8.1 Nature of Acids and Bases

Arrhenius Theory of Acids and Bases

• acids dissociate to produce hydrogen ions (H+(aq))
  • Ex: HCl, H2SO4
• bases dissociate to produce hydroxide ions (OH-(aq))
  • Ex: NaOH, LiOH
• Drawbacks: only includes bases which contain hydroxide ions, and assumes all acid/base reactions occur in aqueous solutions.
  • NH3 (g) + H2O → NH4 + OH

Bronsted-Lowry Theory

• An acid is a hydrogen ion donor
  • Arrhenius theory predicts that dissolving hydrogen fluoride in water dissociates the molecule into a hydrogen ion and fluoride ion. However, the hydrogen ion is not stable, and it reacts with water to form hydronium (H3O(aq)).
• A base is a hydrogen ion acceptor.
  • Instead of being hydroxide producers, they’re hydrogen acceptors.
  • When ammonia dissolves in water, it pulls one of the hydrogen off the water making a hydroxide ion, giving the solution basic properties.

Conjugate Acid-Base Pairs:

• Since the reaction can occur in the reverse direction, we can have an acid/base pair in the reverse direction (opposite as the forward direction).

  

Amphiprotic (amphoteric) substances:

• Able to donate or accept a hydrogen ion, therefore it can act as an acid or a base.
  • Ex: water.

The Acid Ionization Constant, Ka

• We make two simplifications:

  • Since water is a liquid, its concentration is a constant, therefore it becomes part of K.
  • We can simplify the reaction by using hydrogen ions instead of hydronium ions: