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QLDChemistry2024

QCE Chemistry 2024

Walkthrough of the 2024 QCE chemistry exam: what it assessed, strategy tips, and the common errors flagged in the official marker report.

Marks
110
Time
180 min
Authority
QCAA
Updated
Open the official PDF Official marking notes
Key fact

This paper was set on the 2019 QCAA syllabus. From the 2026 external assessments the 2025 syllabus applies, so some content and the exam format have changed. It is still useful practice, but check the current QCAA Chemistry syllabus and QCAA's 2026 key changes before relying on it. The mark and timing figures at the top of this page are a guide based on the 2024 papers (two papers, each with 90 minutes of working time), not QCAA's official structure for every year. Marks and question types vary, so use QCAA's own paper.

What this paper assessed

This QCAA Chemistry external assessment is sat as more than one paper. It assesses Unit 3 (Equilibrium, acids and redox reactions) and Unit 4 (Structure, synthesis and design):

  • Unit 3: chemical equilibrium and Le Chatelier's principle (including KeqK_{eq}), acids and bases (Brønsted-Lowry, pHpH, KaK_a, titration), and oxidation-reduction (half-equations, galvanic cells, electrolysis, standard electrode potentials).
  • Unit 4: the chemistry of organic compounds (functional groups, isomerism, reaction pathways such as substitution, addition, esterification and oxidation), polymers, and the use of instrumental analysis (mass spectrometry, IR, NMR) in molecular design and identification.

The paper is calculation-heavy and combines theory with practical technique. The 2024 paper leaned harder on application items where the surface context was unfamiliar but the underlying skill was core.

Structure and timing

QCAA sets the external assessment as more than one paper, and the number of marks, the sections and the question types differ from year to year. Open the official paper for the exact structure of this year; the marks and time shown at the top of this page are a guide only.

  • Pace by marks. Share your working time in proportion to the marks on offer, so a 6-mark item gets about three times the time of a 2-mark item. Flag any multiple choice item that runs long and return to it.
  • Use perusal time. Scan for the data-heavy items and the longest calculations or extended responses so you can plan them early.
  • Subject habits. Use the QCAA data booklet rather than recalling constants. For any calculation chain, label the quantity at each step so method marks are visible even if the final number is wrong, and keep a few minutes at the end to check state symbols, balancing, units and significant figures.

Worked practice questions (exam-style)

Worked example

Question 1 (5 marks): Titration calculation to find the concentration of hydrochloric acid

20.0 mL of hydrochloric acid is exactly neutralised by 18.0 mL of 0.125 mol L−1^{-1} sodium carbonate solution. Find the concentration of the HCl.

Step 1: Write the balanced equation to establish the mole ratio

The balanced equation shows that 2 moles of HCl react with 1 mole of sodium carbonate. This 2:1 ratio is what links the two reactants.

2HCl(aq)+Na2CO3(aq)→2NaCl(aq)+H2O(l)+CO2(g)2\text{HCl}(aq) + \text{Na}_2\text{CO}_3(aq) \rightarrow 2\text{NaCl}(aq) + \text{H}_2\text{O}(l) + \text{CO}_2(g)

Step 2: Calculate the moles of sodium carbonate used

Use n=cVn = cV, noting that volumes must be in litres.

n(Na2CO3)=0.125×0.0180=2.25×10−3 moln(\text{Na}_2\text{CO}_3) = 0.125 \times 0.0180 = 2.25 \times 10^{-3}\ \text{mol}

Step 3: Apply the mole ratio to find moles of HCl

The ratio HCl:Na2CO3=2:1\text{HCl} : \text{Na}_2\text{CO}_3 = 2 : 1 means the moles of HCl are twice the moles of carbonate.

n(HCl)=2×2.25×10−3=4.50×10−3 moln(\text{HCl}) = 2 \times 2.25 \times 10^{-3} = 4.50 \times 10^{-3}\ \text{mol}

Step 4: Calculate the concentration of HCl

Divide moles by the volume of the HCl solution (in litres).

c(HCl)=nV=4.50×10−30.0200=0.225 mol L−1c(\text{HCl}) = \frac{n}{V} = \frac{4.50 \times 10^{-3}}{0.0200} = 0.225\ \text{mol L}^{-1}

Final answer: the concentration of the HCl is 0.225 mol L−1\mathbf{0.225\ \text{mol L}^{-1}} (3 significant figures).

Worked example

Question 2 (4 marks): For the exothermic equilibrium 2SO2(g)+O2(g)⇌2SO3(g)2SO_2(g) + O_2(g) \rightleftharpoons 2SO_3(g), ΔH<0\Delta H < 0, predict and justify the effect on the yield of SO3SO_3 of (i) increasing the total pressure and (ii) increasing the temperature.

(i) Increasing pressure: the system shifts to oppose the change by favouring the side with fewer gas moles. There are 3 mol of gas on the left and 2 mol on the right, so equilibrium shifts right, increasing the yield of SO3SO_3.

(ii) Increasing temperature: the forward reaction is exothermic, so adding heat favours the endothermic (reverse) direction. Equilibrium shifts left, decreasing the yield of SO3SO_3 and lowering KeqK_{eq}.

(This is why the industrial Contact process uses moderately high pressure but a compromise temperature, balancing yield against rate.)

Worked example

Question 3 (6 marks): A compound with molecular formula C₃H₆O shows a strong IR absorption near 1715 cm⁻¹ and no broad absorption above 3000 cm⁻¹. Its ¹H NMR shows a single peak (singlet) only. Deduce the structure and justify each piece of evidence.

Degrees of unsaturation
(2×3+2)−62=1\dfrac{(2\times3+2)-6}{2} = 1, indicating one ring or double bond.
IR
~1715 cm⁻¹ is a C=O (carbonyl) stretch; the absence of a broad 2500-3300 cm⁻¹ band rules out a carboxylic acid O-H, and no peak ~3300 cm⁻¹ rules out an alcohol or aldehyde C-H pattern. This points to a ketone.
¹H NMR
a single peak means all hydrogen atoms are equivalent. For C₃H₆O, propanone (acetone) CH3−CO−CH3CH_3{-}CO{-}CH_3 has two CH₃ groups in identical environments, giving one singlet (6H).

Putting it together: the compound is propanone (acetone), CH3COCH3CH_3COCH_3.

Check: formula C₃H₆O ✓; carbonyl present ✓; single equivalent H environment ✓.

Common errors students made

The 2024 QCAA marker report flagged that students lost marks by:

  • omitting state symbols and not balancing charge in ionic/redox equations;
  • mixing up oxidising and reducing agents;
  • quoting Le Chatelier shifts without naming the driver (moles of gas, exo/endothermic direction);
  • drawing energy diagrams without labels or an activation-energy marker.

Further traps to avoid:

  • Confusing strength and concentration of acids (a concentrated weak acid vs a dilute strong acid).
  • Errors in the mole ratio for diprotic or carbonate titrations.
  • Quoting answers to the wrong significant figures - match the least precise data in the stem.
  • In spectroscopy, jumping to a structure without first calculating degrees of unsaturation and assigning every key peak.

How to use this paper

  1. The 2024 sitting was two papers of 90 minutes working time each, with 10 minutes of perusal in the booklets. QCAA's 2026 timetable has two papers, each with 5 minutes of perusal and 90 minutes of working time (check the current QCAA syllabus for your sitting). Sit one paper at a time under timed conditions, pacing each by marks. Have your calculator and data sheet ready.
  2. Self-mark against the official marking guide and QCAA marker report using the links at the top of this page, awarding method marks line by line.
  3. Re-attempt any calculation you lost marks on, writing the relationship first, then substituting, then evaluating with units and sig figs.
  4. Build a one-page summary of characteristic IR/NMR/mass-spec values and electrode potentials, and review it before each re-sit.

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