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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: 60
Time: 120 min
Authority: QCAA
Updated: 2026-05-29

Open the official PDF ->Official marking notes

What this paper assessed

This QCAA Chemistry external assessment is one 60-mark paper sat over 120 minutes (plus 5 minutes perusal). 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 $K_{eq}$ ), acids and bases (Brønsted-Lowry, $pH$ , $K_a$ , 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

60 marks in 120 minutes is 2 minutes per mark:

Multiple choice (~20 marks): ~30-35 minutes.
Short and extended response (~40 marks): ~80-85 minutes at ~2 min/mark. A 6-mark structure-determination or multi-step stoichiometry item deserves ~12 minutes.
Reserve the final ~10-15 minutes for unit and significant-figure checks.

Use perusal time to locate any structure-determination and multi-step calculation items, which reward early mental planning.

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 $^{-1}$ 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.

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

n(\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 $\text{HCl} : \text{Na}_2\text{CO}_3 = 2 : 1$ means the moles of HCl are twice the moles of carbonate.

n(\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(\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 $\mathbf{0.225\ \text{mol L}^{-1}}$ (3 significant figures).

Worked example

Question 2 (4 marks): For the exothermic equilibrium $2SO_2(g) + O_2(g) \rightleftharpoons 2SO_3(g)$ , $\Delta H < 0$ , predict and justify the effect on the yield of $SO_3$ 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 $SO_3$ .

(ii) Increasing temperature: the forward reaction is exothermic, so adding heat favours the endothermic (reverse) direction. Equilibrium shifts left, decreasing the yield of $SO_3$ and lowering $K_{eq}$ .

(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: $\dfrac{(2\times3+2)-6}{2} = 1$ , 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) $CH_3{-}CO{-}CH_3$ has two CH₃ groups in identical environments, giving one singlet (6H).

Putting it together: the compound is propanone (acetone), $CH_3COCH_3$ .

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

Sit the multiple-choice section in ~30 minutes, then the short/extended responses in ~85 minutes, with calculator and data sheet ready.
Self-mark against the official marking guide and QCAA marker report at the authority links in the frontmatter, awarding method marks line by line.
Re-attempt any calculation you lost marks on, writing the relationship first, then substituting, then evaluating with units and sig figs.
Build a one-page summary of characteristic IR/NMR/mass-spec values and electrode potentials, and review it before each re-sit.

Use this paper well

Sit the paper under exam conditions (120 minutes, 60 marks).
Mark yourself against the official QCAA marking notes.
Compare against the Chemistry hub to find the syllabus dot points this paper tested.

What this paper assessed

Structure and timing

Worked practice questions (exam-style)

Common errors students made

How to use this paper

Use this paper well

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