Skill Align SACE Stage 2 Chemistry - Free Online Pack 0 Booklet 1
An independently authored SACE Chemistry examination booklet with worked answers, atomic marking guidance and diagnostic review prompts.
- Paper
- Booklet 1
- Reading
- No separate reading time
- Writing
- Approximately 65 minutes
- Assessment
- 59 marks
Approved calculators may be used. A Skill Align-formatted data sheet and periodic table based on the current SACE Chemistry reference material are supplied in this booklet.
Booklet 1 - Questions 1 to 3
Answer every question and every part. Show working for calculations, include units and use the supplied chemical evidence where required.
Question 1
18 marksThree monitoring stations recorded afternoon ozone concentrations of 28, 46 and 71 ppb from the city centre to a downwind foothills site. NO₂ photolysis initiates ozone formation. The local advisory trigger is 65 ppb, and the final value has an uncertainty of ±4 ppb.
Question 2
20 marksA 25.00 mL wine sample required 12.40 mL of 0.01000 mol L⁻¹ I₂. The reaction is I₂ + SO₃²⁻ + H₂O → 2I⁻ + SO₄²⁻ + 2H⁺. A second aliquot gave 12.46 mL. Assume the sample preparation converts all free sulfite to SO₃²⁻.
Question 3
21 marksA pilot Haber loop uses N₂(g) + 3H₂(g) ⇌ 2NH₃(g), ΔH = -92 kJ mol⁻¹ as written. It operates at high pressure and a moderate compromise temperature with an iron catalyst. A single pass converts 18.0% of 250 mol N₂. Ammonia is condensed before unreacted gases are recycled, and the hydrogen is supplied by electrolysis using contracted renewable electricity.
Worked Solutions And Marking Guide
General marking principles
- Award each listed raw mark independently when the required chemical evidence is demonstrated.
- Accept chemically correct equivalent equations, representations, working and wording.
- A conclusion or Science as a Human Endeavour judgement must be supported by relevant evidence and acknowledge material limitations or trade-offs.
Question 1
(a)(i) - The plotted station values rise monotonically from 28 ppb in the city centre through 46 ppb at the intermediate site to 71 ppb at the downwind foothills site. - The downwind foothills value of 71 ppb is the highest measured concentration.
(a)(ii) - The downwind result is 6 ppb above the 65 ppb advisory trigger. - The stated ±4 ppb interval around the downwind value is 67-75 ppb and remains above the trigger. - The intermediate-site value is 46 ppb, between the city-centre and downwind measurements.
(b) - The absolute increase is 71-28 = 43 ppb. - The percentage increase is 43/28 × 100 = 154%, which is dimensionless.
(c) - Sunlight photolyses NO₂ to NO and an oxygen atom: NO₂ + light → NO + O. - The oxygen atom combines with molecular oxygen to form O₃: O + O₂ → O₃. - VOC oxidation supplies radicals that convert NO back to NO₂ without consuming O₃. - Regenerated NO₂ can photolyse again during the downwind travel time, permitting ozone production to continue. - Ozone formed through this sequence is a secondary pollutant, so the higher foothills reading does not mean ozone was directly emitted there.
(d) - The evidence supports an afternoon ozone advisory at the foothills site. - The complete uncertainty interval remains above the operational trigger. - One afternoon cannot establish the frequency of high-ozone events. - Repeat measurements across seasons and comparable wind conditions. - Co-locate NOx and VOC measurements to test the proposed mechanism. - Public warnings reduce exposure while longer-term precursor controls address formation.
Detailed marking criteria
Part (a)(i) (1 mark)
Awards one mark for establishing this question-specific chemical point: The plotted station values rise monotonically from 28 ppb in the city centre through 46 ppb at the intermediate site to 71 ppb at the downwind foothills site.
Part (a)(i) (1 mark)
Awards one mark for establishing this question-specific chemical point: The downwind foothills value of 71 ppb is the highest measured concentration.
Part (a)(ii) (1 mark)
Awards one mark for establishing this question-specific chemical point: The downwind result is 6 ppb above the 65 ppb advisory trigger.
Part (a)(ii) (1 mark)
Awards one mark for establishing this question-specific chemical point: The stated ±4 ppb interval around the downwind value is 67-75 ppb and remains above the trigger.
Part (a)(ii) (1 mark)
Awards one mark for establishing this question-specific chemical point: The intermediate-site value is 46 ppb, between the city-centre and downwind measurements.
Part b (1 mark)
Awards one mark for establishing this question-specific chemical point: The absolute increase is 71-28 = 43 ppb.
Part b (1 mark)
Awards one mark for establishing this question-specific chemical point: The percentage increase is 43/28 × 100 = 154%, which is dimensionless.
Part c (1 mark)
Awards one mark for establishing this question-specific chemical point: Sunlight photolyses NO₂ to NO and an oxygen atom: NO₂ + light → NO + O.
Part c (1 mark)
Awards one mark for establishing this question-specific chemical point: The oxygen atom combines with molecular oxygen to form O₃: O + O₂ → O₃.
Part c (1 mark)
Awards one mark for establishing this question-specific chemical point: VOC oxidation supplies radicals that convert NO back to NO₂ without consuming O₃.
Part c (1 mark)
Awards one mark for establishing this question-specific chemical point: Regenerated NO₂ can photolyse again during the downwind travel time, permitting ozone production to continue.
Part c (1 mark)
Awards one mark for establishing this question-specific chemical point: Ozone formed through this sequence is a secondary pollutant, so the higher foothills reading does not mean ozone was directly emitted there.
Part d (1 mark)
Awards one mark for establishing this question-specific chemical point: The evidence supports an afternoon ozone advisory at the foothills site.
Part d (1 mark)
Awards one mark for establishing this question-specific chemical point: The complete uncertainty interval remains above the operational trigger.
Part d (1 mark)
Awards one mark for establishing this question-specific chemical point: One afternoon cannot establish the frequency of high-ozone events.
Part d (1 mark)
Awards one mark for establishing this question-specific chemical point: Repeat measurements across seasons and comparable wind conditions.
Part d (1 mark)
Awards one mark for establishing this question-specific chemical point: Co-locate NOx and VOC measurements to test the proposed mechanism.
Part d (1 mark)
Awards one mark for establishing this question-specific chemical point: Public warnings reduce exposure while longer-term precursor controls address formation.
Question 2
(a) - The two titres differ by only 0.06 mL and are concordant. - The mean of both concordant titres is the appropriate analytical volume. - I₂ and SO₃²⁻ react in a one-to-one mole ratio. - Both titres refer to separate 25.00 mL aliquots under the same stated conditions. - The calculation assumes that sample preparation converts all free sulfite to SO₃²⁻.
(b) - Mean titre = (12.40 + 12.46)/2 = 12.43 mL. - n(I₂) = 0.01000 × 0.01243 = 1.243 × 10⁻⁴ mol. - The one-to-one equation gives 1.243 × 10⁻⁴ mol SO₃²⁻. - c(SO₃²⁻) = 1.243 × 10⁻⁴ / 0.02500 = 0.004972 mol L⁻¹. - The mean titre is converted from 12.43 mL to 0.01243 L before substitution.
(c)(i) - SO₃²⁻ + H₂O → SO₄²⁻ + 2H⁺ + 2e⁻.
(c)(ii) - Sulfur is oxidised from +4 in sulfite to +6 in sulfate. - Iodine is reduced from 0 in I₂ to -1 in iodide. - The balanced overall equation conserves atoms and charge. - Two electrons lost by sulfur match two electrons gained by iodine.
(d) - The concordant titres support repeatability for this prepared sample under the stated sulfite model, but do not establish sulfite specificity. - Oxidation of sulfite by air before titration would bias the result low. - Analyse promptly in a closed vessel to reduce oxygen exposure. - A reagent blank without wine measures reagent-background iodine consumption only; it cannot correct the titre contribution of other wine reductants, which requires a sulfite-selective comparison. - Certified sulfite reference material would test accuracy.
Detailed marking criteria
Part a (1 mark)
Awards one mark for establishing this question-specific chemical point: The two titres differ by only 0.06 mL and are concordant.
Part a (1 mark)
Awards one mark for establishing this question-specific chemical point: The mean of both concordant titres is the appropriate analytical volume.
Part a (1 mark)
Awards one mark for establishing this question-specific chemical point: I₂ and SO₃²⁻ react in a one-to-one mole ratio.
Part a (1 mark)
Awards one mark for establishing this question-specific chemical point: Both titres refer to separate 25.00 mL aliquots under the same stated conditions.
Part a (1 mark)
Awards one mark for establishing this question-specific chemical point: The calculation assumes that sample preparation converts all free sulfite to SO₃²⁻.
Part b (1 mark)
Awards one mark for establishing this question-specific chemical point: Mean titre = (12.40 + 12.46)/2 = 12.43 mL.
Part b (1 mark)
Awards one mark for establishing this question-specific chemical point: n(I₂) = 0.01000 × 0.01243 = 1.243 × 10⁻⁴ mol.
Part b (1 mark)
Awards one mark for establishing this question-specific chemical point: The one-to-one equation gives 1.243 × 10⁻⁴ mol SO₃²⁻.
Part b (1 mark)
Awards one mark for establishing this question-specific chemical point: c(SO₃²⁻) = 1.243 × 10⁻⁴ / 0.02500 = 0.004972 mol L⁻¹.
Part b (1 mark)
Awards one mark for establishing this question-specific chemical point: The mean titre is converted from 12.43 mL to 0.01243 L before substitution.
Part (c)(i) (1 mark)
Awards one mark for establishing this question-specific chemical point: SO₃²⁻ + H₂O → SO₄²⁻ + 2H⁺ + 2e⁻.
Part (c)(ii) (1 mark)
Awards one mark for establishing this question-specific chemical point: Sulfur is oxidised from +4 in sulfite to +6 in sulfate.
Part (c)(ii) (1 mark)
Awards one mark for establishing this question-specific chemical point: Iodine is reduced from 0 in I₂ to -1 in iodide.
Part (c)(ii) (1 mark)
Awards one mark for establishing this question-specific chemical point: The balanced overall equation conserves atoms and charge.
Part (c)(ii) (1 mark)
Awards one mark for establishing this question-specific chemical point: Two electrons lost by sulfur match two electrons gained by iodine.
Part d (1 mark)
Awards one mark for establishing this question-specific chemical point: The concordant titres support repeatability for this prepared sample under the stated sulfite model, but do not establish sulfite specificity.
Part d (1 mark)
Awards one mark for establishing this question-specific chemical point: Oxidation of sulfite by air before titration would bias the result low.
Part d (1 mark)
Awards one mark for establishing this question-specific chemical point: Analyse promptly in a closed vessel to reduce oxygen exposure.
Part d (1 mark)
Awards one mark for establishing this question-specific chemical point: A reagent blank without wine measures reagent-background iodine consumption only; it cannot correct the titre contribution of other wine reductants, which requires a sulfite-selective comparison.
Part d (1 mark)
Awards one mark for establishing this question-specific chemical point: Certified sulfite reference material would test accuracy.
Question 3
(a) - Kc = [NH₃]²/([N₂][H₂]³) for the stated gas reaction. - Removing ammonia shifts the equilibrium towards further ammonia formation by Le Chatelier's principle. - Compressing the mixture also favours the side with fewer gas moles. - Very low temperature would reduce production rate despite improving equilibrium yield. - The iron catalyst provides a lower-activation-energy pathway.
(b) - The theoretical ammonia amount is 2 × 45.0 = 90.0 mol. - M(NH₃) = 14.01 + 3(1.008) = 17.034 g mol⁻¹. - m(NH₃) = 90.0 × 17.034 = 1.53 × 10³ g. - The answer is 1.53 kg NH₃ to three significant figures. - Stoichiometry uses two moles NH₃ per mole N₂ consumed.
(c) - After the 18.0% first-pass conversion, 250 - 45.0 = 205 mol N₂ remains unreacted. - Converting the 45.0 mol N₂ in that pass consumes 3 × 45.0 = 135 mol H₂. - If sufficient H₂ is supplied and every remaining N₂ molecule eventually reacts, the 205 mol could form at most an additional 410 mol NH₃. - The original 250 mol N₂ feed therefore sets an upper bound of 500 mol NH₃ over all passes, including the 90.0 mol already calculated in Part (b). - Neither bound is an achieved overall yield: the H₂ feed amount, later-pass conversions and product-recovery losses are not supplied.
(d) - Measure ammonia production rate and single-pass nitrogen conversion at matched feed composition across candidate temperatures to quantify the rate-yield compromise. - Compare conversion and recovered-ammonia throughput at matched feed rate across candidate pressures. - Measure compression electricity per kilogram of recovered ammonia for each pressure condition. - Record condenser duty and recovered ammonia amount to assess the separation benefit and cost. - Measure recycle-gas flow and composition to distinguish single-pass conversion from overall loop utilisation. - Monitor iron-catalyst activity and lifetime under the candidate conditions before changing the operating point.
Detailed marking criteria
Part a (1 mark)
Awards one mark for establishing this question-specific chemical point: Kc = [NH₃]²/([N₂][H₂]³) for the stated gas reaction.
Part a (1 mark)
Awards one mark for establishing this question-specific chemical point: Removing ammonia shifts the equilibrium towards further ammonia formation by Le Chatelier's principle.
Part a (1 mark)
Awards one mark for establishing this question-specific chemical point: Compressing the mixture also favours the side with fewer gas moles.
Part a (1 mark)
Awards one mark for establishing this question-specific chemical point: Very low temperature would reduce production rate despite improving equilibrium yield.
Part a (1 mark)
Awards one mark for establishing this question-specific chemical point: The iron catalyst provides a lower-activation-energy pathway.
Part b (1 mark)
Awards one mark for establishing this question-specific chemical point: The theoretical ammonia amount is 2 × 45.0 = 90.0 mol.
Part b (1 mark)
Awards one mark for establishing this question-specific chemical point: M(NH₃) = 14.01 + 3(1.008) = 17.034 g mol⁻¹.
Part b (1 mark)
Awards one mark for establishing this question-specific chemical point: m(NH₃) = 90.0 × 17.034 = 1.53 × 10³ g.
Part b (1 mark)
Awards one mark for establishing this question-specific chemical point: The answer is 1.53 kg NH₃ to three significant figures.
Part b (1 mark)
Awards one mark for establishing this question-specific chemical point: Stoichiometry uses two moles NH₃ per mole N₂ consumed.
Part c (1 mark)
Awards one mark for establishing this question-specific chemical point: After the 18.0% first-pass conversion, 250 - 45.0 = 205 mol N₂ remains unreacted.
Part c (1 mark)
Awards one mark for establishing this question-specific chemical point: Converting the 45.0 mol N₂ in that pass consumes 3 × 45.0 = 135 mol H₂.
Part c (1 mark)
Awards one mark for establishing this question-specific chemical point: If sufficient H₂ is supplied and every remaining N₂ molecule eventually reacts, the 205 mol could form at most an additional 410 mol NH₃.
Part c (1 mark)
Awards one mark for establishing this question-specific chemical point: The original 250 mol N₂ feed therefore sets an upper bound of 500 mol NH₃ over all passes, including the 90.0 mol already calculated in Part (b).
Part c (1 mark)
Awards one mark for establishing this question-specific chemical point: Neither bound is an achieved overall yield: the H₂ feed amount, later-pass conversions and product-recovery losses are not supplied.
Part d (1 mark)
Awards one mark for establishing this question-specific chemical point: Measure ammonia production rate and single-pass nitrogen conversion at matched feed composition across candidate temperatures to quantify the rate-yield compromise.
Part d (1 mark)
Awards one mark for establishing this question-specific chemical point: Compare conversion and recovered-ammonia throughput at matched feed rate across candidate pressures.
Part d (1 mark)
Awards one mark for establishing this question-specific chemical point: Measure compression electricity per kilogram of recovered ammonia for each pressure condition.
Part d (1 mark)
Awards one mark for establishing this question-specific chemical point: Record condenser duty and recovered ammonia amount to assess the separation benefit and cost.
Part d (1 mark)
Awards one mark for establishing this question-specific chemical point: Measure recycle-gas flow and composition to distinguish single-pass conversion from overall loop utilisation.
Part d (1 mark)
Awards one mark for establishing this question-specific chemical point: Monitor iron-catalyst activity and lifetime under the candidate conditions before changing the operating point.
Diagnostic Checklist
| Topic | Questions | Marks | Marks Lost | Action |
|---|---|---|---|---|
| Topic 1 - Analytical chemistry and monitoring | Q2 | 20 | ___ | Review volumetric analysis, chromatography, atomic spectroscopy, calibration, uncertainty and fit-for-purpose monitoring. |
| Topic 1 - Environmental monitoring and atmospheric chemistry | Q1 | 18 | ___ | Review atmospheric reactions, greenhouse and photochemical-smog chemistry, monitoring evidence and environmental interpretation. |
| Topic 2 - Optimising chemical processes | Q3 | 21 | ___ | Review yield, rate, catalysts, energy, separation, recycling and process-condition trade-offs. |