Fix the mistake: Acids and Bases

Definitions of Acids and Bases (Arrhenius and Brønsted)

Acids and bases are not defined only by being “sour” or “bitter”; they are clearly defined chemical concepts. Historically, several definitions have been proposed.

Basic Knowledge

There are two representative definitions.
① Arrhenius definition: A substance that dissolves in water and produces hydrogen ions H+ (oxonium ions H3O+) is defined as an acid, while a substance that produces hydroxide ions OH- is defined as a base.
Examples: HCl → H+ + Cl-, NaOH → Na+ + OH-
② Brønsted–Lowry definition: A substance that donates H+ is an acid, and one that accepts it is a base. This is a broad definition that is not limited to water.
In NH3 + H2O → NH4+ + OH-, H2O donates H+ and is therefore the acid, while NH3 accepts it and is the base.

Important Terms
Arrhenius definition (Produces H+ = acid; produces OH- = base. Limited to aqueous solutions)
Brønsted–Lowry definition (Donates H+ = acid; accepts H+ = base)
Oxonium ion (H3O+. H+ actually bonds with a water molecule to form this structure)
Properties of acids (Sour taste, blue litmus → red, reacts with metals to produce H2)
Properties of bases (Bitter taste, slippery, red litmus → blue)

Further Exploration (Background and Significance)

Arrhenius theorized ionization in aqueous solutions at the end of the 19th century and provided the first scientific definition of acids and bases. However, this definition is limited to aqueous solutions.
The Brønsted–Lowry definition is broader and can deal with acid–base reactions in the gas phase and in nonaqueous solvents. For example, even in a reaction without water such as NH3 + HCl → NH4Cl, HCl functions as the acid and NH3 as the base.
The fact that water itself can be “both an acid and a base” (an amphoteric substance) can be explained clearly for the first time by the Brønsted definition.

Key Points
  • Arrhenius: produces H+/OH- (limited to aqueous solutions)
  • Brønsted: defined by the transfer of H+ (broader)
  • H+ ≒ H3O+ (oxonium ion)
  • HCl, H2SO4, CH3COOH are acids
  • NaOH, Ca(OH)2, NH3 are bases
  • Water can act as both an acid and a base (amphoteric)
  • NH3 has no OH group but is a base according to Brønsted

Points of Caution (Common Confusions)

① NH3 is a base, not an acid. It accepts H+ even though it does not contain OH. ② Arrhenius is limited to aqueous solutions, while Brønsted is general. ③ H+ and H3O+ may be treated as essentially the same. ④ An alkali is a base that dissolves readily in water. Base ⊇ alkali.

Practice

  1. State the acid and base according to the Arrhenius definition.
  2. In NH3 + H2O → NH4+ + OH-, which substance is the Brønsted acid?
  3. Briefly explain why water is called “amphoteric.”

Chemistry (High School Basic): related lessons