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QLDChemistry2022

QCE Chemistry 2022

Walkthrough of the 2022 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

The QCAA Chemistry external assessment (worth 50% of the subject) covers Units 3 and 4. The 2022 paper assessed:

  • Unit 3 - Equilibrium, acids and redox reactions: chemical equilibrium and KeqK_{eq}, Le Chatelier's principle, acid-base reactions (pH, KaK_a, buffers, titrations), and oxidation-reduction including electrochemical cells and E∘E^\circ values.
  • Unit 4 - Structure, synthesis and design: properties of organic compounds, functional groups and reaction pathways, organic synthesis, polymers, and macromolecules/materials design.

Items required balanced equations with state symbols, worked calculations that show the chosen relationship before the arithmetic, and the ability to link practical technique to chemical theory. The paper rewarded clearly set-out multi-step reasoning over unsupported final answers.

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

A 20.00 mL20.00\ \text{mL} sample of hydrochloric acid is titrated against 0.150 mol L−10.150\ \text{mol L}^{-1} NaOH, requiring 24.60 mL24.60\ \text{mL} to reach the endpoint. Calculate the concentration of the HCl.

Solution. The neutralisation is HCl+NaOH→NaCl+H2O\text{HCl} + \text{NaOH} \rightarrow \text{NaCl} + \text{H}_2\text{O} (1 mark), a 1:1 mole ratio.

n(NaOH)=c×V=0.150×0.02460=3.69×10−3 moln(\text{NaOH}) = c \times V = 0.150 \times 0.02460 = 3.69 \times 10^{-3}\ \text{mol}

So n(HCl)=3.69×10−3 moln(\text{HCl}) = 3.69 \times 10^{-3}\ \text{mol} (1 mark).

c(HCl)=nV=3.69×10−30.02000=0.1845 mol L−1≈0.185 mol L−1c(\text{HCl}) = \frac{n}{V} = \frac{3.69 \times 10^{-3}}{0.02000} = 0.1845\ \text{mol L}^{-1} \approx 0.185\ \text{mol L}^{-1}

(2 marks for working, 1 mark for the answer to 3 significant figures with units.)

Worked example

Question 2 (4 marks): Le Chatelier's principle

For the equilibrium 2SO2(g)+O2(g)⇌2SO3(g)2\text{SO}_2(g) + \text{O}_2(g) \rightleftharpoons 2\text{SO}_3(g), ΔH<0\Delta H < 0. Predict and explain the effect on the equilibrium yield of SO3\text{SO}_3 of (a) increasing pressure and (b) increasing temperature.

Solution.
(a) Increasing pressure shifts the system toward the side with fewer gas moles. There are 3 mol of gas on the left and 2 mol on the right, so the equilibrium shifts right, increasing the yield of SO3\text{SO}_3, to partially oppose the increase in pressure (2 marks).
(b) The forward reaction is exothermic. Increasing temperature favours the endothermic (reverse) direction to oppose the added thermal energy, shifting the equilibrium left and decreasing the SO3\text{SO}_3 yield (2 marks). Full marks require naming the driver (opposing the imposed change), not just stating the direction.

Worked example

Question 3 (6 marks): Redox and electrochemical cells

A galvanic cell is constructed from a zinc half-cell and a copper half-cell. Given E∘(Cu2+/Cu)=+0.34 VE^\circ(\text{Cu}^{2+}/\text{Cu}) = +0.34\ \text{V} and E∘(Zn2+/Zn)=−0.76 VE^\circ(\text{Zn}^{2+}/\text{Zn}) = -0.76\ \text{V}, identify the anode and cathode, write the half-equations and overall equation, and calculate the cell potential.

Solution. The half-cell with the lower (more negative) reduction potential is oxidised, so zinc is the anode (oxidation) and copper is the cathode (reduction) - 1 mark.

Half-equations:

Anode (oxidation): Zn(s)→Zn2+(aq)+2e−\text{Anode (oxidation): } \text{Zn}(s) \rightarrow \text{Zn}^{2+}(aq) + 2e^-

Cathode (reduction): Cu2+(aq)+2e−→Cu(s)\text{Cathode (reduction): } \text{Cu}^{2+}(aq) + 2e^- \rightarrow \text{Cu}(s)

(2 marks for both, with state symbols.) Overall:

Zn(s)+Cu2+(aq)→Zn2+(aq)+Cu(s)\text{Zn}(s) + \text{Cu}^{2+}(aq) \rightarrow \text{Zn}^{2+}(aq) + \text{Cu}(s)

(1 mark.) Cell potential:

Ecell∘=Ecathode∘−Eanode∘=0.34−(−0.76)=+1.10 VE^\circ_{\text{cell}} = E^\circ_{\text{cathode}} - E^\circ_{\text{anode}} = 0.34 - (-0.76) = +1.10\ \text{V}

(2 marks.) The positive value confirms the reaction is spontaneous as written.

Common errors students made

The 2022 QCAA marker report flagged that students lost marks by omitting state symbols, mixing up oxidising and reducing agents, and quoting Le Chatelier shifts without naming the driver; energy diagrams were often unlabelled or drawn without an activation-energy marker. Add these recurring traps:

  • Significant figures and units dropped at the final step or carried inconsistently through a calculation chain.
  • Mole-ratio errors - assuming 1:1 when the balanced equation is not (e.g. diprotic acids, or the 2:1:22:1:2 ratio in the SO2\text{SO}_2 equilibrium).
  • Sign errors in Ecell∘E^\circ_{\text{cell}} - reversing cathode/anode or mishandling the negative anode potential.
  • Ambiguous organic structures - missing hydrogens, unclear bonds, or drawing an ester where a carboxylic acid is required.

How to use this paper

The 2022 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 the QCAA data booklet to hand. Mark against the official QCAA marking guide and marker report (linked at the top of this page) to see exactly where method marks fall. Rebuild every multi-step calculation that lost more than half its marks, writing one labelled line per quantity. Drill the functional-group/reaction-pathway map and the E∘E^\circ procedure until cell potentials and half-equations are automatic, then practise stating the Le Chatelier driver explicitly in words.

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