lecture recording on 10 March 2025 at 09.49.00 AM

Introduction

• Introduced first-level approximation to describe strength of acids and bases (07/2005).

• Acid solutions categorized as strong or weak.

Acid Strength vs. Concentration

• Acid strength does not correlate with the concentration (e.g., 1M vs. 10M).

• Strength indicates degree of ionization – extent of ionization process with water.

Ionization Process

• Example of ionization: Hydrochloric acid (HCl) dissolves in water.

  • Transfers hydrogen ion (H+) to water, generating chloride ion (Cl-) and hydronium (H3O+).

• Bronsted-Lowry definition:

  • Acid: Donates H+.

  • Base: Accepts H+.

Equilibrium in Reaction

• Acid-base reactions are in equilibrium:

  • Forward reaction: HCl (acid) + water (base).

  • Reverse reaction: Hydronium (acid) and Cl- (base).

• Distinction between forward and reverse reactions:

  • Conjugate base for forward reaction is Cl-, conjugate acid is H3O+.

Strong Acids vs. Weak Acids

• Position of equilibrium determines strength:

  • Strong acids favor products (e.g., HCl ionizes completely).

  • Weak acids favor reactants (e.g., only partially ionize).

• Refresh knowledge of strong acids (Magnificent Seven):

  • Strong acids include: Sulfuric acid (H2SO4), Nitric acid (HNO3), Hydrochloric acid (HCl), and others.

  • Sulfuric acid is diprotic; it can donate two H+ ions sequentially:

    • First ionization: H2SO4 → HSO4- + H3O+.

    • Second ionization: HSO4- → SO4^2- + H3O+ (weak acid).

Acid Dissociation Constant (Ka)

• Ka: Weak acid ionization constant.

• Assumes concentration of water remains constant:

  • Ka expression: [products] / [reactants].

• Strong acids have Ka > 1 while weak acids have Ka < 1.

Strong Bases

• Strong bases: hydroxide containing compounds (e.g., NaOH).

• Solubility is key for strength; soluble ionic hydroxides are strong bases.

• Common strong bases include:

  • Group 1 hydroxides (e.g., NaOH, KOH), some Group 2 compounds.

  • Hydrides (H-) act as strong bases.

Weak Base Ionization Constant (Kb)

• Kb: Base ionization constant similar to Ka:

  • Strong bases have Kb > 1, weak bases have Kb < 1.

Conjugate Acid-Base Pairs

• Each acid-base reaction produces conjugate pairs:

  • Acid donates H+, base accepts H+.

  • Example: HF (acid) → F- (conjugate base).

• Inverse relationship between strength of acid and strength of conjugate base:

  • Stronger acid means weaker conjugate base.

Relationship Between Ka and Kb

• For conjugate pairs: Ka * Kb = Kw (1.0 x 10^-14).

• Kw: autoionization constant for water, indicates equilibrium position favoring reactants due to small water dissociation:

  • Water can act as both acid and base.

Summary

• Understanding acid-base strength requires considering ionization extent and equilibrium positions.

• Knowledge of strong/weak acids and bases, their characteristics, and constants (Ka, Kb) is crucial for chemical calculations and reactions.