Bases and the Ionic Product of Water (KW)

Introduction

• Lecture focuses on bases and the term KW.
• KW is essential for calculating the pH of bases.

Water Dissociation and Equilibrium

• Water dissociates slightly, relating to equilibrium.
• Water can be represented in a simplified equilibrium form:
  • H₂O ⇌ H⁺ + OH^-
• Equilibrium expressions:
  • Normally, KC = [H⁺][OH^-]/[H₂O]
  • Instead, use KW:
    • KW = [H⁺][OH^-]
    • Water is omitted as it’s nearly constant.

Understanding KW

• KW: Ionic product of water.
• KW is used instead of KC due to the equilibrium lying far to the left.
• At room temperature (298 K or 25°C), KW = 1 x 10^-14 mol² dm⁻⁶.

Calculating pH at Different Temperatures

• Room Temperature (298 K):
  • KW = 1 x 10^-14
  • [H⁺] = √KW = 1 x 10^-7
  • pH = -log[H⁺] = 7 (neutral)
• At 100°C (373 K):
  • KW = 51.3 x 10^-14
  • pH = 6.14 (neutral at this temperature)
  • Neutral point changes with temperature.

Calculating pH with NaOH

• For NaOH (0.5 mol dm⁻³):
  • Use KW to calculate [H⁺].
  • pH = 13.7

Examples

• KOH (6.14 mol dm⁻³):
  • pH = 14.79
• Ba(OH)₂ (0.137 mol dm⁻³):
  • Dissociation: Ba(OH)₂ → Ba²⁺ + 2OH^-
  • Adjust concentration to account for 2:1 ratio.
  • pH = 13.44

Conclusion

• Understanding KW aids in calculating pH for bases.
• pH and neutrality change with temperature.
• Important to consider dissociation ratios for accurate pH calculation.

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• Students are encouraged to reference these notes for further clarification and problem-solving.
• Engage with practice questions to reinforce concepts.