Ch 4 · The reactions of chlorine · AS & A Level Chemistry 9701
Topic 4 of 4 in Group 17 — AS & A Level Chemistry 9701 — 48 lessons.
Introduction
Chlorine reacts with water and with aqueous sodium hydroxide in reactions that share the same underlying feature: chlorine, starting at oxidation number 0, is simultaneously oxidised and reduced. This process, disproportionation, both explains chlorine's use in water purification and produces the everyday household bleach reaction.
Key ideas — Disproportionation: the underlying idea
Disproportionation is a redox reaction in which a single element, in a single species, is simultaneously oxidised and reduced, ending up in two different oxidation states in two different products. In elemental chlorine, Cl_2, every chlorine atom has oxidation number 0. In each reaction below, one chlorine atom is reduced (oxidation number decreases, to -1 as Cl^-) while the other is oxidised (oxidation number increases), so the two changes must be recognised and stated using oxidation numbers, not just observed as a colour change.
🧑🏫 Teacher's explanation
Imagine a group of friends playing a game of musical chairs. In a normal redox reaction, one person gives a gift (electrons) to another. In a disproportionation reaction, it is as if one person splits into two different versions of themselves: one version becomes a 'giver' and the other becomes a 'receiver' at the exact same time. The single starting material is being pulled in two opposite directions simultaneously.
📖 Definitions
🔍 Worked example — Easy
Calculate the change in oxidation state for chlorine in the reaction: Cl_2 + H_2O arrow HCl + HClO.
🔍 Worked example — Standard
Write the balanced equation for the reaction of chlorine with cold, dilute aqueous sodium hydroxide: Cl_2 + NaOH arrow NaCl + NaOCl + H_2O.
🔍 Worked example — Exam-style
Chlorine reacts with hot, concentrated aqueous NaOH to form NaCl and NaClO_3. Write the balanced equation.
⚠️ Common errors
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🎯 Exam question
Key ideas — Reaction with cold, dilute aqueous sodium hydroxide
Chlorine reacts with cold, dilute aqueous sodium hydroxide to give a mixture of chloride and chlorate(I) (hypochlorite) ions, the active ingredient of household bleach:
🧑🏫 Teacher's explanation
Imagine a group of friends playing a game of musical chairs. In a normal reaction, everyone either gains something or loses something. But in a 'disproportionation' reaction, the chlorine atoms are like those friends: some end up sitting down (gaining electrons to become negative), while others are forced to stand up (losing electrons to become positive). The chlorine starts at a neutral state of 0, but the reaction splits them into two different fates.
📖 Definitions
🔍 Worked example — Easy
Calculate the oxidation state of chlorine in NaOCl and NaCl.
🔍 Worked example — Standard
Balance the following equation for the reaction of chlorine with cold, dilute NaOH: Cl_2(g) + NaOH(aq) arrow NaCl(aq) + NaOCl(aq) + H_2O(l)
🔍 Worked example — Exam-style
A 2.00 g sample of Cl_2 reacts completely with excess cold, dilute NaOH. Calculate the mass of NaOCl produced.
⚠️ Common errors
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🎯 Exam question
Key ideas — Reaction with hot, concentrated aqueous sodium hydroxide
With hot, concentrated aqueous sodium hydroxide, chlorine disproportionates further, forming chloride and chlorate(V) ions instead:
🧑🏫 Teacher's explanation
Imagine a group of friends playing a game where some people win big prizes and others lose everything, but the total amount of 'luck' stays the same. In chemistry, this is called disproportionation. A single element starts with one oxidation state and, through a single reaction, splits into two different products: one where it has gained electrons (reduction) and one where it has lost electrons (oxidation).
📖 Definitions
🔍 Worked example — Easy
Calculate the oxidation state of chlorine in NaClO_3.
🔍 Worked example — Standard
Balance the following disproportionation reaction: Cl_2 + OH^- arrow Cl^- + ClO^-
🔍 Worked example — Exam-style
In the reaction 3Cl_2(g) + 6NaOH(aq) arrow 5NaCl(aq) + NaClO_3(aq) + 3H_2 extO(l), calculate the total number of moles of Cl atoms that are oxidized per mole of Cl_2 used.
⚠️ Common errors
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🎯 Exam question
Key ideas — Chlorine in water purification
Chlorine also disproportionates when it dissolves in water, forming a mixture of hydrochloric acid and chloric(I) acid:
🧑🏫 Teacher's explanation
Imagine a person who is simultaneously giving away money and receiving money in the same transaction. This is what happens to chlorine in certain reactions. In chemistry, we call this 'disproportionation'. Instead of just being oxidized or just being reduced, the chlorine atom undergoes both processes at once.
📖 Definitions
🔍 Worked example — Easy
Calculate the oxidation state of chlorine in HCl(aq) and HOCl(aq) in the reaction: Cl_2(g) + H_2O(l) arrow HCl(aq) + HOCl(aq).
🔍 Worked example — Standard
Chlorine gas reacts with cold aqueous sodium hydroxide. Write the balanced equation for this disproportionation reaction.
🔍 Worked example — Exam-style
Chlorine reacts with hot aqueous sodium hydroxide to form sodium chloride and sodium chlorate(III). Write the balanced equation.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
Worked example
Question. Chlorine gas is bubbled into cold, dilute aqueous sodium hydroxide.
🔍 Worked example — Easy
Write the balanced equation for the reaction of chlorine gas with cold, dilute aqueous sodium hydroxide. Include state symbols.
🔍 Worked example — Standard
Calculate the change in oxidation state for chlorine in the reaction: Cl_2 + 2NaOH arrow NaCl + NaOCl + H_2O.
🔍 Worked example — Exam-style
Chlorine is used to purify water. Write the equation for the reaction of Cl_2 with H_2 extO and state the products.
⚠️ Common errors
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Practice
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