Conjugate Acids & Bases - CIE A Level Chemistry

Key Concepts

Brønsted Acids and Bases

• Brønsted Acid: A species that donates a proton (H+).

  • Example: Hydrogen chloride (HCl) donates a proton to form H+ and Cl- ions.

• Brønsted Base: A species that accepts a proton.

  • Example: Hydroxide ion (OH-) accepts a proton to form water (H2O).

Equilibrium Reactions

• At equilibrium, both reactants and products are present.
  • Example: Ethanoic acid (CH3COOH) partially dissociates in water.
  • Equilibrium species: CH3COOH, H2O, CH3COO-, H3O+.

Conjugate Acid-Base Pairs

• Definition: Two species differing by an H+ ion.

  • Conjugate Acid: Formed when a base gains a proton.
  • Conjugate Base: Formed when an acid loses a proton.

• Example Reaction: CH3COOH ↔ CH3COO- + H+

  • CH3COOH is the conjugate acid of CH3COO-.
  • CH3COO- is the conjugate base of CH3COOH.
  • H2O is the conjugate base of H3O+.
  • H3O+ is the conjugate acid of H2O.

Worked Example

• Equilibrium Reaction: NH3 (g) + H2O (l) ↔ NH4+ (aq) + OH- (aq)
  • Forward Reaction:
    • NH4+ is the conjugate acid of NH3.
    • OH- is the conjugate base of H2O.
  • Reverse Reaction:
    • NH3 is the conjugate base of NH4+.
    • H2O is the conjugate acid of OH-.

Conclusion

• Understanding conjugate acid-base pairs is crucial for analyzing proton transfer reactions and equilibrium states in chemistry.
• Recognizing these pairs helps in predicting reaction directions and understanding chemical behaviors under different conditions.