HSC Exam Practice

Sample response. The enthalpy of neutralisation (ΔH_n) is the heat change per mole of water formed when an acid and a base react to completion under standard conditions. For a strong acid + strong base reaction, ΔH_n ≈ −57 kJ mol⁻¹.

Marking notes. 1 mark for correctly defining ΔH_n as heat per mole of water formed; 1 mark for stating −57 kJ/mol (accept −55 to −58 kJ/mol).

Sample response. Net ionic equation: H⁺(aq) + OH⁻(aq) → H₂O(l). All strong acid + strong base combinations give the same ΔH_n because both species are fully ionised before mixing, so the only reaction that occurs is the formation of water from pre-existing H⁺ and OH⁻ ions. The spectator ions (e.g. Na⁺, Cl⁻) take no part in the reaction and contribute nothing to the enthalpy change.

Marking notes. 1 mark for the correct net ionic equation. 1 mark for identifying that both are fully ionised before mixing (no ionisation energy cost). 1 mark for stating spectator ions do not contribute to ΔH.

Sample response. Before mixing, HCl is completely ionised so H⁺ ions are freely available; CH₃COOH is only partially ionised (K_a = 1.8 × 10⁻⁵, so <1% ionised at 1.00 mol/L). When OH⁻ is added to the acetic acid solution, it drives the equilibrium CH₃COOH ⇌ H⁺ + CH₃COO⁻ to the right, requiring the O–H bond in the carboxyl group to be broken. This ionisation is endothermic (ΔH(ionisation) ≈ +1.6 kJ/mol). By Hess’s Law, the net heat released = (energy from H⁺ + OH⁻ → H₂O) − (ionisation energy consumed) = −57 + 1.6 = −55.4 kJ/mol — less exothermic than HCl + NaOH.

Marking notes. 1 mark — HCl fully ionised, CH₃COOH partially ionised before mixing. 1 mark — OH⁻ drives equilibrium right during reaction, requiring O–H bond breaking. 1 mark — ionisation is endothermic. 1 mark — net ΔH_n calculated correctly or stated as −55.4 kJ/mol (less negative than −57). All four points required for full marks.

Sample response. ΔH(ionisation of X) = ΔH_n(X + NaOH) − ΔH_n(strong acid + NaOH) = −47.8 − (−57.0) = +9.2 kJ/mol. Acid X is a weak acid: the measured ΔH_n (−47.8 kJ/mol) is significantly more positive than the strong acid baseline (−57.0 kJ/mol), indicating that 9.2 kJ/mol of energy was consumed in the endothermic ionisation of X during the neutralisation. A strong acid would give ΔH_n within ±2–3 kJ/mol of −57.0 kJ/mol.

Marking notes. 1 mark — correct calculation: ΔH(ionisation) = +9.2 kJ/mol. 1 mark — correctly classifies as weak acid. 1 mark — justification references the significant deviation from −57 kJ/mol and endothermic ionisation. All three required.

Sample response. Ranking (most to least exothermic): (i) HCl + NaOH (−57.0) > (iii) CH₃COOH + NaOH (−55.4) > (ii) HCl + NH₃ (−52.2) > (iv) CH₃COOH + NH₃ (−49.5). Justification for (ii) vs (iii): NH₃ is a weak base with a larger proton-acceptance enthalpy (+4.8 kJ/mol deviation) compared to acetic acid’s ionisation enthalpy (+1.6 kJ/mol deviation), so HCl + NH₃ is less exothermic than CH₃COOH + NaOH.

Marking notes. 2 marks for correct ordering of all four combinations (1 mark if one error). 1 mark for a specific, accurate justification of (ii) vs (iii) referencing ionisation/proton-acceptance enthalpy magnitudes.

Sample response. Mix 50.0 mL of the unknown acid (1.00 mol/L) with 50.0 mL of 1.00 mol/L NaOH in a polystyrene foam cup. Record T_initial and T_max; calculate q = mcΔT (m = 100.0 g) and ΔH_n = −q/n(H₂O). Repeat with 1.00 mol/L HCl + 1.00 mol/L NaOH as a reference. If ΔH_n of the unknown is within ±2–3 kJ/mol of −57 kJ/mol, classify as strong; if significantly more positive, classify as weak. Controlled variables: (1) Concentration held at 1.00 mol/L for both acids and the base — so n(H₂O) is the same in each trial, allowing ΔH_n per-mole comparison. (2) Volume held at 50.0 mL + 50.0 mL — same mass of solution (m = 100.0 g) in both experiments. (3) T_initial must be the same — so ΔT reflects only the reaction enthalpy, not any pre-existing temperature difference. (4) Same calorimeter (foam cup) — same heat capacity correction applies to both.

Marking notes. 1 mark — valid procedure described (mix equal volumes, record T, calculate ΔH_n). 1 mark — comparison criterion stated (ΔH_n close to / significantly more positive than −57 kJ/mol). 2 marks — at least two controlled variables named with correct justification (1 mark per variable + reason; accept any two from concentration, volume, T_initial, same calorimeter).

Part (a) — ΔH_n calculations.
n(H₂O) = 1.00 × 0.0500 = 0.0500 mol for both trials.
HCl + NaOH: ΔT = 26.8 − 20.0 = 6.8°C; q = 100.0 × 4.18 × 6.8 = 2842.4 J = 2.842 kJ; ΔH_n = −2.842 / 0.0500 = −56.8 kJ/mol.
CH₃COOH + NaOH: ΔT = 26.6 − 20.0 = 6.6°C; q = 100.0 × 4.18 × 6.6 = 2758.8 J = 2.759 kJ; ΔH_n = −2.759 / 0.0500 = −55.2 kJ/mol.
Marking notes. 2 marks for each correct ΔH_n (1 for correct q, 1 for correct ΔH_n with sign) = 4 marks total.

Part (b) — ionisation enthalpy and sign.
ΔH(ionisation CH₃COOH) = ΔH_n(CH₃COOH) − ΔH_n(HCl) = −55.2 − (−56.8) = +1.6 kJ/mol. The positive sign indicates the ionisation of CH₃COOH is endothermic: energy must be absorbed from the solution to break the O–H bond in the carboxyl group and release the proton. This energy absorption reduces the net temperature rise measured.
Marking notes. 1 mark — correct value (+1.6 kJ/mol, accept +1–2 kJ/mol range based on rounded ΔH_n values). 1 mark — positive sign explained as endothermic bond-breaking. 1 mark — explicitly states energy is absorbed, reducing temperature rise measured. [3 marks]

Part (c) — heat released per hour.
Heat released = 8000 mol/h × 55.2 kJ/mol = 441 600 kJ/h = 441.6 MJ/h (accept 440–443 MJ/h).
Marking notes. 1 mark — correct multiplication (8000 × ΔH_n). 1 mark — correct unit conversion to MJ (divide by 1000). Total for part (c): 2 marks.

Sample response. Enthalpy of neutralisation data is highly useful for classifying an unknown acid as strong or weak, but cannot alone determine K_a.

The molecular-level mechanism for the enthalpy difference is as follows: when a strong acid is neutralised with NaOH, H⁺ ions are already present in solution before mixing (complete ionisation), so the only enthalpy change is from forming the O–H bonds in liquid water: H⁺(aq) + OH⁻(aq) → H₂O(l), ΔH ≈ −57 kJ/mol. When a weak acid is neutralised, most molecules are intact before mixing. OH⁻ drives the equilibrium ionisation of the weak acid — an endothermic bond-breaking step. By Hess’s Law, the measured ΔH_n = −57 + ΔH(ionisation), giving a less negative value than for a strong acid. The larger the ionisation enthalpy (the weaker the acid), the more positive ΔH_n becomes.

The usefulness of ΔH_n for classification: if ΔH_n ≈ −57 kJ/mol (within ±2–3 kJ/mol foam cup precision), the acid is strong; if significantly more positive (e.g. −50 to −55 kJ/mol), the acid is weak. This is a reliable classification in most school and bench-scale settings.

Limitation of foam cup calorimetry: the precision of (±2–3 kJ/mol) means two weak acids with similar K_a values and similar ΔH(ionisation) values may not be distinguishable by calorimetry alone. For example, two acids with ΔH_n values of −55.4 and −55.8 kJ/mol could not be reliably separated. Additionally, calorimetry provides no information about K_a: knowing ΔH(ionisation) = +2.0 kJ/mol does not allow calculation of K_a, since K_a is a function of [H⁺] at equilibrium (a different thermodynamic quantity).

Role of pH in complementing calorimetry: pH measurement at a known concentration gives [H⁺] directly, from which K_a can be calculated: K_a = [H⁺]² / (c − [H⁺]). pH measurement is also more precise (digital probes to ±0.01 pH units) and can resolve two weak acids that calorimetry cannot. Together, the two methods provide both thermodynamic (ΔH_n) and equilibrium (K_a) characterisation of the acid.

Marking criteria. 1 mark — correct molecular mechanism for strong acid (fully ionised, no ionisation cost, H⁺ + OH⁻ → H₂O only). 1 mark — correct mechanism for weak acid (partially ionised, OH⁻ drives equilibrium ionisation, endothermic bond-breaking reduces net ΔH_n). 1 mark — correct classification criterion (ΔH_n ≈ −57 = strong; significantly more positive = weak). 1 mark — specific limitation of foam cup calorimetry referenced (precision ±2–3 kJ/mol cannot resolve similar weak acids OR cannot determine K_a). 1 mark — pH measurement explained as providing [H⁺] → K_a (equilibrium information not available from calorimetry). 1 mark — explicit evaluative judgement: calorimetry is useful for classification but incomplete for full characterisation; pH complements it by providing K_a.

Scientific flaw. The claim is incorrect. While it is true that neutralisation always forms one mole of water per mole of H⁺ reacted, the heat released per mole of water is not the same for all acids — it depends on whether the acid is strong or weak. For strong acids (e.g. HCl), H⁺ is already present in solution before the reaction, so the only energy change is from forming the O–H bond in water: ΔH_n ≈ −57 kJ/mol. For weak acids (e.g. CH₃COOH), most molecules are un-ionised before reaction. To provide H⁺ for neutralisation, the weak acid must ionise during the reaction — an endothermic step. This endothermic ionisation energy is subtracted from the exothermic H⁺ + OH⁻ → H₂O energy, giving a less negative ΔH_n (e.g. −55.4 kJ/mol for acetic acid). The claim treats neutralisation as a single step involving only water formation, ignoring the additional endothermic ionisation step required for weak acids. The correct statement is: ΔH_n for a weak acid + strong base is always more positive (less exothermic) than for a strong acid + strong base, because the ionisation of the weak acid is endothermic and consumes part of the water-formation energy.

Marking criteria. 1 mark — correctly identifies the flaw: claim treats all acids as giving the same ΔH_n per mole. 1 mark — correctly states weak acid neutralisation gives a less negative (more positive) ΔH_n with an example value. 1 mark — explains the molecular mechanism: weak acid ionisation is endothermic and occurs during neutralisation. 1 mark — uses Hess’s Law framing: net ΔH_n = water-formation energy − ionisation energy (or equivalent). 1 mark — reformulates the correct statement explicitly (weak acid ΔH_n more positive than strong acid ΔH_n).