Chemistry • Year 12 • Module 6 • Lesson 1

Acid-Base Models: Arrhenius to Brønsted-Lowry

Apply both models to real equations and data; interpret a graph; critique a student diagram; compare models on multiple criteria.

Apply · Band 4–5

1. Interpret acid-base classification data

A student tests five substances and records the results below. Use both the Arrhenius and Brønsted-Lowry models to analyse the data. 7 marks

Substance	Contains OH⁻?	Aqueous solution pH	Reaction medium available	Produces H₃O⁺ in water?
HCl	No	1.0	Aqueous and gas-phase	Yes
NH₃	No	11.1	Aqueous and gas-phase	No
NaOH	Yes	13.0	Aqueous only	No
CH₃COOH	No	3.2	Aqueous	Yes (partially)
HCO₃⁻	No	8.3 (0.1 mol L⁻¹)	Aqueous	Yes (partially) and accepts H⁺ from strong acids

1.1 Identify which substance(s) in the table cannot be classified as a base by the Arrhenius model. Justify your answer. 2 marks

1.2 For NH₃, write the Brønsted-Lowry equation showing its reaction with water. Identify the acid, base, conjugate acid, and conjugate base. 3 marks

1.3 Explain why HCO₃⁻ is described as amphiprotic, using evidence from the data table. 2 marks

Stuck? Cards 1–3. Remember: Arrhenius needs OH⁻; Brønsted-Lowry needs proton transfer only.

2. Interpret graph — acid ionisation in Australian wine

Tartaric acid (C₄H₆O₆) is the primary acid in Australian wine and is responsible for most of the wine’s acidity. The graph below shows the percentage of tartaric acid that ionises (loses one H⁺) at various pH values in a model wine solution at 20°C. 9 marks

Figure 2.1 — Percentage of tartaric acid ionised versus pH in model wine solution at 20°C. Adapted from Dartiguenave et al. (2010), Food Chemistry. pK₃₁ = 3.04.

2.1 Describe the relationship between pH and the percentage of tartaric acid that ionises. Quote at least one figure from the graph. 2 marks

2.2 At the typical wine pH of 3.2, estimate the percentage of tartaric acid that has donated a proton. State whether the ionisation is best represented by a single forward arrow (→) or an equilibrium arrow (⇌) and justify your choice. 3 marks

2.3 Write the Brønsted-Lowry equation for tartaric acid (abbreviated TA) donating one proton to water. Identify the conjugate base of TA. 2 marks

2.4 At very high pH (above 5), what does the graph suggest is happening to almost all of the tartaric acid? Explain this in terms of the Brønsted-Lowry model and Le Chatelier’s principle. 2 marks

Stuck? Card 2 for proton transfer logic; Card 4 for when to use ⇌ vs →; the equilibrium arrow indicates partial ionisation.

3. Spot the errors in the student diagram

The diagram below shows a student’s attempt to depict the Brønsted-Lowry proton transfer for NH₃ + H₂O ⇌ NH₄⁺ + OH⁻. The diagram contains three deliberate errors. Identify each error and write the correct version. 6 marks (2 per error)

Diagram pending — see image-prompts.md #chem-y12-m6-l01-w02-fig1 The diagram shows: (1) NH₃ labelled “Arrhenius base — produces OH⁻” without showing proton transfer from water; (2) H₂O labelled only as “solvent”, not as an acid/proton donor; (3) the product NH₄⁺ and OH⁻ have their conjugate labels reversed (NH₄⁺ called “conjugate base” and OH⁻ called “conjugate acid”); (4) the reaction arrow is → not ⇌. Three errors only are intentional: the Arrhenius label on NH₃, the reversed conjugate labels, and the single-headed arrow.

Error 1 — identify:

Error 1 — correct version:

Error 2 — identify:

Error 2 — correct version:

Error 3 — identify:

Error 3 — correct version:

Stuck? Cards 2 and 3 contain the correct labelling logic for this reaction. The misconceptions box also addresses the “NH₃ produces OH⁻” error directly.

4. Compare Arrhenius and Brønsted-Lowry across five criteria

Complete the comparison table. Write a phrase or sentence in each empty cell. Where both models agree, write “same” and explain. 10 marks

Feature / criterion	Arrhenius model	Brønsted-Lowry model
Definition of an acid
Definition of a base
Can classify NH₃ as a base?
Can describe HCl(g) + NH₃(g) → NH₄Cl(s)?
Useful for pH calculations of strong acids/bases in water?

Stuck? Card 1 has the four-model comparison cards. Worked Example 2 shows how to evaluate model limitations.

5. Predict and justify

Incitec Pivot, one of Australia’s largest fertiliser manufacturers, converts ammonia into ammonium sulfate [(NH₄)₂SO₄] by reacting liquid NH₃ with sulfuric acid (H₂SO₄). A year 11 student sees the equation below and says: “This reaction can’t be an acid-base reaction because we haven’t added water.” 4 marks

2 NH₃(g) + H₂SO₄(l) → (NH₄)₂SO₄(s)

Predict whether the student’s claim is correct or incorrect, and justify your prediction using the Brønsted-Lowry model. Identify the acid, base, and the proton transfer that occurs.

Stuck? The Real-World Anchor section and Worked Example 2 in the lesson address exactly this scenario.

Answers — Do not peek before attempting

Q1.1 — Which substances cannot be classified as a base by Arrhenius?

NH₃ and HCO₃⁻. The Arrhenius model defines a base as a substance that produces OH⁻ in aqueous solution. NH₃ contains no OH⁻ (1 mark), and HCO₃⁻ also contains no OH⁻; despite both solutions being basic (pH above 7), neither satisfies the Arrhenius definition of a base (1 mark).

Q1.2 — Brønsted-Lowry equation for NH₃ + H₂O

NH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq)

Acid: H₂O (donates H⁺ to NH₃). Base: NH₃ (accepts H⁺ from water). Conjugate acid: NH₄⁺ (NH₃ after gaining H⁺). Conjugate base: OH⁻ (H₂O after losing H⁺).

Marking: 1 mark for correct balanced equation with equilibrium arrows; 1 mark for correctly labelling acid and base; 1 mark for correctly labelling conjugate acid and conjugate base.

Q1.3 — HCO₃⁻ as amphiprotic

The data table shows HCO₃⁻ both “partially produces H₃O⁺ in water” (it can donate H⁺, acting as an acid) AND “accepts H⁺ from strong acids” (acting as a base) (1 mark). A substance that can act as both a proton donor and a proton acceptor depending on its reaction partner is amphiprotic (1 mark).

Q2.1 — Describe the graph trend

As pH increases, the percentage of tartaric acid that ionises increases in a sigmoidal (S-shaped) pattern. At pH 3.0, approximately 18% of tartaric acid is ionised; by pH 4.5, approximately 93% is ionised (1 mark for trend; 1 mark for at least one supporting figure).

Q2.2 — Estimate at pH 3.2; arrow type

At pH 3.2, approximately 30% of tartaric acid has ionised (donated one H⁺), so approximately 70% remains un-ionised (1 mark for reasonable estimate, 20–35% accepted). The equilibrium arrow (⇌) is correct (1 mark) because tartaric acid does not ionise completely — a mixture of ionised and un-ionised forms co-exists, which is the definition of a partial / weak acid equilibrium (1 mark).

Q2.3 — Brønsted-Lowry equation for tartaric acid

TA(aq) + H₂O(l) ⇌ TA⁻(aq) + H₃O⁺(aq) (where TA = tartaric acid, TA⁻ = hydrogen tartrate ion) (1 mark for correct equation with ⇌). The conjugate base of TA is TA⁻ (hydrogen tartrate ion) — it differs from TA by exactly one H⁺ (1 mark).

Q2.4 — High pH interpretation

At pH above 5, the graph shows almost all (>98%) of the tartaric acid has ionised — effectively all has donated one H⁺ (1 mark). Under the Brønsted-Lowry model, the high [OH⁻] at high pH removes H₃O⁺ from the product side; by Le Chatelier’s principle this shifts the equilibrium to the right (towards products), driving almost complete ionisation (1 mark).

Q3 — Three diagram errors

Error 1 — Arrhenius label on NH₃: NH₃ is incorrectly labelled as “Arrhenius base — produces OH⁻”. Correction: NH₃ is a Brønsted-Lowry base — it accepts H⁺ from water. NH₃ does not produce OH⁻ from its own structure; the OH⁻ comes from water after it donates H⁺ to NH₃.

Error 2 — Reversed conjugate labels: NH₄⁺ is incorrectly labelled “conjugate base” and OH⁻ is incorrectly labelled “conjugate acid”. Correction: NH₄⁺ is the conjugate acid of NH₃ (NH₃ + H⁺); OH⁻ is the conjugate base of H₂O (H₂O − H⁺).

Error 3 — Single arrow instead of equilibrium arrow: The diagram uses → instead of ⇌. Correction: NH₃ is a weak base that only partially reacts with water, so the reaction must be represented with an equilibrium arrow (⇌).

Marking: 1 mark per error correctly identified; 1 mark per correct correction. Max 6 marks.

Q4 — Compare-and-contrast table

Feature	Arrhenius	Brønsted-Lowry
Acid definition	Produces H⁺ in aqueous solution	Proton (H⁺) donor in any reaction
Base definition	Produces OH⁻ in aqueous solution	Proton (H⁺) acceptor in any reaction
Classify NH₃	No — NH₃ has no OH⁻	Yes — NH₃ accepts H⁺ from H₂O
HCl(g) + NH₃(g) → NH₄Cl(s)	No — no aqueous medium; cannot classify either reactant	Yes — HCl donates H⁺ to NH₃ in the gas phase
pH calculations for strong acids/bases	Yes — still valid and used	Same — also valid (both predict HCl produces H⁺ in water)

Marking: 2 marks per row (1 per cell). Where a student writes “same” for row 5 and explains it, accept with explanation.

Q5 — Predict and justify (NH₃ + H₂SO₄)

The student’s claim is incorrect (1 mark). Under the Brønsted-Lowry model, an acid-base reaction requires only a proton transfer — no aqueous medium or water is required (1 mark). In this reaction, H₂SO₄ is the Brønsted-Lowry acid (it donates two H⁺ ions) and NH₃ is the Brønsted-Lowry base (it accepts H⁺ via its nitrogen lone pair, forming NH₄⁺) (1 mark for each acid/base identification, max 2 marks). The gas-phase reaction HCl(g) + NH₃(g) → NH₄Cl(s) demonstrates the same principle: proton transfer without water is still a valid Brønsted-Lowry acid-base reaction. Arrhenius cannot describe this reaction because no aqueous medium is involved (1 additional mark for contrast with Arrhenius).

Marking: 4 marks total — 1 for identifying the claim as incorrect; 1 for invoking B-L model (no water needed); 1 for correctly identifying acid and base (accept combined); 1 for equation or named proton transfer mechanism.