The evolution of carbon dioxide on heating A suggests that it is a
carbonate, lead(II) carbonate; the residue B is therefore an oxide, lead(II) oxide: A
à B: PbCO3
à PbO + CO2
On prolonged heating at 470oC lead(II) oxide (which is also called litharge)
partially oxidises to the mixed oxide Pb3O4 which contains both
lead(II) and lead(IV) ions.
B à C: 6PbO
+ O2 à 2 Pb3O4
Pb3O4 is scarlet and is also known as red lead and as minium.
It behaves as PbO2.2PbO. With nitric acid the basic lead(II) oxide portion
reacts to give a solution of lead(II) nitrate, D, and a residue of lead(IV) oxide, E:
C à D + E: Pb3O4
+ 4HNO3 à 2Pb(NO3)2 + PbO2
+ 2H2O.
With lead(II) nitrate solution sodium hydroxide gives an initial precipitate of lead(II)
hydroxide F; this substance is amphoteric and reacts with more sodium hydroxide to
give a solution of sodium plumbate(II), G, which is colourless:
D à F: Pb(NO3)2
+ 2NaOH à Pb(OH)2 + 2NaNO3
or, ionically, Pb2+ + 2OH- à
Pb(OH)2.
F à G:
Pb(OH)2 + 2NaOH à Na2Pb(OH)4
Or, ionically, Pb(OH)2 + 2OH - à
[Pb(OH)4]2-.
Lead(IV) oxide is a strong oxidising agent and oxidises concentrated HCl to chlorine, J,
and lead(II) chloride I. Lead(II) chloride complexes with chloride ions to give
various species such as PbCl3- and PbCl42-,
which are yellow and water-soluble:
E à I + J: PbO2 + 4HCl à PbCl2 + 2H2O + Cl2.
J à K:
PbCl2 + Cl- à PbCl3-
.
Chlorine oxidises potassium iodide solution to iodine, L, which in the presence of
excess iodide ions gives a brown solution of the tri-iodide ion:
J à L: Cl2 + 2I- à I2 + 2Cl-
I2
+ I- à I3- .
Lead(II) nitrate solution gives a precipitate of lead(II) iodide M with potassium
iodide solution. Lead(II) iodide is bright yellow when solid, but gives a colourless
solution in hot water, from which shimmering yellow plates crystallise on cooling. The
colour arises from interactions in the crystal lattice and not from the ions themselves,
both of which are colourless:
D à M:
Pb2+ +
2I- à PbI2 . |