§-Past paper

QLDChemistry2021

QCE Chemistry 2021

Walkthrough of the 2021 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

The QCAA Chemistry external assessment (Paper, 60 marks in 120 minutes) examines Unit 4: Structure, synthesis and design, drawing on Unit 3 foundations. The 2021 paper covered:

Chemical equilibrium systems: dynamic equilibrium, the equilibrium constant $K_c$ , Le Chatelier's principle, and acid-base equilibria (pH, $K_a$ , buffers).
Oxidation and reduction: half-equations, oxidation numbers, the electrochemical series, galvanic and electrolytic cells, and cell potentials.
Organic chemistry: functional groups, isomers, reaction pathways (substitution, addition, oxidation, esterification, hydrolysis), and polymers.
Chemical synthesis and design: yield and atom economy, and the chemistry behind real-world synthesis problems.

QCAA papers mix multiple choice, short response and calculation-heavy extended items, and reward balanced equations with state symbols, the relationship written before the arithmetic, and significant figures consistent with the data.

Structure and timing

60 marks in 120 minutes gives a comfortable 2 minutes per mark.

Multiple choice + short response. Aim for ~2 minutes per mark; even calculation MCQs need scratch working.
Extended calculation/synthesis items. Allocate time in proportion to marks; a 6-mark stoichiometry chain deserves ~12 minutes including checking units and sig figs.

A workable plan: work steadily at 2 min/mark, then keep a 10-minute buffer to verify every equation is balanced with state symbols and every answer carries correct units and significant figures.

Worked practice questions (exam-style)

Worked example

Question 1 (5 marks): A $2.50\ \text{g}$ sample of calcium carbonate reacts completely with excess hydrochloric acid. Calculate the volume of carbon dioxide produced at $25\ ^\circ\text{C}$ and $100\ \text{kPa}$ . ( $M(\text{CaCO}_3) = 100.1\ \text{g mol}^{-1}$ , molar volume $= 24.8\ \text{L mol}^{-1}$ .)

Balanced equation:

\text{CaCO}_3(s) + 2\,\text{HCl}(aq) \rightarrow \text{CaCl}_2(aq) + \text{H}_2\text{O}(l) + \text{CO}_2(g)

Moles of calcium carbonate:

n(\text{CaCO}_3) = \frac{m}{M} = \frac{2.50}{100.1} = 0.02498\ \text{mol}

From the 1:1 mole ratio, $n(\text{CO}_2) = 0.02498\ \text{mol}$ . Volume of gas:

V = n \times V_m = 0.02498 \times 24.8 = 0.619\ \text{L}\ (3\ \text{sig figs})

The volume of carbon dioxide produced is about 0.619 L (619 mL).

Worked example

Question 2 (4 marks): For the equilibrium $2\text{SO}_2(g) + \text{O}_2(g) \rightleftharpoons 2\text{SO}_3(g)$ , $\Delta H < 0$ , predict and justify the effect on the yield of $\text{SO}_3$ of (a) removing $\text{SO}_3$ as it forms and (b) decreasing the temperature.

(a) Removing $\text{SO}_3$ lowers the concentration of a product. By Le Chatelier's principle, the system shifts to partially replace it, so the equilibrium moves right, increasing the conversion of $\text{SO}_2$ and $\text{O}_2$ - the overall yield of $\text{SO}_3$ produced increases (the driver is the decrease in product concentration).

(b) The forward reaction is exothermic ( $\Delta H < 0$ ). Decreasing temperature removes heat, so the system shifts in the exothermic (forward) direction to release heat, moving right. The yield of $\text{SO}_3$ increases, and $K_c$ increases (the driver is the removal of heat).

Worked example

Question 3 (6 marks): Write balanced half-equations and the overall cell reaction for a galvanic cell made from a zinc electrode in $\text{Zn}^{2+}$ solution and a copper electrode in $\text{Cu}^{2+}$ solution. Identify the anode and calculate the standard cell potential. ( $E^\circ(\text{Zn}^{2+}/\text{Zn}) = -0.76\ \text{V}$ , $E^\circ(\text{Cu}^{2+}/\text{Cu}) = +0.34\ \text{V}$ .)

Zinc has the more negative reduction potential, so it is more easily oxidised and acts as the anode (oxidation):

\text{Zn}(s) \rightarrow \text{Zn}^{2+}(aq) + 2e^- \qquad (\text{oxidation, anode})

Copper is the cathode (reduction):

\text{Cu}^{2+}(aq) + 2e^- \rightarrow \text{Cu}(s) \qquad (\text{reduction, cathode})

Overall cell reaction:

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

Standard cell potential:

E^\circ_{\text{cell}} = E^\circ_{\text{cathode}} - E^\circ_{\text{anode}} = (+0.34) - (-0.76) = +1.10\ \text{V}

The positive value confirms the reaction is spontaneous as written.

Common errors students made

The 2021 QCAA marker report noted that students lost marks by omitting state symbols, mixing up oxidising and reducing agents, quoting Le Chatelier shifts without naming the driver, and drawing energy diagrams without an activation-energy marker. Adding to those:

Mole-ratio errors in stoichiometry chains, especially when the balanced equation was not written first.
Sign and reference errors in electrochemistry - subtracting potentials the wrong way, or mislabelling the anode and cathode.
Significant figures quoted beyond the precision of the least precise datum.
Confusing $K_c$ changes with shifts - stating that adding a catalyst or changing concentration alters $K_c$ , when only temperature changes the value of the constant.

How to use this paper

Work the full paper in a single timed 120-minute block, keeping a 10-minute end buffer to check balancing, state symbols, units and significant figures. Mark every calculation against the official QCAA marking guide at the authority page linked in the frontmatter, paying attention to where method marks sit - they are the most recoverable. Re-attempt any calculation that lost a method mark on a blank page, writing the chosen relationship (the balanced equation, $n = m/M$ , $E^\circ_{\text{cell}}$ formula) before any arithmetic. Keep a one-page error log of recurring slips and re-test those item types after a week.

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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