Boron trifluoride (BF₃) reacts with ammonia (NH₃) to form BF₃·NH₃. BF₃ has no proton to donate and no OH⁻ to release. Under which acid-base framework(s) can BF₃ be classified as an acid?
AArrhenius only — BF₃ is an acid because it dissolves in water
BBrønsted-Lowry only — BF₃ accepts a proton from NH₃
CLewis only — BF₃ accepts an electron pair from NH₃'s lone pair
DNone — BF₃ cannot be an acid because it doesn't donate protons or produce H⁺
BF₃ is a Lewis acid: boron has an empty orbital and accepts the lone pair on nitrogen, forming a coordinate covalent bond. There is no proton transfer (eliminating Brønsted-Lowry) and no H⁺ or OH⁻ production in water (eliminating Arrhenius). Option D represents the exact misconception the Lewis definition was designed to correct — not every acid-base reaction involves protons. The Lewis definition is broader precisely because it captures electron-pair interactions independent of proton transfer.
Question 2 Multiple Choice
Which pair of definitions correctly classifies HF donating a proton to F⁻ to give HF₂⁻?
AArrhenius acid-base only — HF produces H⁺ in water
BBrønsted-Lowry and Lewis — HF donates a proton (Brønsted-Lowry acid), and F⁻ donates a lone pair to the proton (Lewis base)
CLewis only — the reaction involves electron pair donation, not proton transfer
DArrhenius and Brønsted-Lowry, but not Lewis — Lewis requires no proton transfer
HF donating H⁺ to F⁻ is a Brønsted-Lowry acid-base reaction (proton transfer). It is also a Lewis reaction: F⁻ donates a lone pair to the proton (H⁺ is the Lewis acid, F⁻ is the Lewis base). Every Brønsted-Lowry reaction is simultaneously a Lewis reaction — proton transfer is always also an electron-pair donation to the proton. Option D gets the nesting backwards: Lewis is the broadest definition, not the narrowest.
Question 3 True / False
The Lewis definition of acids and bases competes with the Brønsted-Lowry definition — chemists is expected to choose which framework to use because they are incompatible.
TTrue
FFalse
Answer: False
The three definitions are nested like concentric circles, not competing alternatives. Every Arrhenius acid-base reaction is also a Brønsted-Lowry one; every Brønsted-Lowry reaction is also a Lewis acid-base reaction. They are not incompatible — they are progressively broader ways of classifying the same phenomenon. Chemists choose based on what they are analyzing: Brønsted-Lowry for most aqueous and protic chemistry, Lewis when electron-pair transfer is occurring without proton involvement (coordination chemistry, organometallics, many organic mechanisms).
Question 4 True / False
Water is amphoteric, meaning it can act as either an acid or a base depending on what it reacts with.
TTrue
FFalse
Answer: True
Water is the classic example of an amphoteric substance. When water reacts with a stronger acid like HCl, water accepts a proton (H₂O is the Brønsted-Lowry base: HCl → H₃O⁺ + Cl⁻). When water reacts with a stronger base like NH₃, water donates a proton (H₂O is the Brønsted-Lowry acid: H₂O + NH₃ → OH⁻ + NH₄⁺). This also illustrates that acid/base identity is context-dependent — a molecule is not inherently an acid or base, but acts as one relative to its reaction partner.
Question 5 Short Answer
Why is the Lewis acid-base definition considered the broadest of the three definitions? What category of reactions does it capture that Brønsted-Lowry cannot?
Think about your answer, then reveal below.
Model answer: The Lewis definition is broadest because it defines acids as electron-pair acceptors and bases as electron-pair donors, requiring no proton transfer at all. It captures reactions like metal ion coordination (M²⁺ accepting lone pairs from water or ligands), BF₃ accepting a lone pair from NH₃, and many organometallic reactions — none of which involve H⁺. Brønsted-Lowry requires a proton donor and acceptor, so it excludes all reactions where electron pairs are transferred without proton movement. The Lewis definition subsumes Brønsted-Lowry: a proton transfer is always also an electron-pair donation to H⁺ (which is the Lewis acid), but not every electron-pair transfer involves a proton.
The nesting relationship — Lewis ⊃ Brønsted-Lowry ⊃ Arrhenius — is the key conceptual structure. Understanding this prevents the common error of assuming every acid-base reaction is a proton transfer. Coordination chemistry and Lewis acid catalysis (critical in organic synthesis and industrial chemistry) are entirely explained by the electron-pair framework.