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10.4: Oxyacids of Chlorine

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    212675
  • Table \(\PageIndex{1}\) lists the various oxyacids of chlorine. The relative strengths increase with the number of oxygen atoms since the more there are, the greater is the extent to which the negative charge on the resulting anion can be delocalized.

    Table \(\PageIndex{1}\): Relative acidity of oxyacids of chlorine.
    Oxyacid Formula pKa
    Hypochlorous HOCl 7.5
    Chlorous HClO2 1.9
    Chloric HClO3 -2
    Perchloric HClO4 -10

    Hypochlorous acid

    Hypochlorous acid (HOCl) can be made pure in the gas phase, (10.4.1), while strong acid solutions can be made from Cl2O. In contrast, dilute aqueous solutions are obtained with a suspension of mercury oxide to remove the chloride, (10.4.2).

    \[ \rm H_2O_{(g)} + Cl_2O_{(g)} \rightleftharpoons 2 HOCl_{(g)}\]

    \[ \rm 2 Cl_2 + 2 HgO_{(s)} + H_2O \rightarrow 2 HOCl + HgO \cdot HgCl_2 \downarrow \]

    Solutions of the anion, OCl-, are obtained by electrolysis of brine solution; allowing the products to mix at low temperature, (10.4.3).

    \[ \rm Cl_2 + 2 OH^- \rightarrow ClO^- + Cl^- + H_2O\]

    The anion (hypochlorite) is a good oxidant, (10.4.4) and (10.4.5), but can undergo disproportionation, (10.4.6); slowly at 25 °C, but fast above 80 °C.

    \[ \rm ClO^- + NH_3 \rightarrow NH_2Cl + OH^-\]

    \[ \rm ClO^- + 2 I^- + 2 H^+ \rightarrow I_2 + Cl^- + H_2O \]

    \[ \rm 3 ClO^- \rightarrow 2 Cl^- + ClO_3^-\]

    Chlorous Acid

    Chlorous acid (HOClO) is prepared by the reaction of ClO2 with base, (10.4.7), followed by the precipitation of the ClO2- salt with barium chloride, (10.4.8). The barium salt is dried and then reacted with a calculated amount of H2SO4, (10.4.9).

    \[ \rm 2 ClO_2 + 2 OH^- \rightarrow ClO_2^- + ClO_3^- + H_2O\]

    \[ \rm 2 ClO_2^- + BaCl_2 \rightarrow 2 Cl^- + Ba(ClO_2)_2 \downarrow\]

    \[ \rm Ba(ClO_2)_2 + H_2SO_4 \rightarrow Ba(SO_4) + 2 HClO_2\]

    The pure acid is unknown since it is too unstable, however, salts can be prepared directly, e.g., (10.4.10).

    \[ \rm 2 ClO_2 + Na_2O_2 \rightarrow 2 NaClO_2 + O_2\]

    The anion (ClO2-) is stable in alkaline solutions but in acid solutions decomposition occurs, (10.4.11).

    \[ \rm 5 HClO_2 \rightarrow 4 ClO_2 + Cl^- + H^+ + 2 H_2O\]

    As with hypochlorite, the chlorite anion is a strong oxidant, (10.4.12).

    \[ \rm ClO_2^- + 4 I^- + 4 H^+ \rightarrow 2 I_2 + 2 H_2O + Cl^-\]

    Chloric Acid

    The chloric anion (ClO3-) is made from the reaction of chlorine gas with hot alkali (80 °C) or by the electrolysis of hot NaCl solution.

    \[ \rm 3 Cl_2 + 6 OH^- \rightarrow ClO_3^- + 5 Cl^- + 3 H_2O\]

    To obtain a solution of the acid, ClO3- is precipitated as the barium salt, (10.4.14), which is removed, dried, and suspended in water and treated with a calculated amount of H2SO4, (10.4.15). The free acid cannot be isolated and a maximum concentration of only 40% can be obtained in water.

    \[ \rm 2 ClO_3^- + BaCl_2 \rightarrow 2 Cl^- + Ba(ClO_3)_2 \downarrow\]

    \[ \rm Ba(ClO_3)_2 + H_2SO_4 \rightarrow Ba(SO_4) + 2 HClO_3\]

    The ClO3- anion is pyramidal both in solid salts and in solution, and many salts are known; however, those with organic cations are explosive. The anion is a strong oxidizing agent, (10.4.16) and (10.4.17), and it disproportionates slowly in solution, (10.4.18).

    \[ \rm ClO_3^- + 6 I^- + 6 H^+ \rightarrow 3 I_2 + 3 H_2O + Cl^-\]

    \[ \rm ClO_3^- + 3 NO_2^- \rightarrow 3 No_3^- + Cl^-\]

    \[ \rm 4 ClO_3^- \rightarrow 3 ClO_4^- + Cl^-\]

    Perchloric Acid

    The perchlorate anion (ClO4-) is best made by electrolytic oxidation of chlorate in aqueous solution, (10.4.19). Fractional distillation can concentrate the solution to 72.5% which is a constant boiling mixture. This concentration is moderately safe to use, however, 100% perchloric acid may be obtained by dehydration with H2SO4.

    \[ \rm ClO_3^- + H_2O \rightarrow ClO_4^- + 2 H^+ + 2 e^-\]

    WARNING

    Perchloric acid is a very dangerous liquid that will explode if traces of metal ions are present. It is also a very strong oxidizing agent that will convert organic compounds to CO2 and H2O.

    Perchloric acid is a very strong acid that is fully ionized in aqueous solution, such that the salt [H3O][ClO4] can be isolated. Many other perchlorate salts are known, but those with organic cations are explosive. Perchlorate salts of metals are often used when studying complex formation in aqueous solution, because ClO4- is a very weak ligand (PF6- is better) and unlikely to form complexes itself. However, perchlorate does complex with +3 and +4 cations.

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