Chemistry • Year 12 • Module 6 • Lesson 3
Enthalpy of Neutralisation: Practical & Theory
Lock in the key formulas, vocabulary, and relationships for calorimetry and molar enthalpy of neutralisation before tackling harder questions.
1. Term–definition match
The twelve definitions below are shuffled. In the right-hand column write the matching term from this list:
enthalpy of neutralisation, calorimetry, specific heat capacity, molar enthalpy, net ionic equation, limiting reagent, heat released (q), temperature change (ΔT), exothermic, strong acid, weak acid, spectator ions.
12 marks
| # | Definition (shuffled) | Matching term |
|---|---|---|
| 1.1 | The heat energy released per mole of water formed in a neutralisation reaction. | |
| 1.2 | An experimental technique that measures heat released or absorbed by a reaction, using q = mcΔT. | |
| 1.3 | The energy required to raise 1 g of a substance by 1°C; approximately 4.18 J g−1 °C−1 for dilute aqueous solutions. | |
| 1.4 | Tfinal − Tinitial; positive for an exothermic reaction. | |
| 1.5 | The calculated heat transferred (in joules) to or from the solution during a reaction. | |
| 1.6 | An acid that is completely ionised in aqueous solution; produces the maximum H+ ions per mole. | |
| 1.7 | An acid that is only partially ionised in aqueous solution; Ka is much less than 1. | |
| 1.8 | The reactant that is completely consumed first; the number of moles of water formed equals its moles. | |
| 1.9 | A reaction that releases heat to the surroundings; ΔH is negative. | |
| 1.10 | The heat energy measured per mole of a specified substance; expressed in kJ/mol. | |
| 1.11 | The ionic equation that shows only the species that actually react; for strong acid + strong base: H+(aq) + OH−(aq) → H2O(l). | |
| 1.12 | Ions that are present in solution but do not participate in the reaction; they cancel from the full ionic equation. |
2. True or false — with correction
For each statement, circle T or F. If false, write the corrected version on the line. 10 marks (1 for T/F, 1 for each correction)
2.1 ΔHn ≈ −57 kJ/mol applies to all neutralisation reactions, regardless of whether the acid and base are strong or weak. T / F
2.2 When calculating q = mcΔT for a neutralisation experiment, only the mass of the acid solution should be used. T / F
2.3 For a strong acid + strong base reaction, ΔHn is always negative (exothermic). T / F
2.4 ΔT = Tinitial − Tfinal is the correct formula for calculating the temperature change in a neutralisation experiment. T / F
2.5 All systematic errors in a foam-cup calorimetry experiment make the experimental |ΔHn| smaller than the theoretical value. T / F
3. Cloze paragraph — fill the blanks
Complete the paragraph below using the word bank. Each term is used once. 10 marks
Neutralisation reactions are always because forming an O–H bond in liquid releases more energy than is consumed. For a strong acid reacting with a strong base, the molar enthalpy of neutralisation (ΔHn) is approximately kJ/mol, and ΔHn is always (by convention for exothermic reactions). This value is constant for all strong + strong combinations because the (Na+, Cl−, etc.) play no part in the energy change. When a weak acid is involved, the measured |ΔHn| is than 57 kJ/mol, because energy must first be absorbed to complete the of the weak acid. In the formula q = mcΔT, the value c is the of the solution, and m must be the of acid plus base combined. The number of moles of water formed equals the moles of the .
4. Sequence the calculation steps
The seven steps below for calculating ΔHn from experimental data are shuffled. Write the correct order (1–7) in the “Order” column. 7 marks
| Order | Step |
|---|---|
| Calculate n(H2O) formed = moles of the limiting reagent using n = c × V for each reactant. | |
| Convert q from joules to kilojoules by dividing by 1000. | |
| Record the initial temperature Tinitial of both solutions and the maximum temperature Tfinal after mixing. | |
| Calculate q = mcΔT (in joules), using total mass m = (Vacid + Vbase) × 1.00 g/mL. | |
| Measure and record the volumes of acid and base solutions in mL. | |
| Calculate ΔHn = −q(kJ) / n(H2O). | |
| Calculate ΔT = Tfinal − Tinitial. |
5. Build a concept map
Draw labelled arrows between the six terms below to show how they connect. Each arrow must carry a linking phrase. Aim for at least 6 labelled arrows. 6 marks
Supplied terms: heat released (q) • temperature change (ΔT) • moles of water formed • ΔHn (kJ/mol) • strong acid + strong base • weak acid + strong base
6. Function recall
Answer each in 1–2 sentences using precise lesson terms. 8 marks (2 each)
6.1 Why must the total mass of acid plus base be used in q = mcΔT, rather than just the mass of the acid?
6.2 Why does replacing HCl with acetic acid (CH3COOH, a weak acid) in a neutralisation experiment produce a smaller temperature rise?
6.3 What is the function of a polystyrene (foam) cup in a school calorimetry experiment?
6.4 Why is the value of ΔHn for strong acid + strong base constant regardless of which specific acid or base is used?
Q1 — Term–definition match
1.1 enthalpy of neutralisation • 1.2 calorimetry • 1.3 specific heat capacity • 1.4 temperature change (ΔT) • 1.5 heat released (q) • 1.6 strong acid • 1.7 weak acid • 1.8 limiting reagent • 1.9 exothermic • 1.10 molar enthalpy • 1.11 net ionic equation • 1.12 spectator ions.
Q2 — True / false with correction
2.1 False. ΔHn ≈ −57 kJ/mol applies only to strong acid + strong base. Any combination involving a weak acid or weak base gives |ΔHn| < 57 kJ/mol.
2.2 False. The total mass of the combined solution (acid + base) must be used. All heat is absorbed by the entire combined solution, not just the acid component.
2.3 True.
2.4 False. Correct formula: ΔT = Tfinal − Tinitial. For an exothermic reaction, Tfinal > Tinitial, giving a positive ΔT.
2.5 True. Heat loss, thermometer heat capacity, incomplete mixing, and density/SHC approximations all reduce the measured ΔT and therefore make |ΔHn| smaller (less negative) than the true value.
Q3 — Cloze paragraph
exothermic • water • −57 • negative • spectator ions • less • ionisation • specific heat capacity • total mass • limiting reagent.
Q4 — Sequence the steps
Correct order: (1) Measure and record volumes. (2) Record Tinitial and Tfinal. (3) Calculate ΔT = Tfinal − Tinitial. (4) Calculate q = mcΔT using total mass. (5) Calculate n(H2O) = moles of limiting reagent. (6) Convert q to kJ. (7) Calculate ΔHn = −q(kJ)/n(H2O).
Q5 — Concept map (sample)
Valid arrows include: ΔT is used to calculate → heat released (q); heat released (q) divided by moles of water formed gives → ΔHn; strong acid + strong base gives → ΔHn = −57 kJ/mol; weak acid + strong base gives a less negative → ΔHn; moles of water formed depends on limiting reagent; temperature change (ΔT) is measured to find → heat released. Award 1 mark per correctly labelled arrow, maximum 6.
Q6 — Function recall
6.1 All heat released by the neutralisation reaction is absorbed by the combined acid + base solution. Using only the acid mass understates the actual mass that absorbs the heat, giving an incorrectly high q value and a more-negative ΔHn than warranted.
6.2 Acetic acid (CH3COOH) is only partially ionised (~1–5%) in solution before the reaction. During neutralisation, additional energy must be absorbed to complete its ionisation; this energy is drawn from the heat that H+ + OH− → H2O would otherwise release, reducing the net temperature rise.
6.3 The foam cup acts as an insulating vessel, slowing the rate of heat loss from the reacting solution to the surrounding atmosphere. This reduces systematic error in the measured ΔT, giving a more accurate value of q and therefore of ΔHn.
6.4 All strong acids are fully ionised in solution before the reaction, contributing H+(aq); all strong bases are fully dissociated, contributing OH−(aq). The only reaction at the ionic level is always H+(aq) + OH−(aq) → H2O(l). Because the net ionic equation is identical for all strong + strong combinations, the energy change per mole of water formed is always the same.