Acid-Base Models: Arrhenius to Brønsted-Lowry

Q1 — Evaluate student claim (8 marks)

Sample response.

Point 1 — Arrhenius definitions and agreement (Reaction 1): Arrhenius (1884) defined an acid as a substance that produces H⁺ in aqueous solution and a base as a substance that produces OH⁻ in aqueous solution. For Reaction 1 (HCl + NaOH → NaCl + H₂O in aqueous solution), both models agree: HCl produces H⁺ (Arrhenius acid; Brønsted-Lowry acid by donating H⁺ to water) and NaOH provides OH⁻ (Arrhenius base; OH⁻ is the Brønsted-Lowry base accepting H⁺ from H₃O⁺). The student’s claim holds for this case.

Point 2 — Divergence at Reaction 2 (NH₃ in water): Arrhenius cannot classify NH₃ as a base — it contains no OH⁻. The Brønsted-Lowry model correctly identifies NH₃ as a base by proton acceptance: NH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq). Acid: H₂O; base: NH₃. Conjugate pair 1: H₂O / OH⁻. Conjugate pair 2: NH₃ / NH₄⁺. The OH⁻ produced comes from water donating H⁺, not from NH₃. The models diverge: Arrhenius predicts NH₃ is NOT a base; Brønsted-Lowry correctly predicts it IS.

Point 3 — Divergence at Reaction 3 (gas phase): HCl(g) + NH₃(g) → NH₄Cl(s) occurs entirely in the gas phase with no water and no ions in solution. Arrhenius cannot classify either reactant (no aqueous medium exists). Brønsted-Lowry classifies it straightforwardly: HCl donates H⁺ to NH₃ — HCl is the acid, NH₃ is the base. Arrhenius offers no description of this reaction at all.

Point 4 — Conceptual difference: Arrhenius defines acids and bases by which ions they produce in water — a macroscopic, medium-dependent, product-based definition. Brønsted-Lowry defines them by what they do — proton transfer, a mechanistic, process-based definition applicable in any medium. Brønsted-Lowry is a broader framework that subsumes Arrhenius for aqueous strong acids/bases but extends beyond it.

Judgement: The student’s claim is an oversimplification and is substantially incorrect. While both models agree for aqueous strong acid-base reactions such as HCl + NaOH, they diverge for NH₃ (which Arrhenius cannot classify as a base at all) and for gas-phase reactions (which Arrhenius cannot describe). Brønsted-Lowry adds a mechanistic framework that explains why bases produce OH⁻ in water (by accepting H⁺ from water, not by releasing their own OH⁻) and applies beyond aqueous systems. The models are not the same — Brønsted-Lowry is a superior and more general framework.

Marking criteria:

• 1 mark — correct Arrhenius definitions of acid and base, with identification of Reaction 1 as the agreement case
• 1 mark — Arrhenius cannot classify NH₃ as a base (no OH⁻); Brønsted-Lowry can
• 1 mark — correct Brønsted-Lowry equation for Reaction 2 with equilibrium arrow
• 1 mark — correct identification of both conjugate pairs in Reaction 2
• 1 mark — Arrhenius cannot describe Reaction 3 (no aqueous medium); Brønsted-Lowry correctly identifies proton transfer in gas phase
• 1 mark — identifies conceptual difference (ion production vs proton transfer; medium-dependent vs medium-independent)
• 2 marks — evidence-based judgement explicitly refuting “essentially the same” using at least two of the divergence points as evidence (1 mark for refutation claim; 1 mark for evidence-linked reasoning)

Q2 — Source critique (7 marks)

Sample response.

Point 1 — The factual error: The source claims NH₃ “releases hydroxide ions (OH⁻) from its own structure.” This is chemically wrong. The molecular formula of ammonia is NH₃ — it contains one nitrogen and three hydrogen atoms and no oxygen or hydroxide whatsoever. NH₃ cannot release OH⁻ from its own structure because it does not contain OH⁻. The OH⁻ that appears in NH₃(aq) solution originates from water molecules, not from ammonia.

Point 2 — Correct Brønsted-Lowry equation and molecular explanation: NH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq). Acid: H₂O (donates H⁺). Base: NH₃ (accepts H⁺). Conjugate acid: NH₄⁺. Conjugate base: OH⁻. At the molecular level, the nitrogen atom in NH₃ carries a lone pair of electrons. When a water molecule collides with NH₃ with sufficient energy, the lone pair on N forms a coordinate covalent bond with the H of water, accepting that H as a proton. The water molecule thereby loses H⁺, leaving OH⁻ as the conjugate base of water. NH₃ acts as a Brønsted-Lowry base by proton acceptance via its lone pair — no OH⁻ is released from NH₃ at any stage.

Point 3 — The misleading claim about model agreement: The source states “Brønsted-Lowry simply agrees with Arrhenius here, confirming that OH⁻ production is what makes a substance basic.” This is misleading. The two models do not agree on NH₃. The Arrhenius model cannot classify NH₃ as a base at all (it predicts NH₃ is NOT a base, because NH₃ has no OH⁻ to supply). The Brønsted-Lowry model correctly classifies NH₃ as a base by proton acceptance. Furthermore, Brønsted-Lowry does not say “OH⁻ production is what makes a substance basic” — it says proton acceptance makes a substance basic. OH⁻ in solution is a consequence of the base (NH₃) accepting H⁺ from water, not a defining criterion.

Point 4 — Experimental test: A student could use isotope labelling. If ¹⁶O-labelled water (H₂¹⁶O) is used as the solvent and unlabelled NH₃ is dissolved in it, the Brønsted-Lowry model predicts all OH⁻ in solution will be ¹⁶OH⁻ (because OH⁻ comes from water losing H⁺, retaining its O). The Arrhenius explanation would predict OH⁻ from NH₃’s own structure, which contains only ¹⁶O atoms would not be expected. If mass spectrometry detects ¹⁶OH⁻ in solution but no unlabelled OH⁻, it confirms the Brønsted-Lowry account. Alternatively, a simpler test: measuring the IR spectrum of solid NH₃ would show no O–H bond stretch, confirming no OH⁻ is present in NH₃’s structure.

Marking criteria:

• 1 mark — identifies the specific factual error: NH₃ does not contain OH⁻ / cannot release OH⁻ from its own structure
• 1 mark — correct Brønsted-Lowry equation with equilibrium arrow and correct identification of acid (H₂O) and base (NH₃)
• 1 mark — correct identification of conjugate acid (NH₄⁺) and conjugate base (OH⁻)
• 1 mark — molecular-level explanation: lone pair on N accepts H⁺ from H₂O; OH⁻ comes from water not from NH₃
• 1 mark — identifies the misleading claim: Brønsted-Lowry does NOT agree with Arrhenius on NH₃; Arrhenius cannot classify NH₃ as a base at all
• 1 mark — explains the correct Brønsted-Lowry position: basicity = proton acceptance, not OH⁻ production
• 1 mark — describes a valid experimental test that distinguishes between the two explanations (isotope labelling, IR spectroscopy, or equivalent valid method)