Chemistry • Year 12 • Module 6 • Lesson 10

Enthalpy of Neutralisation: Comparing Strong & Weak

Lock in the key vocabulary, the ΔH_n baseline, and the energy-budget ranking for all four strong/weak combinations.

Build • Vocab & Recall

1. Term–definition match

The definitions below are shuffled. Write the matching term from this list in the right-hand column: enthalpy of neutralisation (ΔH_n), 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.

Stuck? Revisit the Key Terms panel and the Formula Panel in the lesson.

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 ΔH_n 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 ΔH_n when reacted with NaOH because each acid has different molar mass and Ka. T / F

2.5 A weaker acid (smaller K_a) has a more endothermic ionisation and gives a ΔH_n that is further from −57 kJ/mol (more positive) compared to a stronger weak acid. T / F

Stuck? Revisit Cards 1–3 and the Common Error callouts in the lesson.

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 ΔH_n 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.

Stuck? The answer to each blank corresponds to a sentence from Card 2 (Molecular Mechanism) of the lesson.

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: ΔH_n (−57 kJ/mol baseline) • ionisation enthalpy • weak acid • strong acid • H⁺ + OH⁻ → H₂O • net heat released

ΔH_n baseline (−57 kJ/mol)

H⁺ + OH⁻ → H₂O

net heat released

ionisation enthalpy

weak acid

strong acid

Hints: strong acid → zero ionisation enthalpy cost → ΔH_n = baseline; weak acid → positive ionisation enthalpy → reduces net heat → ΔH_n more positive than baseline.

5. Rank the combinations

Complete the table by filling in the approximate ΔH_n 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. ΔH_n (kJ/mol)	Rank	Cause of deviation from −57 kJ/mol
HCl + NaOH
CH₃COOH + NaOH
HCl + NH₃
CH₃COOH + NH₃

Stuck? Approximate values are given in the lesson’s Card 3 comparison table. The cause always involves the ionisation enthalpy of the weak species.

Answers, Do not peek before attempting

Q1, Term–definition matches

1.1 enthalpy of neutralisation (ΔH_n) • 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 ΔH_n 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 ΔH_n (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 ΔH_n 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 acidhas zero ionisation enthalpy cost, so→ ΔH_n = baseline (−57 kJ/mol)
ΔH_n baselineis produced by→ H⁺ + OH⁻ → H₂O
H⁺ + OH⁻ → H₂Oreleases→ net heat released
weak acidcontributes a positive→ ionisation enthalpy
ionisation enthalpyis subtracted from→ net heat released
net heat releaseddetermines→ ΔH_n 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. ΔH_n (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.