What are Acid-Base Definitions?
Acids and bases are fundamental in chemistry. Over time, scientists developed different definitions to explain their behavior. The two main frameworks are Arrhenius (simple, limited) and Brønsted-Lowry (general, powerful).
Arrhenius definition: An acid produces H⁺ ions in water; a base produces OH⁻ ions. Brønsted-Lowry definition: An acid is a proton (H⁺) donor; a base is a proton acceptor. Brønsted-Lowry is broader and works beyond water.
- •Acid = produces H⁺
- •Base = produces OH⁻
- •Only in aqueous solution
- •Limited scope
- •Acid = proton (H⁺) donor
- •Base = proton acceptor
- •Works in any solvent
- •More general
Step-by-step worked examples
HCl in water: Arrhenius or Brønsted-Lowry?
HCl → H⁺ + Cl⁻ (produces H⁺, so Arrhenius acid) HCl donates H⁺ to water (H⁺ donor, so Brønsted-Lowry acid) Both definitions apply to HCl — Brønsted-Lowry is the broader view.
NH₃ in water: Name its roles.
NH₃ doesn't produce OH⁻ directly (Arrhenius base? No — incorrect!) NH₃ accepts H⁺ from water: NH₃ + H₂O ⇌ NH₄⁺ + OH⁻ (Brønsted-Lowry base) NH₃ is a Brønsted-Lowry base, not Arrhenius.
In ammonia liquid (no water), HCl dissolves and reacts. What framework explains this?
Arrhenius needs water — doesn't apply here. Brønsted-Lowry: HCl (H⁺ donor) reacts with NH₃ (H⁺ acceptor) Brønsted-Lowry explains acid-base chemistry beyond aqueous solutions.
Flashcards
Quick quiz
Q1.HNO₃ in water produces H⁺. Which definition(s) apply?
Q2.What is the conjugate base of HCl (Brønsted-Lowry)?
Q3.Water in HCl(aq) acts as a base (Brønsted-Lowry). Why?
Q4.Which species is NOT a Brønsted-Lowry base?
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Common mistakes
Thinking NH₃ is an Arrhenius base in water. — Correct: NH₃ is a Brønsted-Lowry base; it doesn't produce OH⁻ directly but accepts H⁺.
Confusing acid strength with number of H atoms. — Correct: Acidity depends on willingness to donate H⁺, not the number of H atoms in the formula.
Assuming all bases produce OH⁻. — Correct: Brønsted-Lowry bases accept H⁺; they needn't produce OH⁻.
Using Arrhenius definitions for non-aqueous systems. — Correct: Brønsted-Lowry applies everywhere; Arrhenius only in water.
FAQ
What is the main difference between Arrhenius and Brønsted-Lowry?
Arrhenius requires H⁺/OH⁻ production in water; Brønsted-Lowry uses proton donation/acceptance, working in any solvent.
Is HCl an acid under both definitions?
Yes — it produces H⁺ (Arrhenius) and donates H⁺ (Brønsted-Lowry).
What is a conjugate acid-base pair?
Two species that differ by one proton: HCl and Cl⁻, or NH₄⁺ and NH₃.
Can water be both an acid and a base?
Yes — in Brønsted-Lowry terms, water can donate H⁺ (acting as acid) or accept H⁺ (acting as base).