Lecture Notes: Precipitation Reactions

Introduction

• Precipitation Reaction: A reaction where components in solution interact, resulting in a solid forming and settling out of the solution.
  • Double Displacement/Replacement Reaction: General form is A⁺B⁻ + C⁺D⁻ → A⁺D⁻ + C⁺B⁻.
  • Reactants: Ionic compounds that are soluble.
  • Products: One product is insoluble and forms a lattice (precipitates out).

Solubility

• Solubility: Maximum concentration of a substance that can dissolve in a solvent.
  • Soluble: Substance dissolves readily.
  • Insoluble: Substance does not dissolve appreciably.
  • Precipitate Formation: Occurs when substance concentration exceeds solubility.

Solubility Rules

Group 1 Metals and Ammonium

• Includes lithium, sodium, potassium, etc.
• Compounds with these cations are always water soluble.

Halide Ions

• Larger Halides (Chloride, Bromide, Iodide):
  • Typically water soluble.
  • Exceptions: Soluble except when combined with Silver⁺, Mercury polycations, Lead²⁺.
• Fluoride:
  • Soluble except with Group 2 metals, Lead²⁺, Iron³⁺.

Other Compounds

• Acetate, Bicarbonate, Nitrate, Chlorate:
  • Always water soluble.
• Sulfates:
  • Usually water soluble, except with certain cations.

Insolubility Rules

• Carbonates, Chromates, Phosphates, Sulfides:
  • Generally insoluble except with Group 1 cations or Ammonium.
• Hydroxides:
  • Insoluble except with Group 1 cations or Barium.

Example: Precipitation Reaction

• Formation of Lead Iodide:
  • Lead²⁺ ions and Iodide ions react to form Lead Iodide (PbI₂).
  • Lead is an exception, making the compound insoluble, thus forming a yellow precipitate.

Net Ionic Equations

• Writing net ionic equations helps identify spectator ions and focus on the formation of the precipitate.
• Example: Formation of Silver Fluoride.

Conclusion

• Understanding solubility and precipitation is crucial for predicting the outcome of reactions in solutions.