HSC Exam Practice

Sample response. The enthalpy of neutralisation (ΔH_n) is the heat energy released per mole of water formed when an acid reacts with a base. Because neutralisation is exothermic (heat is released to the surroundings), ΔH_n is always negative by sign convention.

Marking notes. 1 mark for “heat energy released per mole of water formed in a neutralisation reaction”; 1 mark for “ΔH_n is negative because the reaction is exothermic.”

Sample response. Net ionic equation: H⁺(aq) + OH⁻(aq) → H₂O(l). The approximate ΔH_n value is −57 kJ/mol (of H₂O formed).

Marking notes. 1 mark for correct net ionic equation including states; 1 mark for −57 kJ/mol (accept −56 to −58 kJ/mol).

Sample response. For a strong acid + strong base (e.g. HCl + NaOH), both species are fully ionised before the reaction. The only reaction is H⁺(aq) + OH⁻(aq) → H₂O(l), so ΔH_n ≈ −57 kJ/mol. For a weak acid + strong base (e.g. CH₃COOH + NaOH), the weak acid is only partially ionised in solution. During neutralisation, additional energy must be absorbed to drive further ionisation of the weak acid; this energy reduces the net heat released, so |ΔH_n| < 57 kJ/mol.

Marking notes. 1 mark for strong+strong gives −57 kJ/mol with reference to full ionisation; 1 mark for weak acid only partially ionised before reaction; 1 mark for ionisation during reaction is endothermic, reducing net heat → |ΔH_n| < 57 kJ/mol.

Sample response. Source 1: Heat loss through the walls and open top of the foam cup to the surrounding atmosphere. Effect: T_max recorded is lower than the true T_max → ΔT is smaller → q is smaller → |ΔH_n| is underestimated (less negative). Source 2: Heat absorbed by the thermometer or measuring probe. Effect: the thermometer absorbs heat from the solution, so the solution loses some heat that is not measured → smaller ΔT → smaller q → |ΔH_n| underestimated.

Marking notes. 2 marks per source: 1 for naming the source specifically (not “human error”); 1 for correctly tracing effect on ΔT → q → ΔH_n. Accept any two valid sources from: heat loss through cup/lid opening, heat absorbed by thermometer, density/SHC approximation, incomplete mixing, thermometer precision.

Sample response. All heat released by the reaction is absorbed by the combined acid + base solution, not just by one component. Using only the mass of the acid would underestimate the total heat capacity of the solution, giving a q value that is too small and therefore a ΔH_n value that is incorrectly calculated.

Marking notes. 1 mark for “heat is absorbed by the entire combined solution”; 1 mark for correct consequence of using wrong mass on the calculated q or ΔH_n.

Sample response. A data logger records temperature continuously during and after the reaction. After the reaction peak, the temperature decreases as heat is lost to the surroundings (cooling phase). The linear cooling trend from the post-peak region is extrapolated back to the moment of mixing (t = 0 for the reaction). The temperature at the intersection of the extrapolated line and the mixing time gives the true T_max — the temperature that would have been reached if no heat had been lost. This corrected T_max gives a more accurate ΔT and therefore a more accurate q and ΔH_n.

Marking notes. 1 mark for data logger records temperature continuously; 1 mark for extrapolating the cooling portion of the graph back to the moment of mixing; 1 mark for the extrapolated T_max gives a more accurate ΔT because it accounts for heat loss during the rise phase.

(a) Limiting reagent (2 marks): n(HNO₃) = 2.00 mol/L × 0.0300 L = 0.0600 mol H⁺. n(NaOH) = 1.00 mol/L × 0.0500 L = 0.0500 mol OH⁻. OH⁻ is the limiting reagent (0.0500 mol < 0.0600 mol). n(H₂O) = 0.0500 mol.

(b) q (2 marks): Total volume = 30.0 + 50.0 = 80.0 mL → m = 80.0 g. ΔT = 25.8 − 19.5 = 6.3°C. q = 80.0 × 4.18 × 6.3 = 2106.7 J.

(c) ΔH_n (2 marks): ΔH_n = −2106.7 J / 0.0500 mol = −42 134 J/mol ÷ 1000 = −42.1 kJ/mol.

(d) Source + improvement (1 mark): Source: heat loss through the walls and open top of the foam-cup calorimeter to the surrounding atmosphere reduces the recorded T_max, giving a smaller ΔT and therefore a less negative ΔH_n than the theoretical −57 kJ/mol. Improvement: add a tight-fitting lid to the foam cup, or use a data logger with cooling-curve extrapolation to recover the true T_max.

(a) T_max and ΔT (2 marks): Curve A (HCl + NaOH): T_max = 27.4°C; ΔT = 27.4 − 20.0 = 7.4°C. Curve B (CH₃COOH + NaOH): T_max = 25.8°C; ΔT = 25.8 − 20.0 = 5.8°C.

(b) ΔH_n (3 marks — full working for A required): Curve A: q = 100.0 × 4.18 × 7.4 = 3093.2 J; n(H₂O) = 0.0500 mol; ΔH_n = −3093.2/0.0500/1000 = −61.9 kJ/mol (slightly above −57 due to rounding/graph reading; accept −58 to −62). Curve B: q = 100.0 × 4.18 × 5.8 = 2424.4 J; ΔH_n = −2424.4/0.0500/1000 = −48.5 kJ/mol.

(c) Account for difference (3 marks): HCl (strong acid) is fully ionised in solution before the reaction, so all H⁺ ions are immediately available. The only reaction is H⁺(aq) + OH⁻(aq) → H₂O(l), releasing the full ≈ −57 kJ/mol as heat, producing a larger ΔT. CH₃COOH (acetic acid) is a weak acid: only approximately 0.4–1% is ionised before the reaction. When NaOH is added, OH⁻ reacts with the available H⁺, pulling the equilibrium CH₃COOH ⇌ H⁺ + CH₃COO⁻ further to the right. This additional ionisation step is endothermic — it absorbs energy that would otherwise be released as heat — reducing the net heat available to warm the solution. The result is a smaller ΔT and a less negative ΔH_n than for the strong acid combination.

Sample response. The foam-cup calorimeter is a simple and relatively reliable method for determining ΔH_n, but it is not highly accurate due to a consistent pattern of systematic error. Reliability refers to the reproducibility of results when the experiment is repeated under the same conditions; accuracy refers to how close those results are to the true (theoretical) value. The foam-cup method is moderately reliable — repeated trials with the same solutions and equipment tend to give similar results — but it is systematically inaccurate because all sources of error act in the same direction, producing experimental |ΔH_n| values that are consistently less negative than the true value of −57 kJ/mol. The dominant sources of systematic error are: (1) heat loss through the walls and open top of the foam cup to the surrounding atmosphere, which means T_max recorded is lower than the true T_max, reducing ΔT, q, and |ΔH_n|; (2) heat absorbed by the thermometer or probe, which further reduces the apparent temperature rise of the solution; (3) assumptions that solution density = 1.00 g/mL and specific heat capacity = 4.18 J g⁻¹°C⁻¹ introduce small but systematic errors in the calculated mass and q. Because all errors act in the same direction (all reduce |ΔH_n|), a student can never obtain a result more negative than the true value — systematic error is unidirectional in this experiment. Cooling-curve extrapolation substantially improves accuracy. By recording temperature continuously with a data logger, the linear cooling trend after the peak can be extrapolated back to the moment of mixing, recovering the T_max that would have been reached if no heat had been lost. This corrected T_max gives a more accurate ΔT and therefore a less biased ΔH_n. Even with extrapolation, the remaining errors (thermometer heat capacity, density/SHC assumptions) mean the method is still not as accurate as a bomb calorimeter or a calibrated adiabatic calorimeter. In summary: the foam-cup method is reliable (reproducible) but systematically inaccurate; the degree of inaccuracy can be reduced but not eliminated by cooling-curve extrapolation and careful experimental technique.

Marking notes. 1 mark — defines reliability vs accuracy correctly. 1 mark — states that all systematic errors are unidirectional (all reduce |ΔH_n|). 1 mark — names at least two specific sources of systematic error with correct effect on ΔT and ΔH_n. 1 mark — explains the cooling-curve extrapolation procedure (data logger, post-peak cooling region, back-extrapolation to mixing time). 1 mark — explains why extrapolation improves accuracy (“recovers T_max that would have been reached without heat loss”). 1 mark — acknowledges that remaining errors still prevent full accuracy even with extrapolation. 1 mark — reaches an explicit assessment: foam-cup method is reliably reproducible but systematically inaccurate; adequate for ranking experiments but not for precise absolute ΔH_n determination.