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Understanding Brønsted-Lowry Acid-Base Theory

May 6, 2025

Lecture Notes on Brønsted-Lowry Theory of Acids and Bases

Introduction to Theories of Acids, Bases, and Salts

  • Two main theories: Arrhenius Theory and Brønsted-Lowry Theory.
  • Focus of this lecture: Brønsted-Lowry Theory, proposed independently in 1923 by Johan Brønsted and Thomas Lowry.

Basic Chemistry of Hydrogen

  • Hydrogen atom:
    • Atomic number: 1 (1 proton, 1 electron, commonly no neutrons)
    • Removal of electron results in a H+ ion (proton).
    • H+ ion and proton terms are used interchangeably in chemistry.

Brønsted-Lowry Theory

  • Definition of Acid: Species that donates a proton (H+ ion) to another species.
  • Definition of Base: Species that accepts a proton (H+ ion) from another species.

Example: Reaction of HCl and Water

  • HCl (Hydrogen Chloride):
    • Polar molecule with dipole (partial charges: δ+ on H, δ- on Cl).
    • Donates a proton to water, forming Cl- (chloride ion).
    • Considered a Brønsted-Lowry acid.
  • Water (H2O):
    • Acts as a base, accepts a proton becoming H3O+ (hydronium ion).
    • Equation: HCl + H2O → H3O+ + Cl-.

Ionization Process

  • Ionization of HCl: Conversion to ions (H3O+ and Cl-).
  • Example Equations:
    • HNO3 + H2O → H3O+ + NO3- (nitrate ion).
    • HCN + H2O ⇌ H3O+ + CN- (cyanide ion).
  • Use of double arrow indicates equilibrium (e.g., HCN and water reaction).
  • Water is Amphoteric: Can act as acid or base depending on the reaction context.

Types of Acids

  • Monoprotic Acid: Donates one proton (e.g., HCl, HNO3).
  • Diprotic Acid: Donates two protons, e.g., H2SO4.
    • Ionization Steps:
      • First Step: H2SO4 + H2O → H3O+ + HSO4- (hydrogen sulfate).
      • Second Step: HSO4- + H2O ⇌ H3O+ + SO4^2- (sulfate ion).
  • Triprotic Acid: Donates three protons, e.g., H3PO4 (phosphoric acid).
    • Ionizes in three steps, forming hydronium ions and phosphate ions in sequence.

Polyprotic Acids

  • General: Acids that can donate more than one proton.
  • Examples include carbonic acid (diprotic), phosphoric acid (triprotic), pyrophosphoric acid (4 protons).

These notes cover the key points about the Brønsted-Lowry Theory and the behavior of acids and bases according to this theory. The examples provided help in understanding how acids and bases interact with water and the concept of ionization in different contexts.

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