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 7 (Equilibrium), page 20, 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?
In the reaction NH₃ + HF → NH₄⁺ + F⁻, which species acts as the Brønsted-Lowry acid?
Which of the following is a Lewis acid but NOT a Brønsted-Lowry acid?
The conjugate base of H₂O acting as a Brønsted acid is:
Identify the conjugate acid-base pairs in the reaction: HCN + H₂O ⇌ H₃O⁺ + CN⁻
In the reaction BF₃ + F⁻ → BF₄⁻, the Lewis base is:
Water can act as both a Brønsted acid and a Brønsted base. This property is called:
Which of the following correctly ranks the three acid-base theories from narrowest to broadest scope?
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." ---
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. 20pH of acidic buffer in terms of conjugate base/acid concentrations. For basic buffer: pOH = pKb + log10([salt]/[base]).
| Symbol | Quantity | SI Unit |
|---|---|---|
| pKa | -log Ka | - |
| [salt] | conjugate base conc | mol/L |
| [acid] | weak acid conc | mol/L |
Stronger acid → weaker conjugate base, and vice versa. pKa + pKb = 14.
| Symbol | Quantity | SI Unit |
|---|---|---|
| Ka | acid dissociation | - |
| Kb | base dissociation | - |
| Kw | water 10^-14 | - |
Convert between pressure-based and concentration-based equilibrium constants. T in K; R = 0.0821 L atm/mol/K (when K_p in atm).
| Symbol | Quantity | SI Unit |
|---|---|---|
| K_p | pressure constant | - |
| K_c | concentration constant | - |
| Δn | mole change | - |
Equilibrium constant for sparingly soluble salt. Q < K_sp: dissolves; Q > K_sp: precipitates.
| Symbol | Quantity | SI Unit |
|---|---|---|
| K_sp | solubility product | - |
Logarithmic acidity scale. Pure water at 25°C: pH = 7 = pOH.
| Symbol | Quantity | SI Unit |
|---|---|---|
| [H+] | hydrogen ion conc | mol/L |
| [OH-] | hydroxide conc | mol/L |
These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.
Category: Overthinking
Student claims catalyst shifts equilibrium toward products. Catalyst speeds up forward AND reverse equally; equilibrium position unchanged.
Question lists catalyst addition among options for shifting equilibrium.
Catalyst lowers activation energy of BOTH forward and reverse reactions equally. Time to reach equilibrium decreases; equilibrium position is unchanged.
Category: Sign Convention
Δn = (mol gas product) - (mol gas reactant). Sign matters; K_p = K_c (RT)^Δn.
Convert K_p ↔ K_c for gas-phase reaction.
Count moles of gas only (ignore solids/liquids). Δn = product - reactant. If Δn = 0, K_p = K_c. If Δn = +1, K_p = K_c × RT.
Category: Similar Terms
For salt M_aX_b: K_sp = [M⁺]^a [X⁻]^b. Student uses [M⁺][X⁻] regardless of stoichiometry.
K_sp problem with non-1:1 salt (e.g. CaF₂, Mg(OH)₂, Ag₂CrO₄).
Write dissolution: M_aX_b → aM + bX. Then K_sp = [M]^a · [X]^b. For CaF₂ ↔ Ca + 2F: K_sp = s · (2s)² = 4s³.
Root cause: concept gap
Catalyst speeds up forward AND reverse equally. Equilibrium position is unchanged. Time to reach equilibrium decreases.
Root cause: concept gap
Δn = (moles GAS product) - (moles GAS reactant). Solids and liquids excluded.
Root cause: formula misuse
For M_aX_b: K_sp = [M]^a · [X]^b. CaF₂: K_sp = s · (2s)² = 4s³.
Root cause: concept gap
Increase P → shift toward FEWER moles of gas. Decrease P → shift toward MORE moles. Reactions with same number of moles of gas: P doesn't shift equilibrium.
Root cause: concept gap
Weak acid: only fraction ionises. [H+] ≈ √(Ka · C). For 0.1 M acetic acid (Ka ~10⁻⁵): [H+] ~10⁻³ (pH ~3), not 1 (pH=1).
15 questions from NEET 2021, 2022, 2023, 2024, 2025. Answers verified against NTA official keys.
C [A] = [B] = [C] = 2 × 10–3 M. Then, which of the following is correct?
Amongst the given options which of the following molecules/ ion acts as a Lewis acid?
Which complex compound is most stable?
Recurring question shapes from past papers. Each pattern shows why wrong options look tempting.
wrong delta n sign
Counts moles incorrectly
treats catalyst as shifting
Believes catalyst shifts equilibrium
ignores weak vs strong
Treats weak acid as fully dissociated
forgets stoichiometric coefficient power
Uses [M+]^1 [X-]^1 for M(X)_2 salt
Test yourself on this topic with real past-paper questions:
Practice this topic →Get a structured 30-day study plan and a complete formula booklet — delivered to your inbox instantly.