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348 ELECTROCHEMISTRY

Plates of lead, each coated with lead dioxide, are immersed in fairly concentrated
sulphuric acid. Lead is oxidized at the lead anode during discharge:

Pb (s) + HSO − −−→ PbSO 4(s) + H (aq) + 2e − (7.55)
+
4(aq)
The reaction at the cathode during discharge is

+
PbO 2(s) + 2H (aq) + H 2 SO 4(aq) + 2e −−→ PbSO 4(s) + 2H 2 O (7.56)
−
Both half-cell reactions are fully electro-reversible. In practice,
Spongy lead has a there are two types of lead: the ‘collector’ electrode is made of lead
higher surface area alloy ‘mesh’ in order to give it greater structural strength, and is
than normal lead. made with about 5 per cent antimony. ‘Spongy lead’ (Pb + PbO 2 )
is introduced into the holes of the mesh.
Table 7.15 lists the advantages and disadvantages of the lead–acid battery.