Uses of chlorine and chlorate(I) — AQA A-Level Chemistry
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Uses of chlorine and chlorate(I) explained
Chlorine dissolves in water and reacts with it: Cl₂ + H₂O ⇌ HClO + HCl, or ionically Cl₂ + H₂O ⇌ 2H⁺ + Cl⁻ + ClO⁻.
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One chlorine atom ends up as chloride at oxidation state −1 and the other as the chlorate(I) ion, ClO⁻, at +1, both starting from 0 in Cl₂. A single element being oxidised and reduced at the same time is disproportionation. If a question asks you to explain the redox in terms of electrons, say it that way: chlorine gains electrons to form Cl⁻ and loses electrons to form ClO⁻. Chloric(I) acid and the chlorate(I) ion are the species that kill bacteria, which is why this equilibrium, rather than chlorine itself, does the work in water treatment.
The reaction of chlorine with water to form chloride ions and oxygen.
In bright sunlight chlorine water follows a second, slower route: 2Cl₂ + 2H₂O → 4H⁺ + 4Cl⁻ + O₂. Left on a sunny windowsill, the pale green colour of chlorine water fades and a slow stream of oxygen bubbles appears, which is why chlorine water is kept in a dark bottle and why swimming pools lose chlorine fastest on bright days. The redox bookkeeping differs from the chlorate(I) reaction. Here chlorine is only reduced, from 0 in Cl₂ to −1 in Cl⁻, and the species oxidised is the oxygen in water, going from −2 to 0 in O₂. Because chlorine ends in a single oxidation state, this reaction is not disproportionation, even though the two reactants are the same as before.
Appreciate that society assesses the advantages and disadvantages when deciding if chemicals should be added to water supplies.
Adding anything to a public water supply is a judgement about risk against benefit, and it is not one that chemistry can settle alone. Chlorine prevents cholera and typhoid, diseases that killed on a huge scale before treatment; against that, chlorine is toxic and can react with organic matter in the water to form trace chlorinated compounds suspected of being carcinogenic. Fluoride reduces tooth decay; against that are dental fluorosis at higher doses and the objection that nobody can opt out of what comes from their tap. The decisive point in both cases is dose: a substance harmful in quantity may be entirely safe and useful at a few parts per million. Weighing that, and deciding who consents, is a question for society, informed by chemists rather than settled by them.
The use of chlorine in water treatment.
Chlorine is added to drinking water and to swimming pools to kill bacteria and other micro-organisms, which is why water-borne diseases such as cholera and typhoid are rare where supplies are treated. The chlorine is not itself the direct agent: it reacts with water, Cl₂ + H₂O ⇌ HClO + HCl, and the chloric(I) acid or chlorate(I) ion oxidises and destroys the organisms. A small residual concentration is deliberately left in the water so that it stays protected all the way through the pipework to the tap. The amount added must be carefully controlled, because chlorine is itself toxic and in excess would poison the people drinking it. The concentration used is only a few parts per million.
Appreciate that the benefits to health of water treatment by chlorine outweigh its toxic effects.
This is an evaluation, so the answer needs both sides and a conclusion. Against chlorination: chlorine is a toxic gas, used as a chemical weapon in the First World War, and chlorinating water produces traces of chlorinated organic compounds that some studies have linked to cancer. For chlorination: untreated supplies spread cholera, typhoid and dysentery, which killed people in enormous numbers before treatment became routine and still do where water is untreated. The resolution is dose. At the few parts per million used in a public supply, chlorine presents a very small risk, while the disease risk it removes is very large, so the accepted judgement is that the benefit clearly outweighs the harm.
The reaction of chlorine with cold, dilute, aqueous NaOH and uses of the solution formed.
With cold, dilute, aqueous sodium hydroxide chlorine disproportionates again: Cl₂ + 2NaOH → NaCl + NaClO + H₂O, or ionically Cl₂ + 2OH⁻ → Cl⁻ + ClO⁻ + H₂O. Chlorine starts at 0 and finishes at −1 in the chloride and +1 in the sodium chlorate(I), so it is oxidised and reduced in the same reaction. The solution formed is household bleach: the chlorate(I) ion is a strong oxidising agent that destroys coloured compounds and kills bacteria, so the same product serves as a bleach and as a disinfectant. The conditions are part of the answer, because warm, concentrated alkali drives the reaction further, to sodium chlorate(V), NaClO₃, which is used as a weedkiller.
Your focus
- Write the equation for chlorine dissolving in water and name both products.
- Deduce the oxidation state of chlorine in Cl₂, in Cl⁻ and in ClO⁻, and use them to justify the term disproportionation.
- Explain, in terms of electron transfer, why the reaction of chlorine with water is a redox reaction.
Show all 18 objectives
- Write the balanced equation for chlorine reacting with water in sunlight.
- Deduce which element is oxidised and which is reduced in 2Cl₂ + 2H₂O → 4H⁺ + 4Cl⁻ + O₂.
- Explain why chlorine water is stored in a dark bottle.
- Give one benefit and one risk of adding chlorine to drinking water and reach a justified conclusion.
- Explain why the decision to fluoridate a water supply is not one that chemists can take alone.
- Explain why the risk from chlorination depends on the concentration used rather than on the presence of chlorine alone.
- State why chlorine is added to drinking water and name one disease that its use prevents.
- Explain why the concentration of chlorine used in water treatment must be carefully controlled.
- Identify the species formed when chlorine dissolves in water that is responsible for killing bacteria.
- Evaluate the use of chlorine in drinking water, giving a named benefit, a named risk and a conclusion.
- Explain why 'chlorine is toxic' is not on its own a sufficient argument against chlorinating water.
- Use the idea of dose to explain why a substance once used as a chemical weapon is added to drinking water.
- Write the balanced equation for chlorine with cold dilute sodium hydroxide and state the conditions required.
- Deduce the two oxidation states of chlorine in the products and use them to show that the reaction is disproportionation.
- Name the compound responsible for the bleaching action of the solution formed and state one household use.
Uses of chlorine and chlorate(I) exam tips
Marking Points
- one mark for the equation Cl₂ + H₂O ⇌ HClO + HCl, or the ionic form Cl₂ + H₂O ⇌ 2H⁺ + Cl⁻ + ClO⁻
- one mark for stating that chlorine gains electrons to form Cl⁻
- one mark for stating that chlorine loses electrons to form ClO⁻
- one mark for identifying the change as disproportionation, with chlorine going from 0 to −1 and from 0 to +1
- one mark for naming ClO⁻ as the chlorate(I) ion
- one mark for the balanced equation 2Cl₂ + 2H₂O → 4H⁺ + 4Cl⁻ + O₂
- one mark for stating that sunlight or ultraviolet light is needed
- one mark for identifying chlorine as reduced from 0 to −1
- one mark for identifying the oxygen in water as oxidised from −2 to 0
- one mark for stating that this reaction is not disproportionation, unlike the one forming chlorate(I)
- one mark for a named benefit together with the harm it prevents, such as chlorine killing the bacteria that cause cholera
- one mark for a named risk, such as chlorine being toxic or forming chlorinated organic compounds
- one mark for referring to the fact that individuals cannot choose whether to receive treated water
- one mark for a reasoned judgement that weighs the two sides rather than listing them
- one mark for stating that chlorine kills bacteria or micro-organisms in the water
- one mark for naming a disease such as cholera or typhoid that is prevented
- one mark for stating that the amount added must be controlled because chlorine is toxic or poisonous
- one mark for identifying chloric(I) acid or the chlorate(I) ion as the species that kills the organisms
- one mark for stating that a residual amount keeps the water safe along the pipework
- one mark for stating that chlorination prevents water-borne disease such as cholera or typhoid
- one mark for acknowledging that chlorine is toxic or that chlorinated organic compounds can form
- one mark for stating that the concentration used is far below a harmful level
- one mark for a conclusion that the benefit outweighs the risk at the concentration used
- one mark for the equation Cl₂ + 2NaOH → NaCl + NaClO + H₂O
- one mark for stating the conditions as cold, dilute aqueous sodium hydroxide
- one mark for naming NaClO as sodium chlorate(I)
- one mark for identifying the reaction as disproportionation, with chlorine going to −1 and +1
- one mark for naming a use of the solution as bleach or as a disinfectant
Examiner Tips
- 💡If the question says 'with reference to electrons', the words gains and loses must appear, or the mark is lost.
- 💡Work out the oxidation state in ClO⁻ from oxygen at −2 and the overall charge of −1.
- 💡Check the conditions in the stem: sunlight points to the oxygen route, ordinary conditions to the chlorate(I) route.
- 💡Assign oxidation states to every element before deciding whether disproportionation has happened.
- 💡These questions are marked for balance, so give at least one point on each side plus a conclusion.
- 💡Use the word dose or concentration; it is usually the pivot on which the argument turns.
- 💡'Toxic' or 'poisonous' is the key word for the dose-control mark; vaguer answers about taste do not score.
- 💡Name a specific disease; 'stops you getting ill' is not enough for the mark.
- 💡Structure the answer as: risk, benefit, then judgement. The judgement sentence is often the mark students miss.
- 💡Quantify where you can, for instance parts per million, because it shows why the risk is small.
- 💡Always write the Roman numeral in chlorate(I); leaving it out is treated as a different compound.
- 💡Quote the conditions in your answer even when only the equation is asked for, since cold and dilute decide the product.
Common Mistakes
- explaining the redox by saying chlorine is oxidised and reduced when the question asks for an answer in terms of electrons, which mark schemes ignore
- giving the oxidation state of chlorine in ClO⁻ as −1
- writing the product as chlorate(V), ClO₃⁻, which forms only with hot concentrated alkali
- leaving hydrogen unbalanced in the equation
- using a single arrow when the reaction is an equilibrium
- calling this reaction disproportionation, when chlorine is only reduced
- giving the oxidation state of oxygen in water as 0
- balancing with a single Cl₂, which leaves oxygen unbalanced
- assuming the reaction proceeds at the same rate in the dark
- confusing this reaction with the chlorate(I) route and quoting the wrong products
- writing that chlorine is bad for you with no named effect or concentration
- ignoring that risk depends on dose, so a controlled amount can be safe
- giving only scientific arguments when the question is about how society decides
- confusing chlorination with fluoridation and giving the wrong purpose for each
- saying chlorine removes dirt or makes the water clear rather than killing bacteria
- stating that all the chlorine is removed before the water reaches the tap
- giving the smell or taste of chlorine as the reason for controlling the dose, rather than its toxicity
- confusing the role of chlorine with that of fluoride
- naming chloride ions as the species that kills the bacteria
- giving only the benefit, or only the risk, when the command word is evaluate
- treating any amount of a toxic substance as equally dangerous, ignoring the dose
- claiming that chlorinated water carries no risk at all
- arguing the case without naming a disease or a named harmful effect
- giving sodium chlorate(V), NaClO₃, which requires hot concentrated alkali
- naming NaClO as sodium chlorate without the Roman numeral (I)
- omitting the water from the right-hand side, so hydrogen and oxygen do not balance
- stating the oxidation state of chlorine in NaClO as −1
- describing the product as chlorine dissolved in alkali rather than as a new compound