Inorganic answer 4
The colours involved in this reaction scheme should suggest that A is iron. With dilute sulphuric acid iron gives a pale green solution of iron(II) sulphate B, and hydrogen, C:
A à B + C: Fe + H2SO4 à FeSO4 + H2
The pale green ion is [Fe(H2O)6]2+.
If solution B is allowed to crystallise, the iron(II) sulphate (also called green
vitriol) gives iron(III) oxide H on strong heating, and sulphur dioxide
(sulphur(IV) oxide) I and sulphur trioxide (sulphur(VI) oxide) J:
B à H + I + J: 2FeSO4 à Fe2O3 + SO2 + SO3.
The iron(III) oxide formed is a very fine powder, and is used as a polishing powder in the jewellery trade. It is sometimes called jewellers’ rouge.
If J is passed into a solution of barium chloride there is a vigorous reaction
since sulphuric acid is formed rather violently from sulphur trioxide and water. That is
why, in the Contact Process, the sulphur trioxide is absorbed in concentrated sulphuric
acid and the mixture then diluted. The white precipitate K is barium sulphate:
J à K: SO3 + H2O à 2H+ + SO42- (but see my article on the pH of sulphuric acid); followed by Ba2+ + SO42- à BaSO4.
Iron reacts with steam in an equilibrium reaction to give tri-iron tetroxide M,
which contains iron in oxidation states (+2) and (+3); Fe2O3.FeO;
and hydrogen, C. This reaction was at one time used industrially to make hydrogen.
The iron oxide formed was returned to the blast furnace.
A à M + C: 3Fe + 4H2O à Fe3O4 + 4H2
With gaseous HCl on heating iron gives white K, anhydrous iron(II) chloride, and
hydrogen:
A à K: Fe + 2HCl à FeCl2 + H2
Hydrated iron(II) chloride is pale green. With chlorine on heating iron gives brown
covalent iron(III) chloride L which sublimes on heating:
A à L: 2Fe + 3Cl2 à 2FeCl3
Aqueous solutions of iron(III) chloride react with copper metal to give iron(II) ions and
copper(II) ions, and for that reason are used to etch printed-circuit boards in
electronics:
2 [Fe(H2O)6]3+ + Cu + 6H2O à 2 [Fe(H2O)6]2+ + [Cu(H2O)6]2+
The solution of iron(II) ions, B gives a dirty-green precipitate of iron(II)
hydroxide D if sodium hydroxide is added; this precipitate remains in excess sodium
hydroxide since it is not amphoteric. The same reaction is seen with the addition of
ammonia, the latter acting simply as an alkali.
B à D: [Fe(H2O)6]2+ + 2OH- à Fe(OH)2 + 6H2O
If D is allowed to stand in air it forms a foxy-red compound E, which is
iron(III) hydroxide produce by air oxidation of iron(II) hydroxide. This is also obtained
if sodium hydroxide solution is added to solutions of L:
L à E: [Fe(H2O)6]3+ + 3OH- à Fe(OH)3 + 6H2O
Acidified solutions of iron(II) ions decolourise potassium manganate(VII) solution being
oxidised to iron(III) and giving the hexaquamanganese(II) ion N also:
B à F + N 5[Fe(H2O)6]2+ + 8H+ + MnO4- à 5[Fe(H2O)6]3+ + Mn2+ + 4H2O
© JRG Beavon 1999
Problem 4 Inorganic problems contents Home page