Modern inorganic chemistry

96 ACIDS AND BASES: OXIDATION AND REDUCTION

oxygens, each -2. total —8; overall charge on ion = -L hence
the oxidation state of Mn = x 7, i.e. manganate(Vll) ion. (b)
Chlorine in the chlorate ions, ClO^ — there are three oxygens, each
— 2, total -6; overall charge on ion = — 1, hence the oxidation
state of Cl = -1-5 and the ion is a chlorate(V) ion. Chromium in
the dichromate ion C^O^"; there are seven oxygens each —2,
total = — 14; overall charge on ion = — 2, hence chromium atoms
share 12 formal positive charges and so the oxidation state of
chromium is 4-6, and the ion is dichromate(VI).
Oxidation states can be used to establish the stoichiometry for an
equation. Consider the reaction between the manganate(VII) (per-
manganate) and ethanedioate (oxalate) ions in acidic solution.
Under these conditions the MnOjfaq) ion acts as an oxidising
agent and it is reduced to Mn^2 r (aq), i.e.

Mnvn+ 5e~ -> Mn^2 +

The full half equation is

(i) MnO4(aq) + 8H 3 O+ + 5e~ -> Mn2+(aq) + 12H 2 O

The ethanedioate (oxalate) ion C 2 O|"(aq) is oxidised to carbon

dioxide, i.e.

(ii) C 2 O|-(aq)-+2CO 2 + 2e~

To maintain electrical neutrality in the reaction we need to multiply
(i) by 2 and (ii) by 5, ten electrons being transferred. The overall
reaction then becomes

2MnO 4 (aq) + 16H 3 O+ + 10e" -> 2Mn^2 + (aq) + 24H 2 O

5C 2 O2"(aq)-> 1QCQ 2 + 10g~ _______

2^6"4laq)"TT6H^OT"~+ 5C 2 Of:r^ 2MnIT(aq)

-f 24H 2 O + 10CO 2

Consider also the oxidation of iron(II) ions by dichromate(VI)
ions in acidic solution. The Q^O^" is reduced to Cr3+(aq)

Cr 2 O?"(aq) -h 6e~ -» 2Cr^3 + (aq)

The full half equation is

(i) Cr 2 O?~(aq) -f 6e" 4- 14H 3 O+ ^ 2Cr^3 + (aq) -h 21H 2 O

The Fe^2 + (aq) is oxidised to Fe^3 ^(aq), i.e.

(ii) Fe^2 + (aq) -» Fe^3 + (aq) -he"