Acid-Base Theories — Chemistry Mnemonic
Target: mnemonic for Arrhenius Bronsted Lewis acid base theory
Why is this hard to memorize?
There are three progressively broader acid-base theories: Arrhenius (acid = produces H⁺ in water, base = produces OH⁻), Brønsted-Lowry (acid = proton donor, base = proton acceptor), and Lewis (acid = electron pair acceptor, base = electron pair donor). Each theory encompasses the previous: all Arrhenius acids are Brønsted acids, and all Brønsted acids are Lewis acids, but not vice versa. Lewis theory is the broadest — BF₃ is a Lewis acid (accepts electron pair) but has no H⁺ to donate. NEET and JEE test identifying acids/bases under each definition.
Classic mnemonics you should know
"Arrhenius: H⁺ and OH⁻ in water. Brønsted: proton transfer (donor/acceptor). Lewis: electron pair (acceptor/donor). "ABL = Aqueous, Beyond, Lone-pair""
Arrhenius is limited to aqueous solutions. Brønsted-Lowry works in any solvent (proton transfer). Lewis is most general (electron pair interaction). ABL in order of increasing breadth.
"Acid donates H⁺ → becomes conjugate base. Base accepts H⁺ → becomes conjugate acid. "Acid - H = Conjugate Base""
HCl(acid) + H₂O(base) → Cl⁻(conjugate base) + H₃O⁺(conjugate acid). Every Brønsted reaction has two conjugate pairs. Strong acid → weak conjugate base and vice versa.
"Lewis acids = electron pair ACCEPTORS: BF₃, AlCl₃, FeCl₃, H⁺, metal cations (incomplete octet or empty orbital)"
BF₃ has only 6 electrons on B (empty p-orbital) → accepts electron pair. AlCl₃ is also electron-deficient. All metal cations (Fe³⁺, Cu²⁺) accept electron pairs from ligands. H⁺ is a Lewis acid (accepts lone pair from base). Any electrophile is a Lewis acid.
The complete list
- Arrhenius acid: produces H⁺ in water
- Arrhenius base: produces OH⁻ in water
- Brønsted acid: proton (H⁺) donor
- Brønsted base: proton (H⁺) acceptor
- Lewis acid: electron pair acceptor
- Lewis base: electron pair donor
- Conjugate pair: differ by one H⁺
- Lewis > Brønsted > Arrhenius (breadth)
Frequently asked questions
Why do we need three different acid-base theories?
Arrhenius only works in water — it can't explain NH₃ acting as a base in gas phase. Brønsted-Lowry extends to non-aqueous solvents and proton transfer, but can't explain BF₃ acting as an acid (no proton involved). Lewis theory covers all acid-base interactions including metal-ligand bonding, organic electrophile-nucleophile reactions, and reactions without H⁺ at all. Broader theory = more reactions explained.
How is water both an acid and a base?
Water is amphoteric: with HCl, water acts as a Brønsted base (accepts H⁺ → H₃O⁺). With NH₃, water acts as a Brønsted acid (donates H⁺ → OH⁻). This is because water has both a lone pair (can accept H⁺) and an O-H bond (can donate H⁺). Autoionization: H₂O + H₂O ⇌ H₃O⁺ + OH⁻ shows both roles simultaneously.
What is a Lewis acid-base reaction that doesn't involve protons?
BF₃ + NH₃ → F₃B-NH₃. BF₃ is the Lewis acid (B has an empty p-orbital, accepts the electron pair). NH₃ is the Lewis base (N donates its lone pair). No proton transfer occurs — this can't be explained by Arrhenius or Brønsted-Lowry. Another example: AlCl₃ + Cl⁻ → AlCl₄⁻ (AlCl₃ accepts electron pair from Cl⁻).
Explore more Chemistry mnemonics
We have 100+ memory tricks for Chemistry — periodic table groups, reactions, exceptions, and more.
Browse all Chemistry mnemonics