4.
PbO
2
<->Pb
4+
+ 2O
2-
Pb
4+
+2e
-
<->Pb
2+
Pb
2+
+ SO4
2-
<-> PbSO4
Pb<->Pb
2+
+ 2e
-
Pb
2+
+ SO4
2-
<-> PbSO4