In simple terms
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Chlorine's Double Life
Chlorine doesn't just react; it can simultaneously oxidise and reduce itself in a process called disproportionation, creating useful substances like bleach. It also reacts with organic molecules under UV light to form halogenoalkanes.
Imagine a juggler who, in a single trick, gives one ball away to a friend (reduction) while simultaneously taking a new ball from a spectator (oxidation). Chlorine atoms do something similar in disproportionation reactions, with some atoms in the Cl₂ molecule being reduced and others being oxidised in the same reaction.
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When chlorine dissolves in water, it forms a mixture of hydrochloric acid (HCl) and chloric(I) acid (HOCl). This is a reversible disproportionation reaction where chlorine is both oxidised (+1 in HOCl) and reduced (-1 in HCl).
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In cold, dilute sodium hydroxide, chlorine disproportionates to form sodium chloride (Cl⁻) and sodium chlorate(I) (ClO⁻), the active ingredient in household bleach. The reaction goes to completion because the alkali neutralises the acidic products.
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The addition of chlorine to drinking water is a major public health measure. The chloric(I) acid (HOCl) formed is a powerful oxidising agent that kills harmful bacteria and pathogens.
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With alkanes like methane, chlorine undergoes free-radical substitution in the presence of UV light. This reaction breaks the Cl-Cl bond homolytically, initiating a chain reaction that substitutes hydrogen atoms with chlorine atoms.
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Full topic notes
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Disproportionation Reactions of Chlorine
A disproportionation reaction is a specific type of redox reaction where an element from a single reactant is simultaneously oxidised and reduced to form two different products. For this to occur, the element must have an intermediate oxidation state that can both increase and decrease. Chlorine, in its elemental form (Cl₂) with an oxidation state of 0, is perfectly poised for this behaviour.
1. Reaction with Water
When chlorine gas is bubbled through water, it dissolves sparingly and a reversible reaction occurs. The resulting solution is often called 'chlorine water'. In this reaction, chlorine undergoes disproportionation.
Cl₂(aq) + H₂O(l) ⇌ HCl(aq) + HOCl(aq)
The oxidation state of chlorine changes from 0 in Cl₂ to -1 in hydrochloric acid (HCl) and +1 in chloric(I) acid (HOCl).
The reaction is reversible, indicated by the ⇌ symbol. In sunlight, the equilibrium is disturbed as HOCl decomposes to HCl and O₂.
Chloric(I) acid is a weak acid but a powerful oxidising agent, responsible for the bleaching action of chlorine water.
2. Reaction with Cold, Dilute Sodium Hydroxide
If chlorine is reacted with an alkali, such as cold, dilute sodium hydroxide, a similar disproportionation reaction occurs. However, because the sodium hydroxide is alkaline, it neutralises the acidic products (HCl and HOCl) as they are formed. This pulls the equilibrium to the right, and the reaction goes to completion. The resulting solution is a mixture of sodium chloride, sodium chlorate(I), and water, and is sold commercially as household bleach.
Cl₂(aq) + 2NaOH(aq) → NaCl(aq) + NaClO(aq) + H₂O(l)
3. Chlorine in Water Treatment
The disinfection of drinking water is one of chlorine's most important uses. When a small, controlled amount of chlorine is added to water supplies, the formation of chloric(I) acid (HOCl) and hydrochloric acid (HCl) kills harmful bacteria and other pathogens, preventing the spread of waterborne diseases like cholera and typhoid. The benefits of this process are widely considered to outweigh the risks.
Benefit: Effective sterilisation of water, preventing disease.
Mechanism: HOCl acts as a powerful oxidising agent that destroys enzymes and cell structures in microorganisms.
Risk: Chlorine can react with natural organic compounds in water to form trihalomethanes (THMs), such as trichloromethane (CHCl₃), which are suspected carcinogens. Water companies must monitor these levels carefully.
4. Reaction with Alkanes
Chlorine reacts with alkanes, such as methane, in a free-radical substitution reaction. This reaction requires initiation by ultraviolet (UV) light, which provides the energy to break the Cl-Cl bond homolytically, forming two highly reactive chlorine free radicals (Cl•). This initiates a chain reaction leading to the formation of halogenoalkanes.
Overall reaction: CH₄(g) + Cl₂(g) \xrightarrow{UV,light} CH₃Cl(g) + HCl(g)
Worked examples
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By assigning oxidation states, show that chlorine is disproportionated in its reaction with cold, dilute aqueous sodium hydroxide.
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Write the balanced equation:
A swimming pool is treated with chlorine. The reaction with cold, dilute alkali is used to produce the treatment chemical. 1.20 dm³ of chlorine gas, measured at room temperature and pressure (RTP), is reacted with an excess of cold, dilute sodium hydroxide solution. Calculate the mass of sodium chlorate(I) (NaClO) produced. [Molar volume of gas at RTP = 24.0 dm³ mol⁻¹, Ar: Na=23.0, Cl=35.5, O=16.0]
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Write the balanced equation:
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What is disproportionation?
A redox reaction in which an element in a single species is simultaneously oxidised and reduced.
Key takeaways
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The oxidation state of chlorine changes from 0 in Cl₂ to -1 in hydrochloric acid (HCl) and +1 in chloric(I) acid (HOCl).
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The reaction is reversible, indicated by the ⇌ symbol. In sunlight, the equilibrium is disturbed as HOCl decomposes to HCl and O₂.
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Chloric(I) acid is a weak acid but a powerful oxidising agent, responsible for the bleaching action of chlorine water.
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Test your understanding of chlorine's reactions
Test your understanding of chlorine's reactions
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