Inorganic answer 3
The evolution of carbon dioxide C from A suggests that A is a carbonate; the yellow residue that turns white on cooling is zinc oxide, C. This colour change is unique, and depends on zinc ions moving a little in the crystal lattice as it is heated, returning to their original positions on cooling. You might therefore say that A is ZnCO3. This substance does not exist; zinc carbonate is a basic salt, ZnCO3.2Zn(OH)2, that is has zinc hydroxide incorporated in the lattice. Zinc hydroxide loses water on heating and also forms zinc oxide. Its presence accounts for the water vapour that was observed:
A à B + C: ZnCO3.2Zn(OH)2 à 3ZnO + 2H2O + CO2.
Zinc oxide gives a solution of zinc sulphate, D, with dilute sulphuric acid, but
all the subsequent reactions are of the zinc ion [Zn(H2O)6]2+,
so I shall use ionic equations and leave the spectator sulphate ion out:
B à D: ZnCO3.2Zn(OH)2 + 6H+ + 15 H2O à 3 [Zn(H2O)6]2+ + CO2.
This is perhaps a bit daunting, and since basic salts are not in the Edexcel syllabus a satisfactory equation is
B à D: ZnCO3 + 2H+ + 6H2O à [Zn(H2O)6]2+ + H2O + CO2.
Solution D with sodium hydroxide gives a precipitate of zinc hydroxide, E,
which being amphoteric reacts further with hydroxide ions to give the zincate ion F,
which is soluble and colourless:
D à E: [Zn(H2O)6]2+ + 2OH- à Zn(OH)2 + 6H2O
E à F: Zn(OH)2 + 2OH- à [Zn(OH)4]2-
Ammonia also precipitates zinc hydroxide G from aqueous solutions of zinc ions. In
excess ammonia this reacts further to give the colourless and soluble tetramminezinc(II)
ion:
Zn(OH)2 + 4NH3 à [Zn(NH3)4]2+ + 2OH-
Basic zinc carbonate occurs naturally as the mineral calamine, which has long been
used as part of a lotion for skin disorders, and which is used to promote healing. Zinc
oxide, as an ointment or in plasters or dressings, is used for the same purpose, and
although poisonous in large quantities zinc is a necessary trace element for health.
© JRG Beavon 1999
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