AQA · A-Level · Chemistry

    Course version: 7405 · Version from Sept 2016

    Balanced equations and associated calculations: Equations (full and ionic).

    A full equation shows every substance taking part, with correct formulae and, where required, state symbols.

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

    • Point 1 of 3
    • 3 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    A full equation shows every substance taking part, with correct formulae and, where required, state symbols. An ionic equation shows only the species that actually change, with spectator ions cancelled from both sides: silver nitrate and sodium chloride solutions give Ag⁺(aq) + Cl⁻(aq) → AgCl(s), and any strong acid neutralised by a strong alkali reduces to H⁺(aq) + OH⁻(aq) → H₂O(l). Two conditions must hold in an ionic equation: the atoms must balance and the total charge on the left must equal the total charge on the right. Only substances that are fully dissociated, that is strong acids, strong alkalis and soluble ionic salts, may be split into ions; solids, gases, liquids and weak acids stay whole.

    What earns the marks

    • Correct formulae for every species in the equation.
    • The atoms balanced on both sides.
    • The total charge balanced in an ionic equation.
    • Spectator ions cancelled so they do not appear in the ionic equation.
    • Correct state symbols where the question requires them.

    Common mistakes and how to put them right

    • Changing a subscript inside a formula in order to balance an equation.

    • Leaving spectator ions such as sodium and nitrate in an ionic equation.

    • Writing an ionic equation in which the charges do not balance.

    • Splitting an insoluble solid or a weak acid into separate ions.

    Exam tips

    • Balance the full equation first, then cancel the spectators; building the ionic equation directly invites errors.
    • Add up the charges on each side and write the totals in the margin as a check.
    • Only add state symbols when asked, but make them correct when you do.

    Try the lesson

    The start of the Balanced equations and associated calculations: Equations (full and ionic). lesson, made from checked teaching notes for this specification point.

    Have a go first

    Which equation is correctly balanced for the reaction of aluminium with dilute sulfuric acid?

    Listen

    A full equation shows every substance taking part, with correct formulae and, where required, state symbols. An ionic equation shows only the species that actually change, with spectator ions cancelled from both sides: silver nitrate and sodium chloride solutions give Ag⁺(aq) + Cl⁻(aq) → AgCl(s), and any strong acid neutralised by a strong alkali reduces to H⁺(aq) + OH⁻(aq) → H₂O(l). Two conditions must hold in an ionic equation: the atoms must balance and the total charge on the left must equal the total charge on the right. Only substances that are fully dissociated, that is strong acids, strong alkalis and soluble ionic salts, may be split into ions; solids, gases, liquids and weak acids stay whole. A strong answer earns marks for these points. correct formulae for every species in the equation. the atoms balanced on both sides. the total charge balanced in an ionic equation. spectator ions cancelled so they do not appear in the ionic equation. Exam tip: Balance the full equation first, then cancel the spectators; building the ionic equation directly invites errors.

    See it

    A top-down flowchart. The first box reads 'Full equation: every substance, correct formulae' and leads to 'Balance the full equation first'. That leads to 'Split only fully dissociated species into ions: strong acids, strong alkalis, soluble ionic salts', which leads to 'Keep solids, gases, liquids and weak acids whole'. That leads to 'Cancel spectator ions from both sides', which leads to 'Ionic equation: only the species that change'. From this final box two branches lead to two checks: 'Check atoms balance' and 'Check total charge on left equals total charge on right'.
    The sequence from a full equation to a checked ionic equation, showing which species may be split into ions and the two checks that must hold.

    Key words

    Say what each word means before you turn the card.

    full equations

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

    ionic equations

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

    charge balance

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

    See every definition
    full equations
    Shows every substance taking part, with correct formulae and, where required, state symbols.
    ionic equations
    Shows only the species that actually change, with spectator ions cancelled from both sides.
    charge balance
    In an ionic equation the total charge on the left must equal the total charge on the right.

    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 'Full equation: every substance, correct formulae' and leads to 'Balance the full equation first'. That leads to 'Split only fully dissociated species into ions: strong acids, strong alkalis, soluble ionic salts', which leads to 'Keep solids, gases, liquids and weak acids whole'. That leads to 'Cancel spectator ions from both sides', which leads to 'Ionic equation: only the species that change'. From this final box two branches lead to two checks: 'Check atoms balance' and 'Check total charge on left equals total charge on right'.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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