Balanced equations and associated calculations — AQA A-Level Chemistry
Test yourself on Balanced equations and associated calculations with AQA A-Level practice questions.
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Balanced equations and associated calculations explained
A full equation shows every substance taking part, with correct formulae and, where required, state symbols.
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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.
Students should be able to: write balanced equations for reactions studied balance equations for unfamiliar reactions when reactants and products are specified.
Balancing is a bookkeeping exercise on coefficients only; the formulae themselves are fixed by the chemistry and must never be altered. Work through the elements that appear in fewest species first, leave oxygen and hydrogen until last, and clear any fraction at the end by doubling the whole equation, though examiners will accept fractions and multiples unless the question says otherwise. An unfamiliar reaction is no harder provided the question names the reactants and products: copy their formulae, then balance. For an ionic equation you must also balance charge, which is what makes 4 Mn²⁺ + MnO₄⁻ + 8 H⁺ → 5 Mn³⁺ + 4 H₂O work: the total charge is +15 on each side.
Required practical 1 Make up a volumetric solution and carry out a simple acid–base titration.
The practical has two halves. Making the standard solution: weigh the solid by difference, dissolve it in a beaker in less water than the final volume, then transfer the solution and the beaker washings into a volumetric flask, make up to the graduation mark with distilled water so the bottom of the meniscus sits on the line at eye level, stopper and invert repeatedly to mix. The washings matter because solute left in the beaker would lower the concentration. Titrating: rinse the burette with the solution it will hold, pipette a fixed volume into a conical flask, add a few drops of indicator, and run in until the first permanent colour change. Repeat until titres agree within 0.10 cm³ and average only those.
Your focus
- Write the ionic equation for a precipitation or neutralisation reaction by cancelling the spectator ions from the full equation.
- Check an ionic equation by confirming that both atoms and total charge balance.
- Decide which species in a full equation may be written as separate ions and justify the choice.
Show all 9 objectives
- Balance an unfamiliar ionic equation for both atoms and total charge when the reactants and products are supplied.
- Construct a half-equation by adding water, hydrogen ions and electrons in the correct order.
- Explain why balancing must be done by changing coefficients and never by changing a formula.
- Describe, in order, the steps needed to make up 250 cm³ of a solution of accurately known concentration from a weighed solid.
- Explain why the beaker washings must be transferred to the volumetric flask.
- Select concordant titres from a results table and calculate the mean titre from them alone.
Balanced equations and associated calculations exam tips
Marking Points
- one mark for correct formulae for every species in the equation
- one mark for the atoms balanced on both sides
- one mark for the total charge balanced in an ionic equation
- one mark for spectator ions cancelled so they do not appear in the ionic equation
- one mark for correct state symbols where the question requires them
- one mark for correct formulae for every reactant and product named in the question
- one mark for the numbers of atoms of each element balanced on both sides
- one mark for the total charge balanced in an ionic or half-equation
- one mark for an equation accepted in multiples or with fractions cleared consistently
- one mark for transferring the solution together with the beaker washings into a volumetric flask
- one mark for making the solution up to the graduation mark with distilled water
- one mark for inverting or shaking the stoppered flask to mix thoroughly
- one mark for averaging only concordant titres, those within 0.10 cm³ of each other
- one mark for selecting an indicator with a sharp colour change over the vertical section of the titration curve
Examiner 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.
- 💡Tally the atoms of each element under the equation and update the tally after every change.
- 💡Balance charge last in a half-equation by adding electrons to whichever side needs them.
- 💡State symbols are usually ignored unless the question asks for them, so spend your time on the coefficients.
- 💡Quote burette readings to two decimal places ending in 0 or 5, and subtract to give the titre.
- 💡Rinse the burette with the titrant and the pipette with the solution it will deliver, but the conical flask with distilled water only.
- 💡When asked to improve accuracy, name a specific step such as washing the beaker into the flask.
Common Mistakes
- 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
- altering a formula, such as writing water as H₂O₂, in order to make the atoms balance
- balancing the atoms but leaving the charges unequal in an ionic equation
- introducing a species that was not named among the reactants or products
- forgetting to rebalance hydrogen and oxygen after adding water to one side
- rinsing the conical flask with the solution being titrated, which adds extra moles and raises the titre
- including the rough titre when calculating the mean
- using an indicator for a weak acid against a weak base, where the pH change is too gradual to judge
- overshooting the graduation mark and then removing liquid with a pipette
- reading the top of the meniscus rather than the bottom