hypochlorite

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The hypochlorite ion

The hypochlorite ion is ClO⁻. A hypochlorite compound is a chemical compound containing this group, with chlorine in oxidation state +1. Because of this oxidation state, the hypochlorite ion can be alternately named the chlorate(I) ion.

Hypochlorites are the salts of hypochlorous acid. Common examples include sodium hypochlorite (chlorine bleach or bleaching agent) and calcium hypochlorite (bleaching powder). Hypochlorites are frequently quite unstable — for example, sodium hypochlorite is not available as a solid, since removal of the water from NaClO solution converts it to a mixture of sodium chloride and sodium chlorate. Heating of NaClO solution also causes this reaction. Hypochlorites decompose in sunlight, giving chlorides and oxygen.

Due to their low stability, hypochlorites are very strong oxidizing agents. They react with many organic and inorganic compounds. Reaction with organic compounds is very exothermic and may cause ignition, so hypochlorites should be handled with care. They can oxidize manganese compounds, converting them to permanganates.

Preparation

The sodium salt of the hypochlorite ion, NaClO, is formed when chlorine is bubbled through dilute aqueous sodium hydroxide at room temperature.

Cl₂(g) + 2NaOH(aq) → NaCl(aq) + NaClO(aq) + H₂O(l)

In this reaction chlorine undergoes disproportionation, both reduction and oxidation. Chlorine, oxidation number 0, forms chloride Cl^- (oxidation number -1) and hypochlorite ClO^- (oxidation number +1). The same redox changes occur when chlorine is dissolved in water to form hydrochloric acid (HCl(aq))and hypochlorous acid (HClO(aq)).

Reacting chlorine with hot, concentrated sodium hydroxide forms chlorates of higher oxidation state.

3 Cl₂(g) + 6 NaOH (aq) → 5 NaCl(aq) + NaClO₃(aq) + 3 H₂O(l)

Chemistry

Acid reaction

Hypochlorites generate chlorine when mixed with dilute acids. For example, the reaction of hypochlorous and hydrochloric acids.

H⁺(aq) + OCl^-(aq) + H⁺(aq) + Cl^-(aq) → Cl₂(g) + H₂O(l)

This reaction can be made to proceed from right to left. If alkali, which introduces OH^- ions, is added, the direction of this reaction is reversed. That is, if alkali is added chlorine dissolves in water to form the chloride, Cl^-, and hypochlorite, OCl^-, ions. This is an example of Le Chatelier's principle predicting the direction of a reversible reaction according to the relative concentration of reactant and products. In this reaction, the alkali OH^- ions absorb the H⁺ ions to form water, driving the reaction to the left.

Bleaching action

Hypochlorites remove the colour from a wide range of dye stuffs.

Strength of oxidation

Hypochlorite is the strongest oxidizer of the generalized chlorates; thus it can oxidize almost any reducing agent. For example, hypochlorite can create the the stable oxidizing agent permanganate.

2 MnCl₂ + 5 NaClO + 6 NaOH → 2 NaMnO₄ + 9 NaCl+ 3 H₂O

Stability

Many hypochlorite compounds exist only in solution, and are nonexistent in a pure form, as is hypochlorous acid (HClO) itself.

Hypochlorite is unstable with respect to disproportionation (that is, it will oxidize and reduce itself). Upon heating hypochlorite will degrade to a mixture of chloride, oxygen and other chlorates.

2OCl^-(aq) → 2Cl^-(aq) + O₂(g)

3OCl^-(aq) → 2Cl^-(aq) + ClO₃^-(aq)

Hypochlorite is the least stable of the generalized chlorates.

Other oxyanions

If a Roman numeral in brackets follows the word "chlorate", this indicates the oxyanion contains chlorine in the respective oxidation state, namely:

Using this convention, "chlorate" means any chlorine oxyanion. An unqualified "chlorate" typically refers only to the +5 oxidation state.

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