Arrhenius, Brønsted-Lowry, and Lewis: three lenses on the same reaction.
NEET doesn't just ask you to define these three models — it asks you to distinguish them. The trap is treating them as interchangeable when they are nested frameworks with different scopes.
Arrhenius (narrowest scope). An acid produces H⁺ in water; a base produces OH⁻ in water. This works only for aqueous solutions. HCl → H⁺ + Cl⁻ is an Arrhenius acid. NaOH → Na⁺ + OH⁻ is an Arrhenius base. Limitation: it cannot explain why NH₃ is basic (NH₃ doesn't contain OH⁻) or why BF₃ is acidic (no H⁺ involved). NCERT Class 11 Chemistry Chapter 6 (Equilibrium), page 190, introduces this hierarchy explicitly.
Brønsted-Lowry (broader). An acid is a proton (H⁺) donor; a base is a proton acceptor. This extends beyond water. In NH₃ + H₂O → NH₄⁺ + OH⁻, water donates a proton to NH₃ — water acts as the Brønsted acid, NH₃ as the Brønsted base. Every Brønsted acid-base reaction produces a conjugate pair.
Lewis (broadest). An acid is an electron-pair acceptor; a base is an electron-pair donor. This covers reactions with no proton transfer at all. BF₃ + NH₃ → BF₃·NH₃: BF₃ accepts a lone pair (Lewis acid), NH₃ donates one (Lewis base). All Brønsted acids are Lewis acids, but not all Lewis acids are Brønsted acids.
Watch-out for NEET: When a question asks "Which of the following is a Lewis acid but NOT a Brønsted acid?", the answer is always a species that accepts electron pairs without donating or accepting protons — metal cations (Fe³⁺, Al³⁺), BF₃, AlCl₃. If you default to the Arrhenius definition, you will pick the wrong option.
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
According to the Arrhenius concept, which of the following is a base?
Answer: A. NaOH dissociates in water to produce OH⁻ ions, which is the defining criterion for an Arrhenius base (NCERT Class 11 Chemistry Chapter 6, page 190).
Why B is wrong: B — HCl produces H⁺ in water, making it an Arrhenius acid, not a base.
Why C is wrong: C — BF₃ has no OH⁻ to release in water. It is a Lewis acid, not an Arrhenius base.
Why D is wrong: D — NH₃ is a Brønsted-Lowry base (proton acceptor) but not an Arrhenius base because it does not directly produce OH⁻ from its own structure. The OH⁻ forms only when NH₃ reacts with water.
In the reaction NH₃ + HF → NH₄⁺ + F⁻, which species acts as the Brønsted-Lowry acid?
Answer: D. HF donates a proton to NH₃, so HF is the Brønsted-Lowry acid. The definition is proton donor = acid (NCERT Class 11 Chemistry Chapter 6, page 190).
Why A is wrong: A — NH₃ accepts the proton, making it the Brønsted-Lowry base, not the acid.
Why B is wrong: B — F⁻ is the conjugate base of HF. It has already lost access to the donated proton.
Why C is wrong: C — NH₄⁺ is the conjugate acid formed after NH₃ accepts a proton. It is a product, not the acid in this forward reaction.
Which of the following is a Lewis acid but NOT a Brønsted-Lowry acid?
Answer: D. BF₃ accepts an electron pair (Lewis acid) but has no proton to donate, so it is not a Brønsted-Lowry acid. HCl, H₂SO₄, and CH₃COOH can all donate protons (NCERT Class 11 Chemistry Chapter 6, page 192).
Why A is wrong: A — HCl donates a proton, so it qualifies as both a Brønsted-Lowry acid and a Lewis acid. The question asks for Lewis-only.
Why B is wrong: B — H₂SO₄ is a proton donor (Brønsted acid) as well as a Lewis acid. Not Lewis-only.
Why C is wrong: C — CH₃COOH donates H⁺ from its –COOH group, making it a Brønsted acid as well.
The conjugate base of H₂O acting as a Brønsted acid is:
Answer: C. When H₂O donates a proton (acts as Brønsted acid), it loses H⁺ and becomes OH⁻. Conjugate base = parent acid minus one proton (NCERT Class 11 Chemistry Chapter 6, page 191).
Why A is wrong: A — H₃O⁺ is formed when H₂O accepts a proton (conjugate acid of H₂O as a base). The question asks for conjugate base.
Why B is wrong: B — O²⁻ would require removing two protons from H₂O. Conjugate base involves removal of exactly one proton.
Why D is wrong: D — H₂O₂ is hydrogen peroxide, a completely different compound. It is not formed by proton removal from water.
Identify the conjugate acid-base pairs in the reaction: HCN + H₂O ⇌ H₃O⁺ + CN⁻
Answer: B. HCN donates a proton to become CN⁻ (pair 1: HCN/CN⁻). H₂O accepts a proton to become H₃O⁺ (pair 2: H₂O/H₃O⁺). Each conjugate pair differs by exactly one proton (NCERT Class 11 Chemistry Chapter 6, page 191).
Why A is wrong: A — HCN and H₃O⁺ differ by more than one proton and are not conjugates. Similarly H₂O and CN⁻ are not related by a single proton transfer.
Why C is wrong: C — HCN/H₂O are the two reactants, not a conjugate pair. Conjugate pairs cross the reaction arrow (one reactant, one product).
Why D is wrong: D — H₃O⁺/HCN are not conjugates (different molecular identity beyond one proton). This pairing confuses reactant-product with acid-conjugate base.
In the reaction BF₃ + F⁻ → BF₄⁻, the Lewis base is:
Answer: A. F⁻ donates a lone pair of electrons to boron in BF₃. Electron-pair donor = Lewis base (NCERT Class 11 Chemistry Chapter 6, page 192).
Why B is wrong: B — BF₄⁻ is the product (adduct) formed after the electron pair is shared. It is neither the acid nor the base in this reaction.
Why C is wrong: C — BF₃ accepts the electron pair from F⁻. It is the Lewis acid, not the Lewis base.
Why D is wrong: D — B³⁺ is a free boron ion, which is not a species in this reaction. BF₃ (intact molecule) is the Lewis acid here.
Water can act as both a Brønsted acid and a Brønsted base. This property is called:
Answer: B. A substance that can both donate AND accept a proton is amphiprotic. While "amphoteric" refers broadly to reacting with both acids and bases (includes oxide reactions), "amphiprotic" specifically means proton donor and acceptor — the precise Brønsted-Lowry term (the term amphiprotic goes beyond NCERT; NCERT Class 11 Chemistry Chapter 6, page 191 describes this dual role of water as an acid and a base).
Why A is wrong: A — Neutralisation is the reaction between an acid and a base to form salt and water. It does not describe the dual proton-donor/acceptor nature of water itself.
Why C is wrong: C — Amphoteric means a substance reacts with both acids and bases (e.g. Al₂O₃), but it is a broader term that includes oxide chemistry. The question specifically asks about Brønsted proton donation/acceptance, which is amphiprotic.
Why D is wrong: D — Autoprotolysis is the self-ionisation of water (H₂O + H₂O → H₃O⁺ + OH⁻). It is a specific reaction, not the name for the general dual-nature property.
Which of the following correctly ranks the three acid-base theories from narrowest to broadest scope?
Answer: C. Arrhenius is limited to aqueous solutions. Brønsted-Lowry extends to any proton-transfer reaction (aqueous or not). Lewis encompasses all electron-pair acceptance, including reactions with no proton transfer. So Arrhenius ⊂ Brønsted-Lowry ⊂ Lewis (NCERT Class 11 Chemistry Chapter 6, page 190).
Why A is wrong: A — This reverses the order completely. Lewis is the broadest, not the narrowest.
Why B is wrong: B — Lewis is broader than Brønsted-Lowry, not intermediate. Lewis acids like BF₃ and AlCl₃ have no proton involvement, which Brønsted-Lowry cannot classify.
Why D is wrong: D — Arrhenius is not broader than Brønsted-Lowry. Arrhenius cannot explain why NH₃ is basic (no OH⁻ in its formula), but Brønsted-Lowry can.
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Pattern: Identifying acid-base roles across all three models (highest-relevance concept for this topic — no quantitative PYQ pattern is in-scope after Rule 7 filtering).
Given
Consider the reaction: AlCl₃ + Cl⁻ → AlCl₄⁻
Classify AlCl₃ and Cl⁻ under each applicable acid-base theory.
Required
Identify AlCl₃ and Cl⁻ as acid/base under (a) Arrhenius, (b) Brønsted-Lowry, (c) Lewis.
Concept
Arrhenius: acid produces H⁺ in water, base produces OH⁻. Brønsted-Lowry: acid donates proton, base accepts proton. Lewis: acid accepts electron pair, base donates electron pair.
Formula
No quantitative formula applies. This is a classification problem using definitions.
Substitution / Application
(a) Arrhenius: AlCl₃ does not produce H⁺ in water. Cl⁻ does not produce OH⁻ in water. Neither qualifies as an Arrhenius acid or base in this reaction.
(b) Brønsted-Lowry: No proton is transferred in AlCl₃ + Cl⁻ → AlCl₄⁻. Neither qualifies as a Brønsted-Lowry acid or base.
(c) Lewis: AlCl₃ has an incomplete octet on Al (electron-deficient). Cl⁻ has lone pairs. Cl⁻ donates an electron pair to Al → Lewis base. AlCl₃ accepts the electron pair → Lewis acid.
Calculation
Not applicable (qualitative classification).
Final answer
AlCl₃ is a Lewis acid only. Cl⁻ is a Lewis base only. Neither fits Arrhenius or Brønsted-Lowry in this reaction.
Common trap
Students often force-classify all reactions into Brønsted-Lowry by looking for any hydrogen atom. AlCl₃ + Cl⁻ involves zero proton transfer — Brønsted-Lowry simply does not apply. Recognising the boundaries of each model is the skill NEET tests.
Similar NEET-style question
"In the reaction BF₃ + NH₃ → BF₃·NH₃, identify the Lewis acid and Lewis base. Explain why this reaction cannot be classified under the Arrhenius framework."
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Arrhenius: H⁺ producer (acid) / OH⁻ producer (base) in water. Brønsted-Lowry: H⁺ donor (acid) / H⁺ acceptor (base). Lewis: electron-pair acceptor (acid) / electron-pair donor (base).
-- NCERT Class 11 Chemistry, Ch. 6, p. 190More in Equilibrium: 8 exam traps and mistakes · 5 formulas · 4 question patterns from its other lessons.
1 question from NEET 2023. Answers verified against NTA official keys.
Amongst the given options which of the following molecules/ ion acts as a Lewis acid?
Page numbers are the ones printed in the current NCERT textbook (2023 rationalised edition), unless marked pre-2023. The books are free at ncert.nic.in.
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