Lecture Notes: Autoionization of Water and pH Concepts

Introduction to Water as Amphoteric

• Water is amphoteric: can act as both an acid and a base.
  • Bronsted-Lowry Acid: Proton donor.
  • Bronsted-Lowry Base: Proton acceptor.

Autoionization of Water

• Process:
  • A water molecule donates a proton to another water molecule.
  • Forms hydronium ion (H3O+) and hydroxide ion (OH-).
  • Involves lone pairs of electrons on oxygen.
• Chemical Reaction:
  • H2O + H2O ↔ H3O+ + OH-
• Kw - Autoionization Constant:
  • Special equilibrium expression for water: Kw = [H3O+][OH-]
  • At 25°C, Kw = 1.0 x 10^-14
  • Indicates equilibrium lies far to the left (low concentration of ions).

Concentration and Neutrality

• At equilibrium in pure water:
  • [H3O+] = [OH-] = 1.0 x 10^-7 M
  • Water is neutral when [H3O+] = [OH-].
• Acidic Solution:
  • [H3O+] > [OH-]
• Basic Solution:
  • [OH-] > [H3O+]

Example Problem: Lemon Juice

• Given:
  • [H3O+] = 2.2 x 10^-3 M
• Find [OH-]:
  • Use equation: [H3O+][OH-] = Kw
  • Substitute: (2.2 x 10^-3 M)[OH-] = 1.0 x 10^-14
  • Solve for [OH-]:
    • [OH-] = 4.5 x 10^-12 M
• Results:
  • [H3O+] > [OH-]
  • Lemon juice is acidic.

Conclusion

• The autoionization constant (Kw) helps determine the nature of a solution: acidic, basic, or neutral.
• Understanding the balance of hydronium and hydroxide ions is essential to classify solutions.