Chemistry • Year 12 • Module 6 • Lesson 10
Enthalpy of Neutralisation: Comparing Strong & Weak
Lock in the key vocabulary, the ΔHn baseline, and the energy-budget ranking for all four strong/weak combinations.
1. Term–definition match
The definitions below are shuffled. Write the matching term from this list in the right-hand column: enthalpy of neutralisation (ΔHn), net ionic equation, ionisation enthalpy, strong acid, weak acid, spectator ions, Hess’s Law, calorimetry, endothermic, exothermic. 10 marks
| # | Definition (shuffled) | Matching term |
|---|---|---|
| 1.1 | The energy change per mole of water formed when an acid and a base react to completion under standard conditions. | |
| 1.2 | An ionic equation that excludes spectator ions, showing only the reacting species: H+(aq) + OH−(aq) → H2O(l). | |
| 1.3 | The energy absorbed when a weak acid or base ionises; this is an endothermic process that reduces net heat released during neutralisation. | |
| 1.4 | An acid that completely ionises in dilute aqueous solution; for example, HCl → H+ + Cl−. | |
| 1.5 | An acid that partially ionises in dilute aqueous solution; for example, CH3COOH ⇌ H+ + CH3COO−. | |
| 1.6 | Ions present in solution that do not participate in the reaction and cancel from both sides of the full ionic equation. | |
| 1.7 | The principle that total enthalpy change is independent of the reaction pathway taken; allows multi-step energy budgets to be summed. | |
| 1.8 | The experimental technique of measuring heat changes in a solution using q = mcΔT. | |
| 1.9 | A reaction or process that releases heat to the surroundings; ΔH is negative. | |
| 1.10 | A reaction or process that absorbs heat from the surroundings; ΔH is positive. |
2. True or false — with correction
Circle T or F. If false, write the corrected version on the line below. 10 marks (1 T/F, 1 correction each)
2.1 The enthalpy of neutralisation for strong acid + strong base is approximately −57 kJ/mol and represents the energy released solely from the reaction H+(aq) + OH−(aq) → H2O(l). T / F
2.2 When a weak acid is neutralised by a strong base, the measured ΔHn is more negative (more exothermic) than −57 kJ/mol because extra energy is released during ionisation. T / F
2.3 The ionisation of a weak acid during neutralisation is an endothermic process that reduces the net heat measured in the calorimeter. T / F
2.4 All strong acids give different values of ΔHn when reacted with NaOH because each acid has different molar mass and Ka. T / F
2.5 A weaker acid (smaller Ka) has a more endothermic ionisation and gives a ΔHn that is further from −57 kJ/mol (more positive) compared to a stronger weak acid. T / F
3. Fill in the blanks — the energy budget for weak acid neutralisation
Use the word bank to complete the paragraph. Each word is used once only. 8 marks
Word bank: endothermic • −57 • intact • ionisation • bond-breaking • Hess’s Law • more positive • weaker
The standard enthalpy of neutralisation for strong acid + strong base is approximately kJ/mol. This value is also called the “baseline” because strong acids and bases are already fully ionised before mixing — no energy is needed to produce H+ or OH−. When a weak acid such as CH3COOH meets NaOH, most acid molecules are before mixing. During the reaction, OH− drives the equilibrium right, forcing the weak acid to . This step is because it is a process — the O–H bond in the carboxyl group must break to release H+. By , the net heat measured = (heat from H+ + OH− → H2O) − (heat absorbed in ionisation). Therefore the measured ΔHn for weak acid + strong base is than the baseline. A acid gives a larger positive deviation from −57 kJ/mol because its O–H bond is harder to break.
4. Concept map — connect the key ideas
Draw labelled arrows between the six terms below to show how they relate. Each arrow must carry a linking phrase. Aim for at least 6 labelled arrows. 6 marks
Supplied terms: ΔHn (−57 kJ/mol baseline) • ionisation enthalpy • weak acid • strong acid • H+ + OH− → H2O • net heat released
5. Rank the combinations
Complete the table by filling in the approximate ΔHn value and the ranking (1 = most exothermic, 4 = least exothermic). Explain the cause of the deviation from −57 kJ/mol for each combination. 8 marks
| Combination | Approx. ΔHn (kJ/mol) | Rank | Cause of deviation from −57 kJ/mol |
|---|---|---|---|
| HCl + NaOH | |||
| CH3COOH + NaOH | |||
| HCl + NH3 | |||
| CH3COOH + NH3 |
Q1 — Term–definition matches
1.1 enthalpy of neutralisation (ΔHn) • 1.2 net ionic equation • 1.3 ionisation enthalpy • 1.4 strong acid • 1.5 weak acid • 1.6 spectator ions • 1.7 Hess’s Law • 1.8 calorimetry • 1.9 exothermic • 1.10 endothermic
Q2 — True / False with corrections
2.1 True. The baseline −57 kJ/mol arises entirely from H+ + OH− → H2O; spectator ions contribute nothing.
2.2 False. Correction: when a weak acid is neutralised by a strong base, the measured ΔHn is more positive (less exothermic) than −57 kJ/mol, because the endothermic ionisation of the weak acid consumes part of the exothermic H+ + OH− → H2O energy.
2.3 True. The ionisation step (bond-breaking) is endothermic and reduces the temperature rise measured.
2.4 False. Correction: all strong acids give the same ΔHn (approximately −57 kJ/mol) when reacted with a strong base, because the net ionic equation H+ + OH− → H2O is the same regardless of which spectator ions are present.
2.5 True. A weaker acid has a stronger, harder-to-break O–H bond — larger ionisation enthalpy — giving a ΔHn further from the baseline.
Q3 — Cloze paragraph
In order: −57 • intact • ionisation • endothermic • bond-breaking • Hess’s Law • more positive • weaker
Q4 — Concept map (sample links)
- strong acid —has zero ionisation enthalpy cost, so→ ΔHn = baseline (−57 kJ/mol)
- ΔHn baseline —is produced by→ H+ + OH− → H2O
- H+ + OH− → H2O —releases→ net heat released
- weak acid —contributes a positive→ ionisation enthalpy
- ionisation enthalpy —is subtracted from→ net heat released
- net heat released —determines→ ΔHn baseline (−57 kJ/mol) (or the deviation from it)
Award 1 mark per correctly directed, labelled arrow. Any biologically valid linking phrase is acceptable.
Q5 — Ranking table
| Combination | Approx. ΔHn (kJ/mol) | Rank | Cause of deviation |
|---|---|---|---|
| HCl + NaOH | −57.0 | 1st | No deviation — both fully ionised; net ionic equation only. |
| CH3COOH + NaOH | ≈ −55.4 | 2nd | Ionisation enthalpy of CH3COOH (+1.6 kJ/mol) consumed during reaction. |
| HCl + NH3 | ≈ −52.2 | 3rd | Proton-acceptance energy of NH3 (+4.8 kJ/mol) consumed during reaction. |
| CH3COOH + NH3 | ≈ −49.5 | 4th | Both ionisation enthalpy of CH3COOH and proton-acceptance energy of NH3 add to the positive deviation. |