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 + SO₄^2-(aq)  → PbSO₄(s) + 2e^- 

Cathode/ reduction: 4H⁺ + PbO₂  + SO₄^2-  + 2e^- → PbSO₄(s) + 2H₂O

Adding both equations 

Pb + SO₄^2-(aq) + 4H⁺ + PbO₂  + SO₄^2-  + 2e-  → PbSO₄(s) + 2e^-  + PbSO₄(s) + 2H₂O

then simplify

The overall equation is

Pb(s) +  PbO₂(s) + 2H₂SO₄ (aq) → 2 PbSO₄(s) + 2H₂O(l)      E^o = 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.