In 1923, Johannes Brønsted in Copenhagen and Thomas Lowry in London published the same proton-transfer theory of acids and bases simultaneously — in different journals, in different languages, 1,200 km apart — neither aware of the other. The single molecule that forced both of them to abandon Arrhenius's model was ammonia: a base with no OH⁻ ion at all.
Four printable worksheets that build from the foundations up to exam-style questions — start at whatever level suits you.
Ammonia (NH₃) is one of the most important industrial chemicals on Earth — used to make the fertilisers that feed roughly half the global population. But here's the problem: dissolve ammonia in water and the solution is unmistakably basic. Litmus turns blue. pH climbs above 7. Every test confirms it is a base. According to one of the most famous acid-base theories in the history of chemistry, that should be impossible — because ammonia contains no hydroxide ions whatsoever.
Question 1: Before reading on, write down your best guess — what do you think a substance needs to contain or do in order to make a solution basic?
Question 2: The same problem applies to this gas-phase reaction: HCl(g) + NH₃(g) → NH₄Cl(s). No water, no ions in solution — yet chemists call this an acid-base reaction. How do you think that's possible?
Hold your answers. You will come back to test them — and almost certainly revise them — at the end of this lesson.
No calculation formulas this lesson — models and definitions are conceptual.
Core Content
Each model replaced by the one anomaly it could not explain
Every scientific model is replaced not when scientists grow bored of it, but when a single observation appears that it cannot explain — the history of acid-base theory shows this process operating three times in just over a century.
The story begins with Antoine Lavoisier in the late 1700s, who proposed that all acids contain oxygen. Given the acids he studied — sulfuric (H₂SO₄), nitric (HNO₃), phosphoric (H₃PO₄) — this was a reasonable induction. His model was overturned in 1810 when Humphry Davy demonstrated conclusively that hydrochloric acid (HCl) is strongly acidic yet contains no oxygen whatsoever. Davy's revision was straightforward: the essential ingredient is not oxygen but hydrogen — acids must contain hydrogen.
This improved model survived longer but still lacked a definition for bases, and it could not explain which hydrogen-containing compounds were acidic (CH₄ contains hydrogen but is not acidic). In 1884, Svante Arrhenius produced the first rigorous quantitative model: an acid produces H⁺ in aqueous solution, and a base produces OH⁻ in aqueous solution. This model worked beautifully for common laboratory acids and alkali metal hydroxides, and it underpinned the first quantitative treatments of neutralisation. Its limitation was structural: it was locked to aqueous solution and to OH⁻-containing bases. It could not explain ammonia (no OH⁻), could not describe gaseous acid-base reactions, and could not account for the basicity of carbonate or bicarbonate ions.
In 1923, Johannes Brønsted and Thomas Lowry independently proposed the model that resolves all of these cases: an acid is a proton donor, and a base is a proton acceptor, in any reaction in any medium.
Acid-Base Model Timeline — each model replaced by the one anomaly it could not explain
Acid-base models evolved through four stages — Lavoisier (acids contain O), Davy (acids contain H), Arrhenius (acid → H⁺(aq); base → OH⁻(aq), aqueous only) — each replaced by one anomaly it couldn't explain; the Brønsted-Lowry model (acid = H⁺ donor; base = H⁺ acceptor) works in any medium and correctly classifies NH₃ as a base.
Pause — copy the highlighted definition into your book before moving on.
Which acid-base model was overturned because it could not explain why ammonia (NH₃) is a base?
Model before formula — picture the proton moving before writing any equation
We just saw that the Brønsted-Lowry model defines acids and bases by proton donation/acceptance in any medium. That raises a question: What does a proton transfer actually look like at the molecular level? This card answers it → by tracing exactly which H⁺ moves, from which atom, to which partner, in real equations.
Before writing any equation, picture what is physically happening: a proton — a bare hydrogen nucleus — detaches from one species and bonds to another, and that single event is the entire definition of a Brønsted-Lowry acid-base reaction.
In the Brønsted-Lowry model, every acid-base reaction is a proton transfer event. The species that releases the proton is the acid; the species that receives it is the base. These roles are defined by what happens in the specific reaction, not by the identity of the substance in isolation — the same molecule can be an acid in one reaction and a base in another.
Consider HCl dissolving in water: HCl donates H⁺ to H₂O, making HCl the acid and H₂O the base. The product H₃O⁺ forms because water has accepted the proton. Since HCl ionises completely, we write a single forward arrow (→):
HCl(aq) + H₂O(l) → H₃O⁺(aq) + Cl⁻(aq)
Now consider ammonia dissolving in water: NH₃ + H₂O ⇌ NH₄⁺ + OH⁻. Here, water donates H⁺ to NH₃ — water is now the acid and NH₃ is the base. The OH⁻ does not come from NH₃; it is the water molecule's remains after it donated its proton to NH₃. This is why ammonia is a base in the Brønsted-Lowry model: it accepts a proton from water, releasing OH⁻ as a consequence of the water's proton donation — not because ammonia itself contains OH⁻. Since NH₃ only partially ionises, we write the equilibrium arrow (⇌):
NH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq)
The two reactions above also demonstrate that water is amphoteric — it can act as either an acid or a base depending on its reaction partner. This property of water is central to understanding pH, Kw, and buffer systems in later lessons.
Brønsted-Lowry proton transfer: HCl donates H⁺ and water accepts it, forming H₃O⁺ and Cl⁻.
A Brønsted-Lowry acid donates H⁺ and a base accepts H⁺ in any reaction and any medium: HCl + H₂O → H₃O⁺ + Cl⁻ (strong, single arrow); NH₃ + H₂O ⇌ NH₄⁺ + OH⁻ (weak, equilibrium arrow) — water is amphoteric, acting as base with HCl and as acid with NH₃.
Add the highlighted point to your notes before the check below.
In the reaction NH₃ + H₂O ⇌ NH₄⁺ + OH⁻, which species is the Brønsted-Lowry acid?
Differ by exactly one H⁺ · Always on opposite sides of the equation
We just saw that every Brønsted-Lowry reaction involves a proton transfer from an acid to a base. That raises a question: What happens to those species after the proton moves — do they just disappear? This card answers it → the donor becomes its conjugate base and the acceptor becomes its conjugate acid, forming pairs linked by exactly one H⁺.
Every Brønsted-Lowry acid-base reaction produces a new acid and a new base on the product side — and these conjugate species are always related to their parent by exactly one proton, nothing more.
When a Brønsted-Lowry acid donates a proton, the species it becomes is its conjugate base — it has one fewer H and one more negative charge than the original acid. When a Brønsted-Lowry base accepts a proton, the species it becomes is its conjugate acid — it has one more H and one less negative charge than the original base.
A conjugate acid-base pair consists of two species on opposite sides of the equation that differ by exactly one proton.
Consider the full equation: CH₃COOH + H₂O ⇌ H₃O⁺ + CH₃COO⁻
A crucial strength relationship flows from this pairing: a strong acid has a very weak conjugate base (if an acid donates its proton very readily, the resulting conjugate base has very little tendency to accept it back). Conversely, a weak acid has a relatively stronger conjugate base. This inverse relationship governs which direction an acid-base equilibrium favours, and it underpins buffer theory in L13.
| Reaction | Acid (left) | Conjugate base (right) | Base (left) | Conjugate acid (right) |
|---|---|---|---|---|
| HCl + H₂O → H₃O⁺ + Cl⁻ | HCl | Cl⁻ | H₂O | H₃O⁺ |
| NH₃ + H₂O ⇌ NH₄⁺ + OH⁻ | H₂O | OH⁻ | NH₃ | NH₄⁺ |
| CH₃COOH + H₂O ⇌ H₃O⁺ + CH₃COO⁻ | CH₃COOH | CH₃COO⁻ | H₂O | H₃O⁺ |
| HCO₃⁻ + H₂O ⇌ H₂CO₃ + OH⁻ | H₂O | OH⁻ | HCO₃⁻ | H₂CO₃ |
| HPO₄²⁻ + H₂O ⇌ H₂PO₄⁻ + OH⁻ | H₂O | OH⁻ | HPO₄²⁻ | H₂PO₄⁻ |
A conjugate base is formed by removing one H⁺ from the acid (one fewer H, one more negative charge); a conjugate acid by adding one H⁺ to the base — conjugate pairs always appear on opposite sides of the equation, differ by exactly one H⁺, and have inversely related strengths (strong acid → very weak conjugate base).
Pause — write the highlighted definition into your book.
In the reaction HCO₃⁻ + OH⁻ → CO₃²⁻ + H₂O, what is the conjugate base of HCO₃⁻?
The hydronium ion · Band 5–6 molecular-level explanations require H₃O⁺
We just saw that conjugate pairs differ by exactly one H⁺ — which means protons move between species. That raises a question: If H⁺ is being transferred, can a bare proton actually exist in solution? This card answers it → a free H⁺ has a lifetime of less than 10⁻¹³ s in water; it instantly bonds to H₂O to form H₃O⁺, which is why notation matters in extended responses.
Writing H⁺(aq) is a chemist's shorthand that hides a physical reality — a bare proton cannot exist in water for even a fraction of a nanosecond before bonding to a water molecule, and understanding this is essential for any molecular-level explanation.
A hydrogen ion, H⁺, is simply a proton — a hydrogen atom stripped of its single electron, leaving behind only the nucleus. A bare proton has an extraordinarily high charge density and is one of the most reactive species in chemistry. In aqueous solution, a free H⁺ ion immediately forms a coordinate covalent bond with a lone pair on a water molecule, producing the hydronium ion:
H⁺(aq) + H₂O(l) → H₃O⁺(aq)
The lifetime of a free H⁺ ion in water is estimated at less than 10⁻¹³ seconds — it effectively does not exist as an isolated species. This is why Brønsted-Lowry equations for aqueous acid reactions correctly show H₃O⁺ as the product rather than H⁺ alone.
In HSC contexts, H⁺(aq) is an accepted shorthand used extensively in calculations — it represents the same species as H₃O⁺(aq). However, in any question asking you to explain what happens at the molecular or ionic level when an acid dissolves in water, writing H₃O⁺ and explaining the proton transfer to water is required for Band 5–6 responses.
Arrhenius describes acids and bases by the ions they produce in water. Brønsted-Lowry explains acid-base behaviour by proton transfer, so it can handle NH₃ and non-aqueous reactions.
H⁺(aq) and H₃O⁺(aq) represent the same species — a bare H⁺ bonds instantly to H₂O (lifetime <10⁻¹³ s) to form the hydronium ion H₃O⁺; either notation is acceptable in pH calculations, but H₃O⁺ is required in Brønsted-Lowry equations and any molecular-level extended response.
Pause — write the highlighted principle into your book.
H⁺(aq) and H₃O⁺(aq) represent different chemical species with different properties.
Ammonia (NH₃) is the chemical that broke Arrhenius's model — and understanding why it is basic is not just a theoretical curiosity. The Haber process synthesises NH₃ from N₂ and H₂ at high temperature and pressure. This ammonia is then reacted with sulfuric acid in a Brønsted-Lowry acid-base reaction: NH₃ + H₂SO₄ → (NH₄)₂SO₄ (ammonium sulfate — a major nitrogen fertiliser). Without Brønsted-Lowry's insight that NH₃ accepts protons (rather than producing OH⁻), chemists would have struggled to explain and optimise the reactions that produce the fertilisers feeding approximately half the global population today.
The gas-phase reaction HCl(g) + NH₃(g) → NH₄Cl(s) — white solid ammonium chloride forming as a smoke when the gases mix — was the definitive experimental proof that acid-base reactions do not require water. It was observations like this that made Arrhenius's aqueous-only model untenable and drove Brønsted and Lowry to develop their proton-transfer framework.
Worked Examples
(a) Identify which species donates a proton. Compare left and right: H₂O (left) → OH⁻ (right). H₂O has lost one H⁺ to become OH⁻. Therefore H₂O is the Brønsted-Lowry acid. HPO₄²⁻ (left) → H₂PO₄⁻ (right). HPO₄²⁻ has gained one H⁺ to become H₂PO₄⁻. Therefore HPO₄²⁻ is the Brønsted-Lowry base.
(b) Identify conjugate species on the right. Conjugate acid = the base after it gains a proton = H₂PO₄⁻ (HPO₄²⁻ + H⁺). Conjugate base = the acid after it loses a proton = OH⁻ (H₂O − H⁺).
(c) State both conjugate pairs explicitly. Conjugate pair 1 — H₂O (acid, left) and OH⁻ (conjugate base, right): differ by one H⁺, on opposite sides ✓. Conjugate pair 2 — HPO₄²⁻ (base, left) and H₂PO₄⁻ (conjugate acid, right): differ by one H⁺, on opposite sides ✓.
Verification: Check charges balance. Left: −2 + 0 = −2. Right: −1 + (−1) = −2 ✓. Check atoms balance: P: 1 = 1 ✓; H: (1+2) = 3 left, (2+1) = 3 right ✓; O: (4+1) = 5 left, (4+1) = 5 right ✓.
Answer: (a) Acid = H₂O; Base = HPO₄²⁻. (b) Conjugate acid = H₂PO₄⁻; Conjugate base = OH⁻. (c) Pair 1: H₂O / OH⁻. Pair 2: HPO₄²⁻ / H₂PO₄⁻.
(a) Why Arrhenius fails: The Arrhenius model defines an acid as a substance that produces H⁺ in aqueous solution, and a base as a substance that produces OH⁻ in aqueous solution. This reaction occurs entirely in the gas phase — there is no solvent, no aqueous solution, and no ions produced in water. Furthermore, NH₃ contains no OH⁻ — it cannot satisfy the Arrhenius definition of a base under any circumstances. The Arrhenius model is therefore completely inapplicable to this reaction.
(b) Brønsted-Lowry analysis: Identify the proton transfer: HCl → Cl⁻ (HCl loses H⁺) → HCl is the acid; Cl⁻ is the conjugate base. NH₃ → NH₄⁺ (NH₃ gains H⁺) → NH₃ is the base; NH₄⁺ is the conjugate acid.
HCl(g) + NH₃(g) → NH₄⁺(aq) + Cl⁻(aq) → NH₄Cl(s)
(c) Molecular-level explanation: The nitrogen atom in NH₃ has a lone pair of electrons. When HCl and NH₃ molecules collide with sufficient energy in the gas phase, the lone pair on N forms a coordinate covalent bond with the H of HCl — the proton transfers from Cl to N. This leaves Cl with the bonding electrons as Cl⁻, and produces NH₄⁺. NH₃ acts as a Brønsted-Lowry base because it accepts the proton via its lone pair — no OH⁻ is involved at any stage.
Answer: (a) Arrhenius requires aqueous solution and OH⁻ for bases — neither applies here. (b) Acid = HCl; conjugate base = Cl⁻; base = NH₃; conjugate acid = NH₄⁺. Equation: HCl(g) + NH₃(g) → NH₄Cl(s). (c) NH₃ accepts H⁺ via its nitrogen lone pair — Brønsted-Lowry base by proton acceptance, no OH⁻ involved.
Identify what the student got partially right: For strong acids and bases in aqueous solution, both models give identical predictions. HCl in water — Arrhenius: produces H⁺; Brønsted-Lowry: donates H⁺ to water producing H₃O⁺. Both correctly identify HCl as an acid. NaOH in water — Arrhenius: produces OH⁻; Brønsted-Lowry: OH⁻ is the base (it accepts H⁺). The student is not entirely wrong for this class of substances.
Where the models diverge — Example 1 (NH₃): Arrhenius cannot classify NH₃ as a base — it contains no OH⁻ and produces none directly. The Arrhenius model would predict NH₃ is not a base, contradicting experimental evidence. Brønsted-Lowry correctly identifies NH₃ as a base: NH₃ + H₂O ⇌ NH₄⁺ + OH⁻, where NH₃ accepts H⁺ from water. The OH⁻ comes from water, not from NH₃. The two models make different predictions — Arrhenius fails here.
Where the models diverge — Example 2 (gas phase): HCl(g) + NH₃(g) → NH₄Cl(s) is a complete acid-base reaction producing a salt. Arrhenius cannot describe it at all — no aqueous solution exists. Brønsted-Lowry describes it straightforwardly: HCl donates H⁺ to NH₃ in the gas phase. The models are NOT equivalent in scope — Brønsted-Lowry is a broader framework.
Evaluate the key conceptual difference: Arrhenius defines acids and bases by what ions they produce in water — a macroscopic, product-based definition. Brønsted-Lowry defines acids and bases by what they do — proton transfer — a mechanistic, process-based definition that applies in any medium. Brønsted-Lowry also explains WHY bases make solutions basic: by accepting H⁺ from water, they leave OH⁻ behind. Both models also have their own limitations — Brønsted-Lowry cannot describe Lewis acid-base reactions (e.g. BF₃ + F⁻ → BF₄⁻) that involve no proton transfer.
Answer: The statement is an oversimplification. For aqueous strong acid-base reactions both models agree — but they diverge for NH₃ (Arrhenius fails; Brønsted-Lowry correctly identifies NH₃ as a proton acceptor) and for non-aqueous reactions (Arrhenius cannot apply; Brønsted-Lowry applies universally). The models differ in scope, in their definition of bases, and in mechanistic depth. Both retain limitations beyond which a further model (Lewis) is required. 6 marks: 1 mark for agreement in aqueous strong acids/bases; 2 marks for NH₃ divergence with equation; 2 marks for gas-phase reaction divergence; 1 mark for evaluating mechanistic difference.
The Arrhenius model defines a base as a substance that produces OH⁻ ions in aqueous solution. NH₃ contains no OH⁻ and releases none directly — it only produces OH⁻ indirectly when it accepts H⁺ from water. The Arrhenius model cannot classify any base that doesn't contain OH⁻, making it structurally incapable of explaining NH₃. The Brønsted-Lowry model resolves this by defining a base as any proton acceptor — NH₃ fits that definition perfectly.
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✏️ Activities
For each reaction below: (A) State whether Arrhenius can classify it as an acid-base reaction. (B) State whether Brønsted-Lowry can classify it. (C) Identify the Brønsted-Lowry acid and base where applicable.
| Reaction | Arrhenius can classify? | BL can classify? | BL acid / BL base |
|---|---|---|---|
| HNO₃(aq) + H₂O(l) → H₃O⁺(aq) + NO₃⁻(aq) | Your answer | Your answer | Your answer |
| NH₃(g) + HCl(g) → NH₄Cl(s) | Your answer | Your answer | Your answer |
| NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l) | Your answer | Your answer | Your answer |
| CH₃COOH(aq) + H₂O(l) ⇌ H₃O⁺(aq) + CH₃COO⁻(aq) | Your answer | Your answer | Your answer |
| BF₃ + F⁻ → BF₄⁻ | Your answer | Your answer | Your answer |
Check Your Understanding
Q1. A student claims: "NH₃ is a base because it produces OH⁻ ions when dissolved in water." Which response best evaluates this claim?
Q2. In the reaction HCO₃⁻ + OH⁻ → CO₃²⁻ + H₂O, which species is the Brønsted-Lowry acid and what is its conjugate base?
Q3. Which of the following correctly identifies a limitation of the Brønsted-Lowry model that is NOT shared by the Arrhenius model?
Q4. Which equation correctly represents the Brønsted-Lowry ionisation of hydrofluoric acid (HF, a weak acid) in water?
Q5. A student writes: "HCl is a Brønsted-Lowry acid. It ionises as HCl → H⁺ + Cl⁻." Which of the following best describes what is incomplete about this response?
Q6. (5 marks) (a) State the Arrhenius definitions of an acid and a base. (b) Explain why the Arrhenius model cannot classify ammonia (NH₃) as a base. (c) Write the Brønsted-Lowry equation that correctly explains why NH₃ produces a basic solution in water. Identify the acid and base in your equation.
Q7. (4 marks) For the reaction: H₂PO₄⁻(aq) + CO₃²⁻(aq) ⇌ HPO₄²⁻(aq) + HCO₃⁻(aq)
(a) Identify both conjugate acid-base pairs. (b) Identify the Brønsted-Lowry acid and base on the left side. (c) Verify that your identified conjugate pairs each differ by exactly one H⁺ and appear on opposite sides of the equation.
Q8. (5 marks) Real-World Application: The industrial synthesis of ammonium sulfate fertiliser [(NH₄)₂SO₄] involves reacting ammonia gas with sulfuric acid: 2NH₃(g) + H₂SO₄(aq) → (NH₄)₂SO₄(aq)
(a) Using the Brønsted-Lowry model, explain why this is classified as an acid-base reaction. Identify the proton donor and proton acceptor. (2 marks) (b) A student says this reaction cannot be described by Arrhenius. Is this correct? Explain your reasoning with reference to the specific definitions involved. (3 marks)
Row 1 (HNO₃ + H₂O): Arrhenius: Yes — HNO₃ produces H⁺ in aqueous solution. BL: Yes — HNO₃ donates H⁺ to H₂O. BL acid = HNO₃; BL base = H₂O. Arrow = → (strong acid).
Row 2 (NH₃ + HCl gas): Arrhenius: No — no aqueous solution, and NH₃ has no OH⁻. BL: Yes — HCl donates H⁺ to NH₃. BL acid = HCl; BL base = NH₃.
Row 3 (NaOH + HCl): Arrhenius: Yes — NaOH produces OH⁻; HCl produces H⁺. BL: Yes — HCl is the acid; OH⁻ is the base. Net ionic: H₃O⁺ + OH⁻ → 2H₂O.
Row 4 (CH₃COOH + H₂O): Arrhenius: Yes — CH₃COOH produces H⁺ in water. BL: Yes — CH₃COOH donates H⁺ to H₂O. BL acid = CH₃COOH; BL base = H₂O. Arrow = ⇌ (weak acid).
Row 5 (BF₃ + F⁻): Arrhenius: No — no H⁺ or OH⁻ involved. BL: No — no proton transfer; F⁻ donates an electron pair to BF₃ (Lewis acid-base, not Brønsted-Lowry). This reaction requires the Lewis model.
(a) Arrhenius acid: a substance that produces H⁺ ions in aqueous solution [1]. Arrhenius base: a substance that produces OH⁻ ions in aqueous solution [1]. (b) Arrhenius cannot classify NH₃ as a base because NH₃ contains no OH⁻ ions and does not produce OH⁻ directly — it has the formula NH₃ with no oxygen present [1]. (c) NH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq) [1]. Acid = H₂O (donates H⁺ to NH₃); Base = NH₃ (accepts H⁺ from H₂O). The OH⁻ in solution comes from the water molecule that donated its proton, not from NH₃ [1].
(a) Pair 1: H₂PO₄⁻ (acid, left) and HPO₄²⁻ (conjugate base, right) [1]. Pair 2: CO₃²⁻ (base, left) and HCO₃⁻ (conjugate acid, right) [1]. (b) BL acid on left = H₂PO₄⁻ (loses H⁺ → HPO₄²⁻); BL base on left = CO₃²⁻ (gains H⁺ → HCO₃⁻) [1]. (c) Pair 1: H₂PO₄⁻ and HPO₄²⁻ differ by one H and one negative charge = one H⁺ ✓. On opposite sides ✓. Pair 2: CO₃²⁻ / HCO₃⁻ — differ by one H⁺; on opposite sides ✓ [1].
(a) This is a Brønsted-Lowry acid-base reaction because a proton (H⁺) is transferred from H₂SO₄ to NH₃ [1]. Proton donor (acid) = H₂SO₄; proton acceptor (base) = NH₃ [1]. (b) Partially correct [1]. Arrhenius defines an acid as producing H⁺ in aqueous solution — H₂SO₄ satisfies this [1]. However, Arrhenius defines a base as producing OH⁻ in aqueous solution. NH₃ contains no OH⁻ and cannot be classified as an Arrhenius base. So the Arrhenius model CAN classify H₂SO₄ as an acid but CANNOT classify NH₃ as a base, making the Arrhenius framework incomplete (not totally inapplicable) for this reaction [1].
Go back to your Think First response at the top of this lesson. In 1923, Brønsted and Lowry each solved the ammonia problem independently — NH₃ accepts a proton (H⁺) from an acid, acting as a base without ever producing OH⁻. Now that you've studied the Brønsted-Lowry model:
Review
What is the Brønsted-Lowry definition of an acid?
Why can't the Arrhenius model classify NH₃ as a base?
What are conjugate acid-base pairs and how do they differ?
When must you write H₃O⁺ rather than H⁺ in a chemistry response?
What is the fatal limitation of the Arrhenius model?
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