Lead acid battery

 

Chemistry

  • There are two types plates in a charged battery: Pb and PbO2 (brown)
  • The electolyte is 5-6 M H2SO4
  • Both plates are converted to PbSO4(s) on discharging. 

 

Therefore (LEARN)

  1. Pb → PbSO4(s)  [Oxidation number of Pb changes from 0 to +2.  Therefore lead is oxidised] Anode
  2. PbO2 → PbSO4(s)  [Oxidation numbersof Pb changes from +4  to +2. Therefore lead dioxide is reduced] Cathode

for a lead acid battery being discharged.

 

From this basic structure the following half equations can be derived.

Anode/ oxidation: Pb + SO42-(aq)  → PbSO4(s) + 2e

Cathode/ reduction: 4H+ + PbO2  + SO42-  + 2e- → PbSO4(s) + 2H2O

 

Adding both equations 

Pb + SO42-(aq) + 4H+ + PbO2  + SO42-  + 2e-  → PbSO4(s) + 2e + PbSO4(s) + 2H2O

then simplify

 

The overall equation is

Pb(s) +  PbO2(s) + 2H2SO(aq) → 2 PbSO4(s) + 2H2O(l)      Eo = 2 volts

 A typical automobile battery consists of 6 cells in series and generates a total of 12 volts.

 

Hazards of recharging:

If a flat battery a re-charged too fast, hydrogen gas will be generated inside the battery as a side reaction.

2H+(aq) + 2e-  → H2(g)

An internal spark may then cause the battery to explode and spray corrosive sulfuric acid into the face and body of a person. People have been blinded by this.

It is therefore strongly advised to re-charge flat batteries slowly overnight with a proper battery charger. This minimizes the production of hydrogen gas and also gives time for any hydrogen produced to dissipate.

 

 

 

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