|
[g cm–3]
100% H2O2 150.2 –0.42 1.443
70% H2O2 125 –40 1.288
60% H2O2 119 –56 1.241
50% H2O2 114 –52 1.196
Water 100 0 0.997
a. Extrapolated values because decomposition will reduce boiling
point continuously.
b. 25 °C.
Fig. 7.116 Configuration of hydrogen peroxide in the solid phase.
The bond length between the two oxygen atoms of the H2O2 molecule is rather
long (Fig. 7.116). Compared to water, the energy content of H2O2 is much higher.
For water, the heat of formation (DH) [Eq. (107)] from the elements is as low as –
286 kJ mol–1, whereas for H2O2 [Eq. (108)] the corresponding value is only –
188 kJ mol–1 [7]. In consequence, H2O2 is less stable and can disproportionate into
water and oxygen:
H 2 _ 0_5 O 2 _ H 2 O D H _ _286 kJ mol _1 _107_
H 2 _ O 2 _ H 2 O 2 D H _ _188 kJ mol _1 _108_
Since the activation energy for the cleavage of the oxygen–oxygen bond is rather
low (DH = –71kJ mol–1) [7], traces of contaminants can start this reaction. Basically,
the decomposition is a redox process, with H2O2 either supplying electrons
and yielding oxygen, or accepting electrons and yielding water. Metal salts of different
states of oxidation can start the decomposition reaction. The first step can
be the reduction according to Eq. (109):
7.6 Hydrogen Peroxide Bleaching 851
2 Me 2_ _ H 2 O 2 _ 2 Me _ _ O 2 _ 2 H _ _109_
The alternative is the oxidation of a metal according to Eq. (110):
2 Me _ _ H 2 O 2 _ 2 H _ _ 2 Me 2_ _ 2 H 2 O _110_
The reaction certainly can also start with the reduced form of metal. The overall
reaction is identical, it being the formation of water and oxygen from H2O2 with
the redox system of the metal is acting as the catalyst [8].
The decomposition of H2O2 is, in addition, catalyzed by alkali, with the reaction
steps being as follows:
H 2 O 2 _ OH _ _ H 2 O _ HOO _ _111_
HOO _ _ H 2 O 2 _ H 2 O _ O 2 _ OH _ _112_
Since bleaching with H2O2 requires alkaline conditions, this decomposition
reaction is very important for its technical application.
Single electron transfer reactions of H2O2 with catalysts yield radicals, these
decomposition reactions taking place with either metals or with enzymes (e.g.,
catalase). Radical formation may also be the result of a thermal cleavage of the
oxygen–oxygen bond:
H 2 O 2 _ Me _ _ OH _ _ _ OH _ Me 2_ _113_
H 2 O 2 _ _ OH _ H 2 O _ _ OOH _114_
H 2 O _ _ OOH _ _ OO _ _ H 3 O _ _115_
The hydroxyl radical, the hydroperoxy radical, and the superoxide anion radical
are important intermediates. Each of these cause side reactions in bleaching processes,
with delignification as a positive and depolymerization of the cellulose as a
negative result. In general, radicals produce more negative effects than positive
results on delignification. Therefore, if present in higher amounts, transition metal
ions must be removed by acid washing or “neutralized” by chelation before and
during a peroxide treatment.
Tab. 7.46 Standard oxidation potential for hydrogen peroxide [7].
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