Chemistry • Year 12 • Module 6 • Lesson 3
Enthalpy of Neutralisation: Practical & Theory
Build HSC Band 5–6 extended-response technique: evaluate multi-variable calorimetry data, critique a scientific claim, and reach evidence-based judgements about experimental design and enthalpy theory.
1. Extended response — evaluate antacid calorimetry data (Band 5–6)
8 marks Band 5–6
Scenario. A pharmaceutical chemistry student at the University of Wollongong is comparing the heat output of three antacid products to model thermal hazard during neutralisation in the stomach. Each antacid is dissolved to give exactly 50.0 mL of solution containing 0.050 mol of base, then mixed with 50.0 mL of 1.00 mol/L HCl at 20.0°C in a foam-cup calorimeter (total solution mass = 100.0 g; c = 4.18 J g−1°C−1). Results below.
| Antacid | Active base (strength) | Tmax (°C) | Experimental ΔHn (kJ/mol) | Theoretical ΔHn (kJ/mol) |
|---|---|---|---|---|
| Product X — Mylanta® formulation | Mg(OH)2 (weak) | 24.6 | −38.5 | −38 to −42 (variable) |
| Product Y — NaOH solution | NaOH (strong) | 27.4 | −54.2 | −57 |
| Product Z — Ca(OH)2 suspension | Ca(OH)2 (strong) | 27.1 | −53.2 | −57 |
Q1. Evaluate the data in the table to determine which antacid is most appropriate for clinical use as an antacid, with specific reference to enthalpy of neutralisation theory. In your response you must:
- Define enthalpy of neutralisation and state the net ionic equation for strong acid + strong base.
- Explain, using a three-step mechanism, why Product X releases less heat per mole of water than Products Y and Z.
- Compare Products Y and Z: both are strong bases yet their experimental ΔHn values are both less negative than −57 kJ/mol — identify two specific sources of error that explain this discrepancy.
- Use the enthalpy data to justify a clinical recommendation, naming the specific safety advantage of Product X over NaOH solution.
- Reach an evidence-based judgement: is the student's foam-cup method adequate for ranking antacid thermal outputs, given the identified errors?
2. Source critique — evaluate a media claim (Band 5–6)
7 marks Band 5–6
“All neutralisation reactions release exactly the same amount of heat per mole of water formed — 57 kJ/mol. This means that any acid, strong or weak, combined with any base will always raise the temperature of the solution by the same amount, so the choice of antacid makes no difference to the thermal effect on the stomach. This is why all antacids are equally safe to use.”
— Paraphrased from a popular-science health blog post, 2023.
Q2. The passage above contains at least three scientific errors or unsupported claims. For each error:
- Identify the specific claim that is incorrect or unsupported.
- Explain the correct chemistry, referencing the net ionic equation, ionisation theory, or experimental evidence from this lesson.
- Describe how the correct understanding could be verified experimentally.
Your response should also address whether the conclusion (all antacids are equally safe) is valid, using enthalpy and stomach physiology arguments.
Q1 — Antacid evaluation (8 marks)
Definition (1 mark): The enthalpy of neutralisation (ΔHn) is the heat energy released per mole of water formed when an acid reacts with a base. For strong acid + strong base: H+(aq) + OH−(aq) → H2O(l), ΔHn ≈ −57 kJ/mol.
Three-step mechanism for Product X (2 marks): (1) Mg(OH)2 is a weak base: it is only sparingly soluble and provides OH− ions slowly — it is not fully dissociated in solution before the reaction. (2) When HCl is added, the H+ ions react with the available OH−, driving further dissociation of Mg(OH)2; this additional dissociation step is endothermic, consuming energy that would otherwise be released as heat. (3) The net heat released is therefore reduced: |ΔHn| = 38.5 kJ/mol < 57 kJ/mol, giving a smaller temperature rise in the solution.
Sources of error for Y and Z (2 marks): (1) Heat loss through the foam cup walls and open top to the surrounding atmosphere reduces the recorded Tmax below the true maximum, making the experimental |ΔHn| less negative than −57 kJ/mol. (2) The thermometer or probe absorbs some heat from the solution (heat capacity of the measurement device), reducing the apparent Tmax; additionally, the assumption that solution density = 1.00 g/mL slightly underestimates the true mass, introducing a small systematic error in q.
Clinical recommendation (2 marks): Product X (Mg(OH)2) is most appropriate clinically. Its |ΔHn| of 38.5 kJ/mol is significantly lower than that of NaOH (54.2 kJ/mol). This means Product X releases substantially less heat per mole of water formed when neutralising stomach acid, reducing the risk of thermal damage to the stomach lining. NaOH (Product Y) releases nearly the full 57 kJ/mol, producing a temperature spike that could cause thermal injury to gastric mucosa — it is not clinically safe as an antacid despite being chemically effective.
Judgement on method (1 mark): The foam-cup method is adequate for ranking antacid thermal outputs (Products X < Z < Y in heat output) because all systematic errors act in the same direction — all reduce |ΔHn| — so the relative ranking is preserved even if the absolute values are underestimates. However, the method is inadequate for reporting accurate absolute ΔHn values (e.g. for pharmacological specification) because systematic heat loss errors mean the foam-cup results consistently underestimate the true enthalpy. A bomb calorimeter or cooling-curve extrapolation with a calibrated probe would be required for precise values.
Q2 — Source critique (7 marks)
Error 1 (2 marks) — “All neutralisations release exactly −57 kJ/mol.”
Incorrect. ΔHn = −57 kJ/mol applies only to strong acid + strong base combinations. When either the acid or base is weak (e.g. acetic acid, Mg(OH)2, NaHCO3), the measured |ΔHn| is < 57 kJ/mol. The reason is that the net ionic equation for strong + strong (H+ + OH− → H2O) does not apply: additional energy must be absorbed during the reaction to complete the ionisation of the weak species, reducing net heat. Experimental verification: perform paired calorimetry experiments with HCl + NaOH vs CH3COOH + NaOH (identical concentrations and volumes) and compare ΔT; the weak-acid experiment will show a smaller ΔT and a less negative ΔHn.
Error 2 (2 marks) — “Temperature rise is always the same.”
Incorrect. The temperature change ΔT depends on ΔHn, the mass of solution, and the specific heat capacity: q = mcΔT, so ΔT = q/(mc). If ΔHn varies (which it does: weak-base antacids give smaller |ΔHn|), q is smaller for the same number of moles of water formed, and ΔT will be smaller. The antacid data in this lesson (Tmax = 24.6°C for Mg(OH)2 vs 27.4°C for NaOH, starting at 20.0°C) directly disproves this claim — the temperature rises are clearly different. Verification: replicate the paired calorimetry above and record ΔT for each.
Error 3 (2 marks) — “Choice of antacid makes no thermal difference / all are equally safe.”
Unsupported and incorrect. Antacids using strong bases (e.g. NaOH) release significantly more heat per mole of water formed than weak-base antacids (e.g. Mg(OH)2). In the stomach, where the reacting volume is small (~100–200 mL of gastric juice) and tissue is directly exposed to the solution, even a few degrees of excess temperature rise can damage the gastric mucosa. This is why no clinical antacid uses NaOH, while Mg(OH)2 and CaCO3 (both weak bases or salts that react via a more complex, lower-heat pathway) are standard pharmaceutical choices. The safety conclusion requires physiological and thermodynamic evidence, not just the claim that neutralisation occurs.
Overall conclusion validity (1 mark): The conclusion “all antacids are equally safe” is not valid. It conflates chemical effectiveness (all neutralise acid) with thermal safety (they release different amounts of heat). The enthalpy data in this lesson, combined with clinical evidence that strong bases cause gastric burns, demonstrates that antacid selection based on ΔHn is a real safety consideration, not a trivial one.