AQA · A-Level · Chemistry

    Course version: 7405 · Version from Sept 2016

    The reaction of chlorine with cold, dilute, aqueous NaOH and uses of the…

    Full specification wording

    Uses of chlorine and chlorate(I): 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…

    Checked against the AQA 7405 specification (opens in a new tab) on .

    • Point 6 of 6
    • 3 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    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.

    What earns the marks

    • The equation Cl₂ + 2NaOH → NaCl + NaClO + H₂O.
    • Stating the conditions as cold, dilute aqueous sodium hydroxide.
    • Naming NaClO as sodium chlorate(I).
    • Identifying the reaction as disproportionation, with chlorine going to −1 and +1.
    • Naming a use of the solution as bleach or as a disinfectant.

    Common mistakes and how to put them right

    • 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.

    Exam tips

    • 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.

    Try the lesson

    The start of the Uses of chlorine and chlorate(I): The reaction of chlorine with cold, dilute, aqueous NaOH and uses of the solution formed. lesson, made from checked teaching notes for this specification point.

    Have a go first

    What is formed when chlorine reacts with cold, dilute, aqueous sodium hydroxide?

    Listen

    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. A strong answer earns marks for these points. the equation Cl₂ + 2NaOH → NaCl + NaClO + H₂O. stating the conditions as cold, dilute aqueous sodium hydroxide. naming NaClO as sodium chlorate(I). identifying the reaction as disproportionation, with chlorine going to −1 and +1. Exam tip: Always write the Roman numeral in chlorate(I); leaving it out is treated as a different compound.

    See it

    A top-down flowchart. The first box reads 'Chlorine + cold, dilute, aqueous NaOH' and points to a second box with the equation 'Cl₂ + 2NaOH → NaCl + NaClO + H₂O'. That equation box branches to two boxes: 'NaCl: chlorine at −1' and 'NaClO: sodium chlorate(I), chlorine at +1'. The NaCl box points to 'Reduced' and the NaClO box points to 'Oxidised'; both of these point to a single box reading 'Disproportionation'. The NaClO box also points to 'Solution formed: household bleach', which branches to two final boxes, 'Bleach' and 'Disinfectant'.
    It shows chlorine reacting with cold, dilute, aqueous sodium hydroxide to give chloride and sodium chlorate(I), with chlorine both reduced and oxidised, and the solution used as a bleach and a disinfectant.

    Key words

    Say what each word means before you turn the card.

    sodium chlorate(I)

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    bleach and disinfectant

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    effect of conditions

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    See every definition
    sodium chlorate(I)
    The compound NaClO, formed with sodium chloride and water when chlorine reacts with cold, dilute, aqueous sodium hydroxide.
    bleach and disinfectant
    The uses of the solution formed, because the chlorate(I) ion is a strong oxidising agent that destroys coloured compounds and kills bacteria.
    effect of conditions
    Cold, dilute alkali gives sodium chlorate(I), whereas warm, concentrated alkali drives the reaction further to sodium chlorate(V), NaClO₃.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A top-down flowchart. The first box reads 'Chlorine + cold, dilute, aqueous NaOH' and points to a second box with the equation 'Cl₂ + 2NaOH → NaCl + NaClO + H₂O'. That equation box branches to two boxes: 'NaCl: chlorine at −1' and 'NaClO: sodium chlorate(I), chlorine at +1'. The NaCl box points to 'Reduced' and the NaClO box points to 'Oxidised'; both of these point to a single box reading 'Disproportionation'. The NaClO box also points to 'Solution formed: household bleach', which branches to two final boxes, 'Bleach' and 'Disinfectant'.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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