Double-Replacement Reactions Study Guide

DOUBLE-REPLACEMENT REACTIONS

Overview

  • A double-replacement reaction (also called a double-displacement reaction) occurs when two ionic compounds react in solution to form two new ionic compounds. These reactions typically involve the exchange of ions between the reactants.
  • To predict the products of double-replacement reactions, solubility rules are used to determine whether a precipitate, a gas, or water is formed. If no such product forms, the reaction is termed "no reaction".

Steps for Completing Reactions

  1. Identify the reactants. Write down the formulas for the reactants and identify their states (aqueous (aq), solid (s), gas (g), etc.).
  2. Use solubility rules to determine the solubility of the product compounds to predict whether a precipitate will form.
  3. Write the products. Swap the cations or anions to write the new compounds formed.
  4. Balance the equation to ensure the law of conservation of mass is followed.
  5. Write the net ionic equation by removing the spectator ions from the full ionic equation. If no reaction occurs, state "no reaction".

Specific Reactions

Example (a)
  • Reaction: CuCl₂ (aq) + Na₂S (aq)
  • Products: Based on solubility rules, sodium compounds (like Na₂S) are soluble, while copper sulfide (CuS) is generally insoluble and will precipitate.
  • Balanced Equation:
    ext{CuCl}_2 (aq) + ext{Na}_2 ext{S} (aq)
    ightarrow ext{CuS} (s) + 2 ext{NaCl} (aq)
  • Net Ionic Equation:
    ext{Cu}^{2+} (aq) + ext{S}^{2-} (aq)
    ightarrow ext{CuS} (s)
Example (b)
  • Reaction: MgBr₂ (aq) + K₂CO₃ (aq)
  • Products: Magnesium carbonate (MgCO₃) is insoluble and will precipitate, while potassium bromide (KBr) is soluble.
  • Balanced Equation:
    ext{MgBr}_2 (aq) + ext{K}_2 ext{CO}_3 (aq)
    ightarrow ext{MgCO}_3 (s) + 2 ext{KBr} (aq)
  • Net Ionic Equation:
    ext{Mg}^{2+} (aq) + ext{CO}_3^{2-} (aq)
    ightarrow ext{MgCO}_3 (s)
Example (c)
  • Reaction: BaCl₂ (aq) + K₂SO₄ (aq)
  • Products: Barium sulfate (BaSO₄) is insoluble and will precipitate, while potassium chloride (KCl) is soluble.
  • Balanced Equation:
    ext{BaCl}_2 (aq) + ext{K}_2 ext{SO}_4 (aq)
    ightarrow ext{BaSO}_4 (s) + 2 ext{KCl} (aq)
  • Net Ionic Equation:
    ext{Ba}^{2+} (aq) + ext{SO}_4^{2-} (aq)
    ightarrow ext{BaSO}_4 (s)
Example (d)
  • Reaction: Hg(NO₃)₂ (aq) + KCl (aq)
  • Products: Both mercury(I) chloride (HgCl₂) and potassium nitrate (KNO₃) are soluble in water, which means no precipitate forms. Therefore, this reaction is classified as:
  • Result:
    No reaction