110 ACIDS AND BASES: OXIDATION AND REDUCTION
- (a) Outline the principles of the method you would use to
measure the standard redox potential for the reaction- 8H+ + 5e' -» Mn2+ + 4H 2 O
(b) The standard redox potentials for Ce^4 + /Ce^3 + (Ce = cerium)
and Fe3+/Fe2+ are + 1,610 V and + 0.771 V respectively.
Deduce the direction of the reaction
Ce3+ -f Fe^3 ^ =Ce4+ + Fe^2 +
and outline an experiment you could use to find the end
point when the reaction is carried out as a titration. (N.B.
Both Ce^4 ^ and Fe3+ ions are yellow in aqueous solution.)
- 8H+ + 5e' -» Mn2+ + 4H 2 O
(c) What explanation can you offer for the fact that the
standard electrode potentials of copper and zinc are
-I- 0.34 V and - 0.76 V respectively, although the sums of
the first two ionisation energies for both metals are approxi-
mately 2640 kJ mol" l (640 kcal mol" ')?
(CA)- The following redox potentials are given for the oxidation of
manganese(II) to manganese(III) in acid and alkaline solution.
Acid
Mn3+ +e = Mn2+ 4- 1.51V
O 2 + 4H+ + 4e-2H 3 O + 1.23V
Alkaline
Mn(OH) 3 + e = Mn(OH) 2 + OH" - 0.40V
O 2 + 2H 2 O + 4e = 4OH + 0.40 V
(a) Would manganese(II) be oxidised to manganese(III) by
atmospheric oxygen under
(i) acid
(ii) alkaline, conditions?
(b) What would you expect to happen if anhydrous MnF 3
were dissolved in water?
(N,Phys.Sci.,PartI) - Discuss the factors which influence the redox potential of a
half-reaction, illustrating your answer by as many examples as
possible.
(Liverpool B.Sc., Part I)