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210 Halogen Chemistry

The iodine oxide I 2 O 5 , prepared by dehydration of iodic potential to its immediate left is more positive than a
acid (HIO 3 ), is a white, crystalline solid at room tempera- reduction potential to its immediate right.
ture, stable to 300 C. This compound is extremely soluble Reduction potentials are altered by changing the
◦
in water, reforming iodic acid. It is an important reagent concentrations of the species present. Figure 1 is strictly
for the determination of carbon monoxide (CO) in gaseous valid only for unit activity (about 1 M concentration of the
mixtures. The reaction, species involved). Two different sets of diagrams are
provided: one for unit activity of acid and a second for
5CO + I 2 O 5 → I 2 + 5CO 2 , (38)
unit activity of base.
proceeds rapidly and quantitatively at room temperature, To illustrate the use of these diagrams, the stability of
and the iodine that is formed is titrated using sodium thio- I 2 in 1 M base will be considered. The diagram for iodine-
sulfate (iodometry). containing species in base shows I 2 will disproportionate
−
−
−
The remaining two oxides of iodine are not well charac- to I and IO [Eq. (15)]. IO should, however, also be
terized. Oxides I 2 O 4 and I 4 O 9 are only moderately stable unstable, undergoing disproportionation to I − and IO −
3
−
and decompose on heating to give I 2 O 5 and iodine, or [Eq. (16)]. Further disproportionation of IO is not ex-
3
iodine and oxygen, respectively. pected because the reduction potential involving forma-
−
tion of IO is more positive than any reduction potential
4
−
to the left of IO . It should be noted that these diagrams
3
VI. OXYACIDS AND THEIR ANIONS only predict what reactions are allowed to happen; they
say nothing about how fast the reactions will occur (see
A. General Survey Section I.G.).
All of the halogens form oxyacids and salts of these acids.
B. Hypohalous Acids and Hypohalite Salts
The names of these compounds are determined by the
oxidation state of the halogen. Hypofluorous acid is formed by passing fluorine gas at
◦
low pressure over water at 0 C:
+1, HOX (hypohalous acid); OX (hypohalite anion)
−
F 2 + H 2 O → HOF + HF. (39)
−
+3, HXO 2 (halous acid); XO (halite anion)
2
−
+5, HXO 3 (halic acid); XO (halate anion) AlthoughtheoxidationstateofFinHOFistechnically+1,
3
+7, HXO 4 (perhalic acid); XO (perhalate anion) the actual charge is estimated to be somewhat intermediate
−
4
to that of F in HF and OF 2 .
The only oxyacid of fluorine is HOF. Numerous oxyacids HOF is only marginally stable at room temperature,
are known for the other halogens (Fig. 1), although most decomposing to HF and O 2 with a half-life of about 30 min
cannot be isolated pure and are stable only in aqueous at room temperature. HOF also reacts rapidly with water,
solution or as salts. the predominant products being hydrogen peroxide and
The oxyacids become stronger acids as the number HF under acidic conditions and O 2 and HF under alkaline
of oxygen atoms increases (pK a : HOCl, 7.52; HClO 2 , conditions.
1.94; HClO 3 , −3; HClO 4 , +10). The perhalic and halic The other hypohalous acids are all very reactive oxidiz-
acids ionize essentially completely in aqueous solution ing agents that are relatively unstable. They are most con-
and are considered strong acids. Halous acids are moder- veniently prepared by the disproportionation of the par-
ately strong, and hypohalous acids are weak. For a series ent halogen, X 2 , in water [Eq. (14)]. Addition of HgO or
of analogous oxyacids, the acid strength increases in the Ag 2 O precipitates the halide ion and shifts the equilibrium
order I < Br < Cl (i.e., HOI < HOBr < HOCl). so as to favor additional hypohalite formation. Attempts
In water, many of the oxyacids and their anions are un- to prepare pure samples from aqueous solution lead to
stable with respect to disproportionation (self-oxidation decomposition.
and reduction). The tendency for this kind of instability All three acids disproportionate in water, the rate and
may be conveniently determined from the reduction products of this reaction being influenced by temperature,
potential diagrams shown in Fig. 1. A large, positive concentration, and solution pH. HOCl is more stable than
standard reduction potential (the number over the arrows) HOBr or HOI. Base converts the acids to the anions, which
indicates a strong tendency for the particular reaction disproportionate less readily. Anion stability decreases in
indicated by the arrow (a reduction). A large, negative the order ClO > BrO > IO .
−
−
−
standard reduction potential indicates a strong tendency HOCl is the most commercially useful hypohalous acid,
for change in the opposite direction (an oxidation). A being employed primarily for bleaching and sterilizing. It
particular oxyacid or its anion will not be stable with is generally manufactured by disproportionation of Cl 2
respect to self oxidation and reduction if a reduction using a strong base such as sodium or calcium hydroxide,