AQA · A-Level · Chemistry

    Course version: 7405 · Version from Sept 2016

    Students should be able to: write balanced equations for the steps in a…

    Full specification wording

    Reaction mechanisms: Students should be able to: write balanced equations for the steps in a free- radical mechanism.

    There are three kinds of step and each has a defining test.

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

    • Point 3 of 5
    • 3 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    There are three kinds of step and each has a defining test. Initiation makes radicals from a non-radical and needs ultraviolet light: Cl₂ → 2Cl•. Propagation steps each have one radical in and one radical out, so the radical count is unchanged and the chain continues: Cl• + CH₄ → •CH₃ + HCl, then •CH₃ + Cl₂ → CH₃Cl + Cl•. Adding the two propagation steps together gives the overall equation, with the radicals cancelling. Termination joins two radicals to give a non-radical, removing radicals from the system: Cl• + Cl• → Cl₂, or •CH₃ + Cl• → CH₃Cl, or •CH₃ + •CH₃ → C₂H₆. The same pattern describes ozone depletion, where Cl• + O₃ → ClO• + O₂ and ClO• + O₃ → Cl• + 2O₂.

    What earns the marks

    • An initiation equation forming two radicals from a covalent molecule, with ultraviolet light stated.
    • Each propagation step having exactly one radical on each side.
    • A termination step combining two radicals to give a single non-radical product.
    • A balanced overall equation obtained by adding the propagation steps.

    Common mistakes and how to put them right

    • Writing a propagation step with no radical on one side, so the chain cannot continue.

    • Giving Cl• + CH₄ → CH₃Cl + H• as a propagation step, which breaks the wrong bond and produces the wrong radical.

    • Using Cl₂ rather than Cl• as the attacking species in a propagation step.

    • Labelling •CH₃ + •CH₃ → C₂H₆ as propagation when it removes radicals and is termination.

    Exam tips

    • Count radicals on each side: one in and one out is propagation, two in and none out is termination.
    • Write ultraviolet light or sunlight beside the initiation step, because the condition often carries the mark.
    • State symbols are generally ignored, so spend your time balancing rather than on phases.

    Try the lesson

    The start of the Reaction mechanisms: Students should be able to: write balanced equations for the steps in a free- radical mechanism. lesson, made from checked teaching notes for this specification point.

    Have a go first

    Chlorine reacts with methane in ultraviolet light by a free-radical mechanism. Which equation represents the initiation step?

    Listen

    There are three kinds of step and each has a defining test. Initiation makes radicals from a non-radical and needs ultraviolet light: Cl₂ → 2Cl•. Propagation steps each have one radical in and one radical out, so the radical count is unchanged and the chain continues: Cl• + CH₄ → •CH₃ + HCl, then •CH₃ + Cl₂ → CH₃Cl + Cl•. Adding the two propagation steps together gives the overall equation, with the radicals cancelling. Termination joins two radicals to give a non-radical, removing radicals from the system: Cl• + Cl• → Cl₂, or •CH₃ + Cl• → CH₃Cl, or •CH₃ + •CH₃ → C₂H₆. The same pattern describes ozone depletion, where Cl• + O₃ → ClO• + O₂ and ClO• + O₃ → Cl• + 2O₂. A strong answer earns marks for these points. an initiation equation forming two radicals from a covalent molecule, with ultraviolet light stated. each propagation step having exactly one radical on each side. a termination step combining two radicals to give a single non-radical product. a balanced overall equation obtained by adding the propagation steps. Exam tip: Count radicals on each side: one in and one out is propagation, two in and none out is termination.

    See it

    A top-down flowchart. The first box is initiation, Cl₂ going to 2Cl• with ultraviolet light. An arrow leads to the first propagation step, Cl• plus CH₄ going to •CH₃ plus HCl. An arrow leads to the second propagation step, •CH₃ plus Cl₂ going to CH₃Cl plus Cl•. From the second propagation step one arrow leads to a box saying the propagation steps are added and the radicals cancel to give the overall equation. Another arrow leads to a termination box saying two radicals join to give a non-radical. From the termination box, three arrows lead to three termination examples: Cl• plus Cl• going to Cl₂; •CH₃ plus Cl• going to CH₃Cl; and •CH₃ plus •CH₃ going to C₂H₆.
    It shows the three kinds of step in a free-radical mechanism: initiation, the two propagation steps that add to the overall equation, and termination.

    Key words

    Say what each word means before you turn the card.

    initiation

    Try answering aloud or in your head before turning the card.

    propagation

    Try answering aloud or in your head before turning the card.

    termination

    Try answering aloud or in your head before turning the card.

    See every definition
    initiation
    A step that makes radicals from a non-radical and needs ultraviolet light.
    propagation
    A step with one radical in and one radical out, so the radical count is unchanged and the chain continues.
    termination
    A step that joins two radicals to give a non-radical, removing radicals from the system.

    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 is initiation, Cl₂ going to 2Cl• with ultraviolet light. An arrow leads to the first propagation step, Cl• plus CH₄ going to •CH₃ plus HCl. An arrow leads to the second propagation step, •CH₃ plus Cl₂ going to CH₃Cl plus Cl•. From the second propagation step one arrow leads to a box saying the propagation steps are added and the radicals cancel to give the overall equation. Another arrow leads to a termination box saying two radicals join to give a non-radical. From the termination box, three arrows lead to three termination examples: Cl• plus Cl• going to Cl₂; •CH₃ plus Cl• going to CH₃Cl; and •CH₃ plus •CH₃ going to C₂H₆.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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