Lecture on Water Equilibrium

Self-Ionization of Water

• Water Molecule Collision: Two water molecules collide, resulting in ionization.
• Ion Formation:
  • One water molecule becomes a hydronium ion (H₃O⁺).
  • The other becomes a hydroxide ion (OH⁻).
• Self-ionization: This process is called self-ionization.
• Reaction Nature:
  • The forward reaction is endothermic (heat-absorbing).
  • Occurs at 25°C (room temperature).

Concentrations at Equilibrium

• Hydronium and Hydroxide Ion Concentrations:
  • Both concentrations are 10⁻⁷ M at 25°C.
  • The concentrations must always be equal in pure water.

Equilibrium Constant for Water (Kw)

• Expression for Kw:
  • Kw = [H₃O⁺][OH⁻]
  • Kw is 10⁻¹⁴ M² at 25°C.
  • Water is not included in the expression because it is in liquid form.

pH and pOH

• pH Formula:

  • pH = -log₁₀ [H₃O⁺]
  • Memorize this formula.

• Hydronium Ion Stability:

  • Hydrogen ions (protons) are unstable in water.
  • Quickly combine with water to form hydronium ions (aqueous).

• Example:

  • When HCl dissociates, H⁺ ions become unstable and form hydronium.

• pOH Formula:

  • pOH = -log₁₀ [OH⁻]
  • May not be familiar, important to know.

Relationship Between pH and pOH

• pH + pOH Relationship:
  • pH + pOH = 14
  • Important to remember.

Important Formulas to Memorize

1. pH = -log₁₀ [H₃O⁺]
2. pOH = -log₁₀ [OH⁻]
3. pH + pOH = 14

Conclusion

• Water Equilibrium: Started with understanding water equilibrium.
• Next Steps: In the next session, will cover calculating pH of solutions.