Neutralization

pH and buffering

Salt hydrolysis

Complex formation

Solvent effects

Acid: increases [H3O+] (often written as H+) in aqueous solution

Base: increases [OH-] in aqueous solution

HNO3(aq) → H3O+ + NO3-

NaOH(aq) → Na+ + OH-

HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)

Simple and operational for aqueous chemistry

Directly tied to pH and classical neutralization

Water-restricted; bases must contain or generate OH- in water

Poor coverage of non-aqueous and gas-phase acid–base reactions

Acid: proton (H+) donor

Base: proton (H+) acceptor

Acid ⇌ conjugate base + H+

Base + H+ ⇌ conjugate acid

Strength relationship: stronger acid ↔ weaker conjugate base

HCl + H2O → H3O+ + Cl-

NH3 + H2O ⇌ NH4+ + OH-

CH3COOH + H2O ⇌ H3O+ + CH3COO-

Aqueous and many non-aqueous protic systems

Equilibria, buffers, acid/base strength (Ka, Kb, pKa)

Requires proton transfer (cannot cover acid–base events without H+ involvement)

Less direct for many coordination/complexation cases

Lewis acid: electron-pair acceptor

Lewis base: electron-pair donor

BF3 + :NH3 → F3B←NH3

AlCl3 + Cl- → AlCl4-

H+ + :OH- → H2O

Coordination (dative) bond formation

Electrophiles (acids) and nucleophiles (bases) as a broad mapping

Metal ions often act as Lewis acids (e.g., Fe3+, Cu2+)

Coordination chemistry and complex formation

Reactions lacking obvious proton transfer

Many organic mechanisms framed as electrophile–nucleophile interactions

Very broad classification; “acid–base” can become over-applied

Strength is not inherently quantified without extra models

Acid: increases the concentration of the solvent’s characteristic cation (from autoionization)

Base: increases the concentration of the solvent’s characteristic anion

Water: 2H2O ⇌ H3O+ + OH-

Liquid ammonia: 2NH3 ⇌ NH4+ + NH2-

“Acid” produces NH4+ (e.g., NH4Cl)

“Base” produces NH2- (e.g., KNH2)

Explains why acid/base behavior changes with solvent

Clarifies leveling effects within a given solvent

Strongly solvent-dependent; harder to transfer across media or mixed solvents

Acid: O2- (oxide ion) acceptor

Base: O2- (oxide ion) donor

SiO2 (acidic oxide) + O2- → SiO3^2-

CaO (basic oxide) + SiO2 → CaSiO3

High-temperature inorganic chemistry

Molten oxide systems, silicates, metallurgy

Specialized to oxide-ion chemistry; not general for molecular aqueous acids/bases

Acid: accepts anions or electrons, or donates cations

Base: donates anions or electrons, or accepts cations

Encompasses many Brønsted, Lewis, and some redox-like processes

Extremely general umbrella classification

Overly inclusive; blurs acid–base vs redox boundaries

Limited predictive power without extra constraints

Not a standalone definition; a compatibility guideline inside the Lewis framework

Hard acids/bases: small, high charge density, less polarizable

Soft acids/bases: larger, more polarizable, lower charge density

Hard acids prefer hard bases

Soft acids prefer soft bases

Hard acid: Al3+, Mg2+ with hard base: F-, OH- → strong interactions

Soft acid: Ag+, Pd2+ with soft base: I-, S2-, phosphines → strong interactions

Predicting complex stability

Rationalizing selectivity in inorganic/organic reactions

Qualitative; exceptions exist and context matters

Arrhenius: H3O+ / OH- (in water)

Brønsted–Lowry: proton (H+)

Lewis: electron pair

Solvent-system: solvent cation/anion (from autoionization)

Lux–Flood: oxide ion (O2-)

Usanovich: anions/cations/electrons (very broad)

Arrhenius: aqueous solutions

Brønsted–Lowry: protic solvents; many H+-involving reactions

Lewis: broad (solutions, gas phase, coordination complexes)

Solvent-system: non-aqueous solvents with notable autoionization

Lux–Flood: molten oxides/salts, high-temperature systems

Usanovich: very broad, including many inorganic/redox-like contexts

Arrhenius: simplicity for water chemistry

Brønsted–Lowry: conjugate-pair logic, equilibrium and pKa reasoning

Lewis: covers complexation and non-proton acid–base chemistry

Solvent-system: explains solvent-dependent acidity/basicity

Lux–Flood: ideal for oxide-ion reactions

Usanovich: unifies many frameworks at the cost of specificity

Arrhenius: water-only; OH- requirement

Brønsted–Lowry: requires protons

Lewis: too broad; strength not intrinsic

Solvent-system: solvent-dependent; less transferable across media

Lux–Flood: narrow domain (oxide ion)

Usanovich: overly inclusive; weak predictive clarity

Use Arrhenius and/or Brønsted–Lowry

Use Brønsted–Lowry (Ka, pKa, buffers)

Use Lewis (and HSAB for preferences)

Use solvent-system theory

Use Lux–Flood

Use Usanovich (with caution)

Arrhenius ⊂ Brønsted–Lowry ⊂ Lewis (often treated as increasing generality)

Arrhenius: H3O+ + OH- → 2H2O (net ionic)

Brønsted–Lowry: acid transfers H+ to base

Lewis: base donates an electron pair to an acid (including H+)

Strength depends on medium (dielectric constant, leveling/differentiating effects)

Reaction direction via equilibrium constants (Brønsted–Lowry)

Complex stability and selectivity (Lewis + HSAB)

High-temperature oxide equilibria (Lux–Flood)