GROUP IV 203
of sulphuric acid. The lead plates are usually perforated and one is
packed with lead(IV) oxide, the other with spongy lead. An inert
porous insulator acts as a separator between the plates. When the
cell is producing current, the following reactions occur :Lead(IV) oxide plate (positive) :
PbO 2 4- 4H" + 2e' -+ Pb2r + 2H 2 O
followed by :
Pb^2 " + SO -Spongy lead plate (negative) :
Pb -> Pb2+ + 2e~
followed by :
Pb2+ + SO~ ....... -> PbSO 4
Hence the overall chemical reaction in the cell during discharge is :PbO 2 + Pb + 2H 2 SO 4 -> 2PbSO 4 4- 2H 2 O
Hence sulphuric acid is used up and insoluble lead(II) sulphate
deposited on both plates. This process maintains a potential
difference between the two plates of about 2 V. If now a larger
potential difference than this is applied externally to the cell (making
the positive plate the anode) then the above overall reaction is
reversed, so that lead dioxide is deposited on the anode, lead is
deposited on the cathode, and sulphuric acid is re-formed. Hence in
the electrolyte, we have :
, t. ._. discharge
sulphuric acid «_.. "> water
charge
The density of the electrolyte, measured by a hydrometer,
forms a useful indicator of the state of charge or discharge of the
battery.
If the charging process continues after all the lead sulphate has
been used up, then the charging voltage rises. Hydrogen is liberated
from the lead electrode, and oxygen is liberated from the lead
dioxide electrode. The accumulator is then said to be "gassing'.CHEMICAL TEST FOR GROUP IV ELEMENTSCarbon
All carbon compounds, if oxidised by either oxygen or an oxide (such