Definitions
High-Yield Summary
- Arrhenius: acid produces H⁺ (as H₃O⁺), base produces OH⁻, both in water only — most restrictive, identifiable by formula (H... acids, ...OH bases).
- Brønsted-Lowry: acid donates a proton, base accepts one — works outside water, generates conjugate acid-base pairs.
- Lewis: acid accepts an electron pair, base donates one, forming a coordinate covalent bond — most inclusive (e.g., BF₃, AlCl₃ have no H to donate but are still Lewis acids).
- Hierarchy: every Arrhenius acid/base is Brønsted-Lowry, every Brønsted-Lowry acid/base is Lewis — not the reverse.
- Amphoteric: acts as acid or base depending on environment. Amphiprotic: subset of amphoteric that specifically gains/loses protons. Water is the classic example of both.
Key Terms
- Conjugate acid-base pair
- An acid and the base left behind after it donates a proton (or a base and the acid formed after it accepts one).
- Coordinate covalent bond
- A bond in which one atom (the Lewis base) supplies both bonding electrons to the other (the Lewis acid).
- Amphoteric
- A species that reacts as an acid in a basic environment and as a base in an acidic environment.
- Amphiprotic
- An amphoteric species specifically able to both donate and accept a proton (Brønsted-Lowry sense).
- Binary acid
- An acid containing hydrogen and one other element, named hydro- + root + -ic acid (e.g., HCl → hydrochloric acid).
- Oxyacid
- An acid containing hydrogen, oxygen, and another element, named from its parent oxyanion.
Three Acid-Base Definitions
| Definition | Acid / Base |
|---|---|
| Arrhenius (most restrictive) | Acid produces H⁺/H₃O⁺ in water; base produces OH⁻ in water. Formula gives it away: H... / ...OH. |
| Brønsted-Lowry | Acid donates a proton; base accepts a proton. Not limited to water (e.g., NH₃, F⁻ are B-L bases). |
| Lewis (most inclusive) | Acid accepts an electron pair; base donates an electron pair, forming a coordinate covalent bond. |
Naming Acids from Their Parent Anion
| Anion pattern | Acid name |
|---|---|
| -ide (binary acid) | hydro- + root + -ic acid — e.g., chloride (Cl⁻) → hydrochloric acid (HCl) |
| -ite (fewer O, oxyacid) | root + -ous acid — e.g., nitrite (NO₂⁻) → nitrous acid (HNO₂) |
| -ate (more O, oxyacid) | root + -ic acid — e.g., nitrate (NO₃⁻) → nitric acid (HNO₃) |
| hypo-/per- prefix | carries over unchanged — hypochlorite → hypochlorous acid; perchlorate → perchloric acid |
Must-Know Points
- Water's autoionization (H₂O + H₂O ⇌ H₃O⁺ + OH⁻) is the clearest Brønsted-Lowry example: one water molecule is the acid, the other is the base.
- BF₃ and AlCl₃ are Lewis acids used as organic chemistry catalysts — they accept electron pairs but have no H to donate, so they fail both Arrhenius and Brønsted-Lowry.
- Amino acids are amphiprotic: –COOH donates a proton, –NH₂ accepts one.
Common MCAT Trap
- Every amphiprotic species is amphoteric, but NOT every amphoteric species is amphiprotic — Al(OH)₃ and ZnO are amphoteric (react with acids and bases) but don't do so via direct proton transfer, so they aren't amphiprotic.
- Don't assume a compound without H is a base by default — Lewis acids like BF₃ have no acidic proton but are still acids in the electron-pair sense.
- HCO₃⁻ and HSO₄⁻ are amphiprotic (partially deprotonated polyprotic acids) — they can both donate their remaining proton and re-accept one.
Quick Recall
Why is NH₃ a Brønsted-Lowry base but not an Arrhenius base?
What makes BF₃ a Lewis acid despite having no hydrogen?
Name the acid derived from the sulfate ion (SO₄²⁻).