Conjugate Acids & Bases - CIE A Level Chemistry

Introduction

• Brønsted Acid: A species that can donate a proton.
  • Example: HCl can lose a proton to form H⁺ and Cl⁻ ions.
  • Reaction: HCl (aq) → H⁺ (aq) + Cl⁻ (aq)
• Brønsted Base: A species that can accept a proton.
  • Example: OH⁻ can accept a proton to form water.
  • Reaction: OH⁻ (aq) + H⁺ (aq) → H₂O (l)

Equilibrium Reactions

• At equilibrium, reactants and products are present in a solution.
• Example: Ethanoic acid (CH₃COOH) partially dissociates in solution.
  • Equilibrium includes CH₃COOH, H₂O, CH₃COO⁻, and H₃O⁺ ions.
  • Acids donate protons; bases accept protons.

Conjugate Acid-Base Pairs

• A conjugate acid-base pair consists of two species differing by an H⁺ ion.
  • Related by the transfer of a proton.
• Example Reaction:
  • CH₃COOH (acid) ↔ CH₃COO⁻ (base)
  • H₂O (base) ↔ H₃O⁺ (acid)
• In forward reaction:
  • CH₃COOH (acid) is linked to CH₃COO⁻ (conjugate base).
  • H₂O (base) is linked to H₃O⁺ (conjugate acid).
• In reverse reaction:
  • CH₃COO⁻ (base) is linked to CH₃COOH (conjugate acid).
  • H₃O⁺ (acid) is linked to H₂O (conjugate base).

Worked Example

Identifying Conjugate Acid-Base Pairs

• Reaction: NH₃ (g) + H₂O (l) ↔ NH₄⁺ (aq) + OH⁻ (aq)
  • Forward reaction:
    • NH₄⁺ (conjugate acid) of NH₃ (base),
    • OH⁻ (conjugate base) of H₂O (acid).
  • Reverse reaction:
    • NH₃ (conjugate base) of NH₄⁺ (acid),
    • H₂O (conjugate acid) of OH⁻ (base).

Additional Information

• Conjugate pairs are essential for understanding acid-base reactions.
• The concept helps in predicting reaction products and understanding pH changes.

Conclusion

Understanding conjugate acid-base pairs is fundamental to mastering equilibrium reactions in chemistry. This knowledge is crucial for predicting the behavior of acids and bases in solutions, especially in the context of A Level Chemistry.