Core Concept

Double displacement reactions involve the exchange of ions between two compounds in solution, resulting in the formation of two new compounds. These reactions follow the general form:

AB+CD→AD+CB\text{AB} + \text{CD} \rightarrow \text{AD} + \text{CB}AB+CD→AD+CB

Where:

• AB and CD are ionic compounds.

• The positive ion (cation) of the first compound (A) swaps with the positive ion of the second compound (C).

Types of Double Displacement Reactions

1. Precipitation Reactions: A solid (precipitate) forms when two aqueous solutions of ionic compounds are mixed.

2. Acid-Base Neutralization Reactions: An acid reacts with a base to produce a salt and water.

3. Gas-Forming Reactions: A gas is produced when certain compounds react, such as when an acid reacts with a carbonate.

Predicting Double Displacement Reactions

To determine if a double displacement reaction will occur, you need to consider the solubility rules and formation of specific products like precipitates, water, or gas.

Solubility Rules

1. Always Soluble:

   • Compounds containing alkali metal ions (Li⁺, Na⁺, K⁺) and ammonium (NH₄⁺).

   • Nitrates (NO₃⁻), acetates (CH₃COO⁻), and perchlorates (ClO₄⁻) are generally soluble.

2. Insoluble Compounds (usually form precipitates):

   • Carbonates (CO₃²⁻), phosphates (PO₄³⁻), and sulfides (S²⁻), unless with alkali metals or ammonium.

   • Halides (Cl⁻, Br⁻, I⁻) are generally soluble except with Ag⁺, Pb²⁺, and Hg₂²⁺.

   • Sulfates (SO₄²⁻) are soluble, except with Ba²⁺, Sr²⁺, and Pb²⁺.

Steps to Predict and Write Double Displacement Reactions

1. Write the Reactants and Swap the Ions:

   • Identify the cations and anions in each reactant.

   • Swap the anions between the two compounds.

2. Write the Products:

   • Combine each cation with the opposite anion to form the products.

   • Be sure to write correct formulas based on the charges of ions.

3. Check Solubility:

   • Use the solubility rules to determine if any of the products are insoluble (forming a precipitate).

   • If a precipitate, gas, or water forms, the reaction will likely occur.

4. Balance the Equation:

   • Adjust coefficients to balance the atoms on both sides of the equation.

Example Problems

Example 1: Precipitation Reaction

Problem: Will a reaction occur if solutions of barium chloride (BaCl2\text{BaCl}_2BaCl2​) and sodium sulfate (Na2SO4\text{Na}_2\text{SO}_4Na2​SO4​) are mixed? Write the balanced equation.

1. Write the Reactants and Swap Ions:

   • Ba2+\text{Ba}^{2+}Ba2+ and Cl−\text{Cl}^-Cl−; Na+\text{Na}^+Na+ and SO42−\text{SO}_4^{2-}SO42−​.

   • Products: BaSO4\text{BaSO}_4BaSO4​ and NaCl\text{NaCl}NaCl.

2. Check Solubility:

   • BaSO4\text{BaSO}_4BaSO4​ is insoluble (forms a precipitate).

   • NaCl\text{NaCl}NaCl is soluble.

3. Write and Balance the Equation:

   BaCl2(aq)+Na2SO4(aq)→BaSO4(s)+2NaCl(aq)\text{BaCl}_2 \text{(aq)} + \text{Na}_2\text{SO}_4 \text{(aq)} \rightarrow \text{BaSO}_4 \text{(s)} + 2\text{NaCl} \text{(aq)}BaCl2​(aq)+Na2​SO4​(aq)→BaSO4​(s)+2NaCl(aq)

Answer: Yes, a reaction occurs, and a precipitate of BaSO4\text{BaSO}_4BaSO4​ forms.

Example 2: Acid-Base Neutralization

Problem: Predict the products and balance the equation for the reaction between hydrochloric acid (HCl\text{HCl}HCl) and potassium hydroxide (KOH\text{KOH}KOH).

1. Write the Reactants and Swap Ions:

   • H+\text{H}^+H+ and Cl−\text{Cl}^-Cl−; K+\text{K}^+K+ and OH−\text{OH}^-OH−.

   • Products: KCl\text{KCl}KCl and H2O\text{H}_2\text{O}H2​O.

2. Check for Solubility:

   • Both KCl\text{KCl}KCl is soluble, and H2O\text{H}_2\text{O}H2​O is a stable liquid.

3. Write and Balance the Equation:

   HCl(aq)+KOH(aq)→KCl(aq)+H2O(l)\text{HCl} \text{(aq)} + \text{KOH} \text{(aq)} \rightarrow \text{KCl} \text{(aq)} + \text{H}_2\text{O} \text{(l)}HCl(aq)+KOH(aq)→KCl(aq)+H2​O(l)

Answer: A neutralization reaction occurs, producing KCl\text{KCl}KCl and water.

Example 3: Gas-Forming Reaction

Problem: Predict the products and balance the equation for the reaction between calcium carbonate (CaCO3\text{CaCO}_3CaCO3​) and hydrochloric acid (HCl\text{HCl}HCl).

1. Write the Reactants and Swap Ions:

   • Ca2+\text{Ca}^{2+}Ca2+ and CO32−\text{CO}_3^{2-}CO32−​; H+\text{H}^+H+ and Cl−\text{Cl}^-Cl−.

   • Products: CaCl2\text{CaCl}_2CaCl2​ and H2CO3\text{H}_2\text{CO}_3H2​CO3​.

2. Decompose the Unstable Product:

   • H2CO3\text{H}_2\text{CO}_3H2​CO3​ is unstable and decomposes into CO2\text{CO}_2CO2​ gas and H2O\text{H}_2\text{O}H2​O.

3. Write and Balance the Equation:

   CaCO3(s)+2HCl(aq)→CaCl2(aq)+CO2(g)+H2O(l)\text{CaCO}_3 \text{(s)} + 2\text{HCl} \text{(aq)} \rightarrow \text{CaCl}_2 \text{(aq)} + \text{CO}_2 \text{(g)} + \text{H}_2\text{O} \text{(l)}CaCO3​(s)+2HCl(aq)→CaCl2​(aq)+CO2​(g)+H2​O(l)

Answer: A gas-forming reaction occurs, producing calcium chloride, carbon dioxide, and water.