) + e– → MnO(OH)(s)  (OS Mn: +3 )+ OH–(aq) ---- (1) 

  • MnO(OH)(s) + H2O(l) + e– → Mn(OH)2(s) (OS Mn: +2) + OH–(aq) ---- (2)

State equations at electrodes at Alkaline–Manganese Battery

1. Cathode (+): Brass

• Reduction: By (1) + (2): MnO2(s) + 2H2O(l) + 2e– → Mn(OH)2(s) + 2OH–(aq)

2. Anode (–): Powdered zinc

• Oxidation: Zn(s) + 2OH–(aq) → Zn(OH)2(s) + 2e–

3. Overall reaction: Zn(s) + MnO2(s) + 2H2O(l) → Mn(OH)2(s) + Zn(OH)2(s); Ecell = +1.58 V

Draw Alkaline–Manganese Battery + state its advantages

Main advantage: ~50% improvement in properties compared to Leclanche cell

Describe Lead–Acid Accumulator

• Stores accumulated electric charge.

• During the discharging process, the cathode (+) is lead(IV) oxide, PbO2, the anode (–) is lead, Pb, and the electrolyte is aqueous H2SO4

State equations at electrodes of Lead–Acid Accumulator

1. Anode (–): Pb

• Oxidation: Pb(s) + SO42–(aq) ⇌ PbSO4(s) + 2e–

2. Cathode (+): PbO2

• Reduction: PbO2(s) + 4H+(aq) + SO42–(aq) + 2e– ⇌ PbSO4(s) + 2H2O(l)

State advantages and disadvantages of Lead–Acid Accumulator

• Main advantages: High voltage / current and rechargeable

• Main disadvantages: Heavy weight and discharge cannot take place completely since PbSO4 is non–conducting (at best 50% of the cell material is converted)

Draw Lead–Acid Accumulator