Chemistry • Year 12 • Module 6 • Lesson 1
Acid-Base Models: Arrhenius to Brønsted-Lowry
Lock in the vocabulary, the historical sequence of models, and the structural logic of Brønsted-Lowry proton transfer before moving to application.
1. Label the acid-base model comparison diagram
The diagram below compares the Arrhenius and Brønsted-Lowry models across five criteria. Write the missing content into boxes A–J. Use the lesson Key Terms panel and Cards 1–3 as your source. 10 marks
| Box | Criterion | Your answer |
|---|---|---|
| A | Arrhenius: acid definition | |
| B | Arrhenius: base definition | |
| C | Arrhenius: medium required | |
| D | Arrhenius: can it classify NH₃ as a base? | |
| E | Arrhenius: main limitation | |
| F | Brønsted-Lowry: acid definition | |
| G | Brønsted-Lowry: base definition | |
| H | Brønsted-Lowry: medium required | |
| I | Brønsted-Lowry: can it classify NH₃ as a base? | |
| J | Brønsted-Lowry: main limitation |
2. Term–definition match
The ten definitions below are shuffled. In the right-hand column write the matching term from this list: Arrhenius acid, Arrhenius base, Brønsted-Lowry acid, Brønsted-Lowry base, conjugate acid, conjugate base, conjugate acid-base pair, amphiprotic substance, hydronium ion, proton transfer. 10 marks
| # | Definition (shuffled) | Matching term |
|---|---|---|
| 2.1 | A substance that donates a proton (H⁺) in any reaction, in any medium. | |
| 2.2 | A substance that produces OH⁻ ions when dissolved in water. | |
| 2.3 | The species formed when a Brønsted-Lowry base gains one proton; it differs from the base by exactly one H⁺. | |
| 2.4 | A substance that can act as both a proton donor and a proton acceptor, depending on its reaction partner. | |
| 2.5 | The H₃O⁺ ion formed when H⁺ bonds to a water molecule; the correct species in Brønsted-Lowry aqueous equations. | |
| 2.6 | A substance that accepts a proton (H⁺) in any reaction, in any medium. | |
| 2.7 | The movement of a proton from one species to another; the fundamental event in every Brønsted-Lowry reaction. | |
| 2.8 | Two species that differ by exactly one H⁺ and appear on opposite sides of a Brønsted-Lowry equation. | |
| 2.9 | A substance that produces H⁺ ions when dissolved in water. | |
| 2.10 | The species formed when a Brønsted-Lowry acid loses one proton; it differs from the acid by exactly one H⁺. |
3. True or false — with correction
For each statement, circle T or F. If the statement is false, write the corrected version on the line below. 8 marks (1 T/F, 1 correction where false)
3.1 The Arrhenius model correctly predicts that ammonia (NH₃) is a base because it produces OH⁻ directly from its own structure when dissolved in water. T / F
3.2 In the Brønsted-Lowry model, the same substance can act as an acid in one reaction and as a base in a different reaction. T / F
3.3 A conjugate acid-base pair consists of two species that are on the same side of a Brønsted-Lowry equation and differ by one proton. T / F
3.4 The Arrhenius model was proved incorrect and should not be used for any acid-base situation today. T / F
4. Function recall
Answer each question in 1–2 sentences using precise terms from the lesson. 8 marks (2 each)
4.1 What is the role of water when HCl dissolves in it, according to the Brønsted-Lowry model?
4.2 What is the function of the lone pair on nitrogen in NH₃ during an acid-base reaction with water?
4.3 Why do chemists write H₃O⁺(aq) rather than H⁺(aq) alone in Brønsted-Lowry equations?
4.4 What single observation about HCl made Lavoisier’s oxygen-based model of acids untenable?
5. Fill in the blanks — proton transfer paragraph
Complete the paragraph by selecting the correct term from the word bank. Each term is used once. 8 marks
Word bank: proton donor • proton acceptor • conjugate base • conjugate acid • equilibrium • hydronium • amphiprotic • Brønsted-Lowry
In the model, an acid is defined as a and a base is defined as a . When NH₃ reacts with water, NH₃ accepts H⁺ from water, forming the ion NH₄⁺. The OH⁻ ion left behind is the of water. Because NH₃ does not fully react, we use an arrow (⇌). The NH₄⁺ formed is the of NH₃. Water is described as because it can act as either an acid or a base depending on its reaction partner.
6. Build a concept map
Draw labelled arrows between the six terms below to show how they connect. Each arrow must carry a linking phrase (e.g. “donates”, “becomes after losing H⁺”, “involves both”). Aim for at least 6 labelled arrows. 6 marks
Supplied terms: Brønsted-Lowry acid · Brønsted-Lowry base · conjugate acid · conjugate base · proton (H⁺) · proton transfer reaction.
Q1 — Model comparison diagram labels
A: A substance that produces H⁺ ions when dissolved in water. B: A substance that produces OH⁻ ions when dissolved in water. C: Aqueous solution only. D: No — NH₃ contains no OH⁻, so it cannot satisfy the Arrhenius definition of a base. E: Limited to aqueous systems; cannot classify bases without OH⁻ (e.g. NH₃) or non-aqueous acid-base reactions.
F: A proton (H⁺) donor in any reaction. G: A proton (H⁺) acceptor in any reaction. H: Any medium — aqueous or non-aqueous. I: Yes — NH₃ accepts H⁺ from water: NH₃ + H₂O ⇌ NH₄⁺ + OH⁻. J: Cannot account for Lewis acid-base reactions that involve no proton transfer (e.g. BF₃ + F⁻ → BF₄⁻).
Q2 — Term–definition matches
2.1 Brønsted-Lowry acid • 2.2 Arrhenius base • 2.3 conjugate acid • 2.4 amphiprotic substance • 2.5 hydronium ion • 2.6 Brønsted-Lowry base • 2.7 proton transfer • 2.8 conjugate acid-base pair • 2.9 Arrhenius acid • 2.10 conjugate base.
Q3 — True / false with correction
3.1 False. The Arrhenius model cannot classify NH₃ as a base at all — NH₃ contains no OH⁻. The OH⁻ in NH₃ solution comes from water donating H⁺ to NH₃, not from NH₃ itself. Arrhenius would incorrectly predict that NH₃ is not a base.
3.2 True. Water is the classic example: it acts as a base (accepts H⁺) with HCl, and as an acid (donates H⁺) with NH₃. This property is called being amphiprotic.
3.3 False. A conjugate acid-base pair always consists of two species on opposite sides of the equation (one left, one right) that differ by exactly one H⁺. Same-side species are never a conjugate pair.
3.4 False. The Arrhenius model was not proved incorrect — it correctly describes strong acids and strong bases in aqueous solution and is still used in pH calculations. The correct language is that it is “limited to aqueous systems” or “cannot account for bases without OH⁻.”
Q4 answers
4.1 Water acts as the Brønsted-Lowry base (proton acceptor): it accepts H⁺ from HCl, forming the hydronium ion H₃O⁺ (water’s conjugate acid). HCl is the acid (proton donor) and Cl⁻ is HCl’s conjugate base.
4.2 The nitrogen lone pair forms a coordinate covalent bond with the donated proton H⁺ from water. This is the mechanism by which NH₃ accepts H⁺, making it a Brønsted-Lowry base without needing to contain any OH⁻.
4.3 A bare H⁺ (proton) cannot exist in water for more than ~10⁻¹³ s — it immediately bonds to a water molecule’s lone pair to form H₃O⁺. Writing H₃O⁺ also makes water’s role as a Brønsted-Lowry base explicit, required for Band 5–6 responses.
4.4 HCl is a strong acid that contains no oxygen. Lavoisier’s model defined all acids as containing oxygen — HCl falsified this. Humphry Davy demonstrated this in 1810.
Q5 — Cloze answers (in order of blanks)
Brønsted-Lowry → proton donor → proton acceptor → conjugate acid (NH₄⁺) → conjugate base (OH⁻) → equilibrium → conjugate acid (of NH₃) → amphiprotic.
Q6 — Sample concept map arrows
- Brønsted-Lowry acid → donates → proton (H⁺)
- proton (H⁺) → accepted by → Brønsted-Lowry base
- Brønsted-Lowry acid → after donation becomes → conjugate base
- Brønsted-Lowry base → after acceptance becomes → conjugate acid
- conjugate acid + conjugate base → form a pair in → proton transfer reaction
- proton transfer reaction → involves → Brønsted-Lowry acid + Brønsted-Lowry base
Award 1 mark per correctly labelled arrow with valid linking phrase. Biologically sound alternatives accepted.