GATE Acid-Base Concepts — Bronsted-Lowry, Lewis and HSAB
GATE Chemistry expects you to switch between three different definitions of "acid" and "base" depending on what a question is testing — a proton-transfer equilibrium, a coordinate-bond-forming reaction with no protons in sight, or a stability preference between two possible products. Each theory is broader than the last, and each answers a question the previous one cannot. This guide covers all three with worked examples, so you can identify which lens a question wants without hesitating.
Bronsted-Lowry — proton transfer
A Bronsted-Lowry acid is a proton (H⁺) donor; a base is a proton acceptor. Every acid-base reaction under this theory produces a conjugate base (the acid minus a proton) and a conjugate acid (the base plus a proton).
pKₐ + pKᵦ = pKw = 14.00 (at 25 °C, for a conjugate acid-base pair only)
Worked example 1 — conjugate pairs. In NH₃ + H₂O ⇌ NH₄⁺ + OH⁻, identify the
two conjugate pairs.
NH₃ gains a proton to become NH₄⁺, so NH₄⁺/NH₃ is one conjugate acid-base pair. H₂O loses a
proton to become OH⁻, so H₂O/OH⁻ is the other. Water is acting as the acid here and ammonia as
the base — the same molecule (water) can be an acid in one reaction and a base in another,
which is the whole point of "conjugate": the label depends on the reaction, not the molecule.
Worked example 2 — pKₐ to pKᵦ. Acetic acid has pKₐ = 4.76. Find the pKᵦ of
its conjugate base, the acetate ion.
pKᵦ(CH₃COO⁻) = 14.00 − pKₐ(CH₃COOH) = 14.00 − 4.76 = 9.24
The relation only ever connects a conjugate pair — you cannot apply it to two acids picked at
random.
Lewis — electron pairs, not protons
A Lewis acid is an electron-pair acceptor; a Lewis base is an electron-pair donor. This definition covers everything Bronsted-Lowry covers — H⁺ itself is a Lewis acid, since accepting a proton is literally donating an electron pair to it — and a great deal more, including reactions with no proton transfer at all.
Worked example 3 — a reaction with no protons. BF₃ + NH₃ → F₃B←NH₃
Boron in BF₃ has only six electrons around it (an incomplete octet) and an empty orbital, so it
accepts the lone pair on nitrogen to form a coordinate (dative) bond. BF₃ is the Lewis acid,
NH₃ the Lewis base. Neither species has a proton to donate or accept, so Bronsted-Lowry has
nothing to say about this reaction — only the Lewis definition classifies it.
Worked example 4 — classify a mixed list. Sort BF₃, NH₃, H₂O, Fe³⁺, CN⁻ and
SO₃ as Lewis acids or Lewis bases.
Lewis acids (electron-pair acceptors, typically electron-deficient or a bare
cation): BF₃ (incomplete octet), Fe³⁺ (empty d-orbitals, high positive charge), SO₃ (sulfur can
expand its octet to accept a pair).
Lewis bases (electron-pair donors, need a lone pair to offer): NH₃, H₂O, CN⁻ —
the same three species that act as ligands in coordination chemistry, because "ligand" and
"Lewis base" describe the same behaviour.
HSAB — which base an acid actually prefers
Bronsted-Lowry and Lewis tell you whether something can act as an acid or base. HSAB (Hard-Soft Acid-Base theory, Pearson) goes one step further and predicts which pairing is more stable when an acid has a choice of bases. Hard acids and hard bases are small, compact, weakly polarisable and highly charged; soft acids and soft bases are large, diffuse and easily polarisable. The rule: hard prefers hard, soft prefers soft.
| Hard | Soft | |
|---|---|---|
| Typical acids | H⁺, Al³⁺, Fe³⁺, Cr³⁺ | Ag⁺, Hg²⁺, Cu⁺, Pt²⁺ |
| Typical bases | F⁻, OH⁻, H₂O, NH₃ | I⁻, CN⁻, S²⁻, PR₃ |
Worked example 5 — predicting the stronger pairing. Which is the more stable
combination: Ag⁺ with F⁻, or Ag⁺ with I⁻?
Ag⁺ is a soft acid (large, polarisable, low charge density for its size). F⁻ is a hard base;
I⁻ is a soft base. Soft prefers soft, so Ag⁺–I⁻ is the more stable pairing — which matches the
real solubility trend: AgI is far less soluble (and the Ag–I bond correspondingly stronger)
than AgF, which is comparatively soluble.
This article keeps HSAB to the classification rule you need to apply it quickly. For the quantitative side — putting an actual number on hardness from ionisation energy and electron affinity, and a worked Ksp comparison — see the dedicated HSAB article linked below.
Comparing all three at a glance
| Theory | Defines acid as | Defines base as | Answers the question |
|---|---|---|---|
| Bronsted-Lowry | Proton donor | Proton acceptor | Which species gives up H⁺? |
| Lewis | Electron-pair acceptor | Electron-pair donor | Can this reaction happen with no protons at all? |
| HSAB | (Applies within Lewis acids) | (Applies within Lewis bases) | Which possible pairing is more stable? |
Common mistakes that cost marks
- Assuming every acid needs a hydrogen. BF₃ and AlCl₃ have no acidic protons and are still textbook Lewis acids.
- Confusing acid "strength" with "hardness". These are unrelated axes. Strength (measured by pKₐ) is about how completely a species donates or accepts a proton; hardness is about polarisability and size, and governs which base a given acid prefers, not how strong the acid is.
- Applying pKₐ + pKᵦ = 14 to two unrelated species. The relation only connects a genuine conjugate pair — an acid and the specific base formed when it loses a proton.
- Forgetting that Bronsted-Lowry is a subset of Lewis, not a separate system. Any proton-transfer reaction is simultaneously a Lewis acid-base reaction; the Lewis definition is the more general one, not a rival one.
- Treating a hard/soft classification as absolute. Hardness is a relative, continuous property (H⁺ is harder than Al³⁺, which is harder than Fe³⁺); the hard/soft table is a working approximation, not a strict binary.
Where this appears in GATE Chemistry
| Question style | What it is testing |
|---|---|
| "Identify the conjugate acid/base of X" | Adding or removing exactly one H⁺ |
| "Which of these is a Lewis acid but not a Bronsted-Lowry acid?" | Recognising electron-deficient species with no acidic proton |
| "Which product is favoured?" (given a choice of ligand/leaving group) | Applying hard-hard / soft-soft preference |
| Numerical: pKₐ ↔ pKᵦ, or pH of a buffer | Direct use of the pKₐ + pKᵦ = 14 relation and the Henderson–Hasselbalch equation |
Check pH and buffer numericals instantly. The ABC Chemistry Calculator Suite includes a pH/pOH calculator and a Henderson–Hasselbalch buffer calculator that pair directly with the Bronsted-Lowry numericals above.
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