108 ACIDS AND BASES: OXIDATION AND REDUCTION
- Change a solution of iron(III) in aqueous solution to iron(II).
Reagents are oxidising if they:
- Liberate iodine from a potassium iodide solution acidified with
dilute sulphuric acid.
- Convert iron(II) to iron(III) in aqueous acid solution.
QUESTIONS
- (a) The following are standard redox potentials in volts in 1 N
acid solution for the reactions
Mn+ + xe~ -> M(n"x)+ (symbolised as Mfl+/M("~-x)+),
where, for example, the process
Na+ + e~ -> Na (symbolised as Na+/Na)
is defined as having a large negative potential:
Cr2+/Cr - 0.9 V, Mn2+/Mn - 1.2 V,
Cr3+/Cr2+ -0.4V, Mn3+/Mn2+ + 1.5V,
Fe^2 + /Fe -0.4V,
Fe3+/Fe2+ + 0.8V.
Use these data to comment upon:
(i) the stability in acid solution of Fe^3 + towards reducing agents
as compared to that of either Cr3+ or Mn3+ ;
(ii) the ease with which metallic iron can be oxidised to iron(II)
(ferrous) ions compared to the similar process for either
metallic chromium or metallic manganese;
(iii) the result of treating a solution containing either chromium(II)
(chromous) or manganese(II) (rnanganous) ions with a
solution containing iron(III) (ferric) ions.
(b) The following equations represent four chemical reactions
involving redox processes:
(i) 3N 2 H 4 + 2BrO 3 ^ -> 3N 2 + 2Br" + 6H 2 O,
(ii) 5As 2 O 3 + 4MnO4 + 12H+ -> 5As 2 O 5 + 4Mn2+ -f 6H 2 O,
(iii) SO 2 + I 2 + 2H 2 O -^ H 2 SO 4 + 2HI,
(iv) VOJ" + Fe2+ + 6H+ -> VO2+ -f Fe3+ + 3H 2 O
