Double Displacement Reactions

What is a Double Displacement Reaction?

• Double Displacement Reactions (also known as metathesis reactions) involve the exchange of ions between two ionic compounds.
• Alternative Name: Precipitation reactions (solid may form as a product).

Key Characteristics

• Reaction between two ionic compounds.
• Ions switch partners to form new compounds.
• Follows a predictable pattern:
  • Compound AB reacts with Compound CD.
  • Resulting compounds: AD and CB (A and C are cations; B and D are anions).

General Example

• Compound AB (where A is + and B is -)
• Compound CD (where C is + and D is -)
• Resulting compounds:
  • AD (A pairs with D)
  • CB (C pairs with B)

Example Reaction: Lead Nitrate and Potassium Chloride

• Reactants: Lead nitrate (Pb(NO3)2) and potassium chloride (KCl)

Steps to Predict Products

1. Identify Ions:

• Lead Nitrate: Pb²⁺ and NO₃⁻ (nitrate)
  • Potassium Chloride: K⁺ and Cl⁻ (chloride)

2. Pair Ions: New compound formations

• Pb pairs with Cl → PbCl₂
  • K pairs with NO₃ → KNO₃

3. Check Solubility:

• PbCl₂ is insoluble (solid precipitate)
  • KNO₃ is soluble (aqueous)

4. Balance the Equation:

• Pb(NO3)2 + 2KCl → PbCl2 + 2KNO3

Write Equations

1. Molecular Equation: Shows complete formulas
2. Ionic Equation: Dissociated ions shown as they exist in solution
   • Pb²⁺ + 2NO₃⁻ + 2K⁺ + 2Cl⁻ → PbCl₂ (s) + 2K⁺ + 2NO₃⁻
3. Net Ionic Equation: Removes spectator ions
   • Pb²⁺ + 2Cl⁻ → PbCl₂ (s)

Example Reaction: FeCl3 and NaOH

• Reactants: Iron(III) chloride (FeCl₃) and sodium hydroxide (NaOH)

Steps to Predict Products

1. Identify Ions:
   • FeCl₃: Fe³⁺ and Cl⁻
   • NaOH: Na⁺ and OH⁻
2. Pair Ions: New compound formations
   • Fe pairs with OH → Fe(OH)₃
   • Na pairs with Cl → NaCl
3. Check Solubility:

• Fe(OH)₃ is insoluble (solid precipitate)
  • NaCl is soluble (aqueous)

4. Balance the Equation:

• FeCl₃ + 3NaOH → Fe(OH)₃ + 3NaCl

Write Equations

1. Molecular Equation: Shows complete formulas
2. Ionic Equation: Dissociated ions shown as they exist in solution
   • Fe³⁺ + 3Cl⁻ + 3Na⁺ + 3OH⁻ → Fe(OH)₃ (s) + 3Na⁺ + 3Cl⁻
3. Net Ionic Equation: Removes spectator ions
   • Fe³⁺ + 3OH⁻ → Fe(OH)₃ (s)

Key Practices: Always practice predictions, balance equations, and understand the solubility rules!