Modern inorganic chemistry

GROUP VI 279

PROPERTIES

Pure hydrogen peroxide is a colourless, viscous liquid, m.p. 272.5 K,

density 1.4 gem"^3. On heating at atmospheric pressure it decom-

poses before the boiling point is reached ; and a sudden increase of

temperature may produce explosive decomposition, since the

decomposition reaction is strongly exothermic :

H 2 O 2 (1) -> H 2 O(1) + fO 2 (g):AH = -9
This is a disproportionation reaction, and is strongly catalysed by
light and by a wide variety of materials, including many metals (for
example copper and iron) especially if these materials have a large
surface area. Some of these can induce explosive decomposition. Pure
hydrogen peroxide can be kept in glass vessels in the dark, or in
stone jars or in vessels made of pure aluminium with a smooth
surface.
The structure of hydrogen peroxide is given below:

Rotation about the O — O bond is relatively easy. Hydrogen

bonding causes even more association of liquid hydrogen peroxide

than occurs in water.

AQUEOUS SOLUTIONS OF HYDROGEN PEROXIDE

Because of the instability of pure and concentrated aqueous solu-
tions of hydrogen peroxide, it is usually used in dilute solution. The
concentration of such solutions is often expressed in terms of the
volume of oxygen evolved when the solution decomposes:

2H 2 O 2 -> 2H 2 O + O 2 t

Thus a^410 volume' solution is such that 1 cm^3 yields 10cm^3 of
oxygen at s.t.p. From the above equation we see that 2 moles H 2 O 2
give 22.41 of oxygen at s.t.p. and using this fact the concentration
of any solution can be calculated.
Aqueous solutions of hydrogen peroxide decompose slowly; the
decomposition is catalysed by alkalis, by light and by hetero-
geneous catalysts, for example dust, platinum black and manganese