Chlorine undergoes disproportionation in key reactions important for water treatment and manufacture of household bleach. Understanding its oxidation changes clarifies why chlorine behaves as both an oxidising and reducing agent.

Chlorine Disproportionation and Uses

What Disproportionation Means

When chlorine reacts and is simultaneously oxidised and reduced, it undergoes disproportionation, a core idea for interpreting halogen chemistry. This behaviour explains chlorine’s role in sanitation and industry.

Chlorine is particularly prone to disproportionation because its atoms can adopt a range of oxidation states from −1 to +7, making multiple electron-transfer pathways possible during reactions with water and alkalis.

Disproportionation of Chlorine with Water

When chlorine gas dissolves in water, it reacts in a reversible equilibrium forming chloric(I) acid (HClO) and hydrochloric acid (HCl). This showcases chlorine acting as an oxidising and reducing agent in the same reaction.

• In this reaction:

  • One chlorine atom is reduced from oxidation state 0 to −1 in HCl.

  • The other chlorine atom is oxidised from oxidation state 0 to +1 in HClO.

The formation of chloric(I) acid is central to chlorine’s antibacterial effect because HClO is a powerful oxidising agent that kills pathogens by disrupting cellular structures.

Formation of Hypochlorite in Sunlight

Although not always emphasised, understanding chlorine’s behaviour in light helps explain why sunlight reduces the effectiveness of chlorinated water.

• UV light accelerates decomposition of chloric(I) acid into chloride and oxygen.

• This reduces the concentration of the active disinfecting species, meaning chlorine-treated water loses potency when exposed to direct sunlight.

Disproportionation of Chlorine with Cold, Dilute Sodium Hydroxide

A second key disproportionation occurs when chlorine reacts with cold, dilute sodium hydroxide. This reaction produces sodium chloride, sodium chlorate(I) (NaClO), and water.

3D ball‑and‑stick diagram of sodium hypochlorite, NaClO, the main oxidising species in many chlorine-based bleaches. The model highlights the arrangement of sodium, chlorine and oxygen atoms in the hypochlorite salt formed by disproportionation in cold, dilute sodium hydroxide. The precise 3D geometry is more detailed than required by the OCR specification but provides useful visual context for the active bleaching ion. Source

Sodium chlorate(I) is the active component in common household bleach.

• Oxidation changes:

  • Chlorine reduced from 0 to −1 in NaCl

  • Chlorine oxidised from 0 to +1 in NaClO

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The product sodium chlorate(I) is a stable hypochlorite and can be stored and transported more easily than chlorine gas, making it highly practical for domestic and industrial uses.

Why Cold, Dilute Conditions Matter

The specification focuses on cold, dilute NaOH because these conditions favour the formation of chlorate(I) rather than higher oxidation-state oxyanions.

• Using hot, concentrated NaOH would instead produce chlorate(V) (NaClO₃), which is beyond the OCR A-Level requirement.

• Cold conditions stabilise the +1 oxidation state, preventing further oxidation of chlorine species.

Uses of Chlorine Based on Its Disproportionation Chemistry

1. Water Treatment

Disproportionation in water generates HClO, the key sterilising agent. Chlorination helps eliminate bacteria, viruses, and organic contaminants.

Aerial photograph of a modern water treatment facility with multiple large tanks and treatment basins. Chlorine-based processes are commonly used at such plants to disinfect water before it enters the public supply, linking the disproportionation chemistry of chlorine to real-world infrastructure. The image does not show the chlorine dosing equipment explicitly but effectively illustrates the industrial setting in which chlorination is carried out. Source

• HClO penetrates cell membranes and oxidises vital cellular components.

• Chlorinated water remains effective due to the establishment of a reversible equilibrium generating small amounts of HClO continually.

However, students should note that chlorination can produce trace amounts of by-products such as chlorinated hydrocarbons, though this is outside the immediate specification’s scope.

2. Household Bleach

Bleach solutions rely on the disproportionation of chlorine with cold sodium hydroxide.

• Active ingredient: NaClO

• Function: Oxidises coloured compounds, breaking chromophores and removing stains.

• pH: Typically alkaline to stabilise the hypochlorite ions, preventing decomposition.

Hypochlorite is widely used for disinfecting surfaces, treating wastewater, and cleaning due to its broad-spectrum antimicrobial activity.

3. Industrial and Environmental Uses

Chlorine’s ability to disproportionate under controlled conditions supports several important applications:

• Paper and textiles: Hypochlorite solutions help remove pigments during processing.

• Waste treatment: Oxidising capability of hypochlorite aids in degrading harmful organic substances.

• Swimming pools: Controlled dosing ensures a steady supply of HClO, maintaining microbiological safety.

Oxidising Behaviour of Chlorine During Disproportionation

Chlorine’s role as an oxidising agent originates from its high electronegativity and ability to accept electrons. In both water and alkali reactions, chlorine provides pathways for electron transfer that generate oxidising species.

• In water: HClO acts as the active oxidant.

• In alkali: Hypochlorite ions (ClO⁻) carry out oxidation.

Chlorine’s ability to switch oxidation states makes these applications versatile and chemically efficient.

Safety Considerations

Although not examined in depth, familiarity with key safety practices is useful when handling chlorine and its disproportionation products.

• Chlorine gas is toxic and corrosive; reactions are typically carried out in fume cupboards.

• Hypochlorite solutions should be stored away from acids to prevent release of chlorine gas.

• Sunlight accelerates decomposition, so bleach is usually sold in opaque containers.

Key Points for OCR A-Level Chemistry

• Disproportionation reactions demonstrate simultaneous oxidation and reduction of chlorine.

• Reaction with water forms HCl and HClO; reaction with cold, dilute NaOH forms NaCl and NaClO.

• These reactions underpin major practical uses including water purification and bleach manufacture.