Chapter 4: Aqueous Reactions and Solutions Stoichiometry

Learning Objectives

• Understand exchange reactions (double displacement reactions).
• Learn to determine ions present in reactions.
• Write chemical formulas for products based on ion exchange.
• Check solubility using provided tables.
• Balance chemical equations.

Exchange Reactions

• Definition: Also called metathesis reactions, involves the exchange or "dancing" of ion partners.
• Example: AX + BY → AY + BX
  • A combines with Y, B combines with X.
  • Example: AgNO3 + KCl → AgCl + KNO3

Steps for Completing and Balancing Exchange Reactions

1. Determine Ions Present: Essential for simplifying process.
2. Write Chemical Formulas for Products: Based on ion exchange.
3. Check Solubility: Use solubility rules; often provided on exams.
4. Balance the Equation: Crucial skill practiced often.

Example Reactions

• Ni(NO3)2 + NaOH

  • Identify ions: Ni²⁺, NO₃⁻, Na⁺, OH⁻
  • Write products: Ni(OH)₂, NaNO₃
  • Check solubility: Ni(OH)₂ solid, NaNO₃ aqueous
  • Balance equation: Ni(NO3)2 + 2NaOH → Ni(OH)2 + 2NaNO₃

• NaOH + K2SO4

  • Identify ions: Na⁺, OH⁻, K⁺, SO₄²⁻
  • Write products: Na2SO4, KOH
  • Check solubility: Both products aqueous
  • Balance equation: 2NaOH + K2SO4 → Na2SO4 + 2KOH

Ionic Equations

• Molecular Equation: Reactants and products written in molecular form.
• Complete Ionic Equation: Dissociate all strong electrolytes (aqueous) into ions.
• Net Ionic Equation: Remove spectator ions (do not change during reaction).

Example Conversion

• Molecular Example: Na2CO3 + MgSO4 → MgCO3 + Na2SO4
• Complete Ionic: 2Na⁺ + CO₃²⁻ + Mg²⁺ + SO₄²⁻ → MgCO3(s) + 2Na⁺ + SO₄²⁻
• Net Ionic: CO₃²⁻ + Mg²⁺ → MgCO3(s)
  • Spectator Ions: Na⁺, SO₄²⁻

Steps to Writing Net Ionic Equations

1. Write balanced molecular equation.
2. Dissociate strong electrolytes.
3. Identify and cross out spectator ions.
4. Write net ionic equation with remaining species.

Practice Problems

• Example Reaction 1: Ni(NO3)2 + 2NaOH → Ni(OH)2 + 2NaNO3

  • Complete Ionic: Ni²⁺ + 2NO₃⁻ + 2Na⁺ + 2OH⁻ → Ni(OH)2(s) + 2Na⁺ + 2NO₃⁻
  • Net Ionic: Ni²⁺ + 2OH⁻ → Ni(OH)₂
  • Spectator Ions: Na⁺, NO₃⁻

• Example Reaction 2: Cu(NO3)2 + Na2S → CuS + 2NaNO3

  • Complete Ionic: Cu²⁺ + 2NO₃⁻ + 2Na⁺ + S²⁻ → CuS(s) + 2Na⁺ + 2NO₃⁻
  • Net Ionic: Cu²⁺ + S²⁻ → CuS(s)
  • Spectator Ions: Na⁺, NO₃⁻

Important Concepts

• Spectator Ions: Ions that do not change during the reaction.
• Practicing these steps will simplify solving problems on exams.
• Understanding these concepts is critical for advancing in chemistry courses.