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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
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 examines Unit 4: Structure, synthesis and design, drawing on Unit 3 foundations. The 2021 paper covered:

  • Chemical equilibrium systems: dynamic equilibrium, the equilibrium constant KcK_c, Le Chatelier's principle, and acid-base equilibria (pH, KaK_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

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): A 2.50 g2.50\ \text{g} sample of calcium carbonate reacts completely with excess hydrochloric acid. Calculate the volume of carbon dioxide produced at 25 ∘C25\ ^\circ\text{C} and 100 kPa100\ \text{kPa}. (M(CaCO3)=100.1 g mol−1M(\text{CaCO}_3) = 100.1\ \text{g mol}^{-1}, molar volume =24.8 L mol−1= 24.8\ \text{L mol}^{-1}.)

Balanced equation:

CaCO3(s)+2 HCl(aq)→CaCl2(aq)+H2O(l)+CO2(g)\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(CaCO3)=mM=2.50100.1=0.02498 moln(\text{CaCO}_3) = \frac{m}{M} = \frac{2.50}{100.1} = 0.02498\ \text{mol}

From the 1:1 mole ratio, n(CO2)=0.02498 moln(\text{CO}_2) = 0.02498\ \text{mol}. Volume of gas:

V=n×Vm=0.02498×24.8=0.619 L (3 sig figs)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 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 justify the effect on the yield of SO3\text{SO}_3 of (a) removing SO3\text{SO}_3 as it forms and (b) decreasing the temperature.

(a) Removing SO3\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 SO2\text{SO}_2 and O2\text{O}_2 - the overall yield of SO3\text{SO}_3 produced increases (the driver is the decrease in product concentration).

(b) The forward reaction is exothermic (ΔH<0\Delta H < 0). Decreasing temperature removes heat, so the system shifts in the exothermic (forward) direction to release heat, moving right. The yield of SO3\text{SO}_3 increases, and KcK_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 Zn2+\text{Zn}^{2+} solution and a copper electrode in Cu2+\text{Cu}^{2+} solution. Identify the anode and calculate the standard cell potential. (E∘(Zn2+/Zn)=−0.76 VE^\circ(\text{Zn}^{2+}/\text{Zn}) = -0.76\ \text{V}, E∘(Cu2+/Cu)=+0.34 VE^\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):

Zn(s)→Zn2+(aq)+2e−(oxidation, anode)\text{Zn}(s) \rightarrow \text{Zn}^{2+}(aq) + 2e^- \qquad (\text{oxidation, anode})

Copper is the cathode (reduction):

Cu2+(aq)+2e−→Cu(s)(reduction, cathode)\text{Cu}^{2+}(aq) + 2e^- \rightarrow \text{Cu}(s) \qquad (\text{reduction, cathode})

Overall cell reaction:

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

Standard 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}

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 KcK_c changes with shifts - stating that adding a catalyst or changing concentration alters KcK_c, when only temperature changes the value of the constant.

How to use this paper

Work the full paper under timed exam conditions, keeping a short buffer at the end to check balancing, state symbols, units and significant figures. Mark every calculation against the official QCAA marking guide at the authority page linked at the top of this page, 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/Mn = m/M, Ecell∘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.

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