280 GROUP VI
(IV) oxide, the latter being used in the common laboratory prepara-
tion of oxygen from hydrogen peroxide (p. 260).ACIDITY
Hydrogen peroxide in aqueous solution is a weak dibasic acid; the
dissociation constant Ka for H 2 O 2 ^ H+ + HO^ is 2.4 x 1CT^12
mol I"^1 , indicating the strength of the acid (pKa = 11.6). The salts,
known as peroxides (e.g. Na 2 O 2 ) yield hydrogen peroxide on acidifi
cation and this reaction provides a useful method of differentiating
between peroxides which contain the O—O linkage, and dioxides.
OXIDISING AND REDUCING PROPERTIES
Hydrogen peroxide has both oxidising properties (when it is con-
verted to water) and reducing properties (when it is converted to
oxygen); the half-reactions are (acid solution):
oxidation: H 2 O 2 (aq) + 2H 3 O+ + 2e" -> 4H 2 O: E^ = +1.77 V
reduction: O 2 (g) + 2H 3 O+ + 2e" -> 2H 2 O 2 (aq):£^ - +0.69 V
The following reactions are examples of hydrogen peroxide used as
an oxidising agent:
- Lead(II) sulphide is oxidised to lead(II) sulphate; this reaction
has been used in the restoration of old pictures where the white lead
pigment has become blackened by conversion to lead sulphide due
to hydrogen sulphide in urban air:
PbS + 4H 2 O 2 -» PbSO 4 + 4H 2 O
black white- Iron(II) is oxidised to iron(III) in acid solutions:
2Fe2+ + H 2 O 2 + 2H+ -* 2Fe3+ + 2H 2 O - Iodide ions are oxidised to iodine in acid solution :
21 ~ + 2H+ + H 2 O 2 -» I 2 + 2H 2 OAs the above redox potentials indicate, only in the presence of very
powerful oxidising agents does hydrogen peroxide behave as a
reducing agent. For example:
- Chlorine water (p. 323) is reduced to hydrochloric acid:
HC1O + HO -> HO + HC1 + O 2 T