AQA · A-Level · Chemistry

    Course version: 7405 · Version from Sept 2016

    Students should be able to: explain why sulfur dioxide can be removed from…

    Full specification wording

    Combustion of alkanes: Students should be able to: explain why sulfur dioxide can be removed from flue gases using calcium oxide or calcium carbonate.

    This is an acid-base reaction and the explanation must say so.

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

    • Point 5 of 5
    • 3 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    This is an acid-base reaction and the explanation must say so. Sulfur dioxide is an acidic oxide, calcium oxide is a basic oxide and calcium carbonate is the salt of a weak acid, so both neutralise it. In flue gas desulfurisation a slurry of powdered calcium oxide or calcium carbonate in water is sprayed into the rising gases, and the solid produced is calcium sulfite: CaO + SO₂ → CaSO₃, or CaCO₃ + SO₂ → CaSO₃ + CO₂. A fine slurry is used so as to maximise the surface area available. Oxidising the sulfite further gives calcium sulfate, sold as gypsum for plasterboard. The carbonate route has an obvious drawback, releasing a mole of carbon dioxide for every mole of sulfur dioxide removed.

    What earns the marks

    • Identifying sulfur dioxide as an acidic oxide and calcium oxide or calcium carbonate as basic.
    • A balanced equation such as CaO + SO₂ → CaSO₃ or CaCO₃ + SO₂ → CaSO₃ + CO₂.
    • Describing the process as neutralisation, or as an acid-base reaction.
    • A comment on the products, such as calcium sulfate being sold as gypsum or carbon dioxide being released by the carbonate route.

    Common mistakes and how to put them right

    • Writing calcium sulfate as the direct product with calcium oxide, when the immediate product is the sulfite.

    • Calling the reaction a redox reaction, when sulfur stays in the +4 oxidation state throughout.

    • Forgetting the carbon dioxide product when calcium carbonate is the reagent used.

    • Describing calcium oxide as acidic because it contains oxygen.

    Exam tips

    • Name the acid-base relationship explicitly, because they react together is not an explanation.
    • Check the oxidation state of sulfur on both sides to confirm this is neutralisation rather than redox.
    • Include carbon dioxide as a product whenever the reagent is the carbonate.

    Try the lesson

    The start of the Combustion of alkanes: Students should be able to: explain why sulfur dioxide can be removed from flue gases using calcium oxide or calcium carbonate. lesson, made from checked teaching notes for this specification point.

    Have a go first

    Calcium oxide is used to remove sulfur dioxide from flue gases. What type of substance is calcium oxide in this reaction?

    Listen

    This is an acid-base reaction and the explanation must say so. Sulfur dioxide is an acidic oxide, calcium oxide is a basic oxide and calcium carbonate is the salt of a weak acid, so both neutralise it. In flue gas desulfurisation a slurry of powdered calcium oxide or calcium carbonate in water is sprayed into the rising gases, and the solid produced is calcium sulfite: CaO + SO₂ → CaSO₃, or CaCO₃ + SO₂ → CaSO₃ + CO₂. A fine slurry is used so as to maximise the surface area available. Oxidising the sulfite further gives calcium sulfate, sold as gypsum for plasterboard. The carbonate route has an obvious drawback, releasing a mole of carbon dioxide for every mole of sulfur dioxide removed. A strong answer earns marks for these points. identifying sulfur dioxide as an acidic oxide and calcium oxide or calcium carbonate as basic. a balanced equation such as CaO + SO₂ → CaSO₃ or CaCO₃ + SO₂ → CaSO₃ + CO₂. describing the process as neutralisation, or as an acid-base reaction. a comment on the products, such as calcium sulfate being sold as gypsum or carbon dioxide being released by the carbonate route. Exam tip: Name the acid-base relationship explicitly, because they react together is not an explanation.

    See it

    A top-down flowchart. At the top, a node labelled 'Sulfur dioxide in flue gases' leads down to 'Acidic oxide', which leads to 'Neutralisation: acid-base reaction'. Two side nodes feed into this neutralisation node: 'Calcium oxide (basic oxide)' and 'Calcium carbonate (salt of weak acid)'. From the neutralisation node a line goes down to 'Calcium sulfite', which leads to 'Oxidise further: calcium sulfate', which leads to 'Sold as gypsum for plasterboard'. Separately, the 'Calcium carbonate (salt of weak acid)' node has a line down to 'Carbon dioxide released (drawback)'.
    It shows sulfur dioxide as an acidic oxide being neutralised by calcium oxide or calcium carbonate to give calcium sulfite, with the carbonate route also releasing carbon dioxide and the sulfite oxidising further to calcium sulfate sold as gypsum.

    Key words

    Say what each word means before you turn the card.

    flue gas desulfurisation

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    acidic and basic oxides

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    carbon dioxide trade-off

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    See every definition
    flue gas desulfurisation
    A slurry of powdered calcium oxide or calcium carbonate in water is sprayed into the rising gases, and the solid produced is calcium sulfite.
    acidic and basic oxides
    Sulfur dioxide is an acidic oxide, calcium oxide is a basic oxide and calcium carbonate is the salt of a weak acid, so both neutralise it.
    carbon dioxide trade-off
    The carbonate route has an obvious drawback, releasing a mole of carbon dioxide for every mole of sulfur dioxide removed.

    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. At the top, a node labelled 'Sulfur dioxide in flue gases' leads down to 'Acidic oxide', which leads to 'Neutralisation: acid-base reaction'. Two side nodes feed into this neutralisation node: 'Calcium oxide (basic oxide)' and 'Calcium carbonate (salt of weak acid)'. From the neutralisation node a line goes down to 'Calcium sulfite', which leads to 'Oxidise further: calcium sulfate', which leads to 'Sold as gypsum for plasterboard'. Separately, the 'Calcium carbonate (salt of weak acid)' node has a line down to 'Carbon dioxide released (drawback)'.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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