Classify Reactions: Double Displacement MCQs Quiz | Class 10

Understanding Double Displacement Reactions

Double displacement reactions are a fundamental type of chemical reaction where two compounds react, and the positive ions (cations) and negative ions (anions) of the two reactants switch places, forming two new compounds. These reactions are often observed in aqueous solutions and can lead to the formation of a precipitate (an insoluble solid), a gas, or water.

Key Characteristics of Double Displacement Reactions:

• Ion Exchange: The most defining feature is the exchange of ions between the reacting compounds. If reactants are AB and CD, products are AD and CB.
• Occur in Solution: Most commonly, these reactions take place when reactants are dissolved in water, allowing their ions to dissociate and move freely.
• Formation of New Products: Always results in the formation of two new compounds.
• Types of Products: Can produce a precipitate (insoluble solid), a gas (e.g., CO2 from carbonates), or water (in acid-base neutralization reactions).
• Not Redox Reactions: Generally, the oxidation states of the elements involved do not change in a double displacement reaction.

The Reaction: Sodium Sulphate and Barium Chloride Solutions

A classic example of a double displacement reaction, specifically a precipitation reaction, occurs when aqueous solutions of sodium sulphate (Na₂SO₄) and barium chloride (BaCl₂) are mixed.

Reactants:

• Sodium Sulphate (Na₂SO₄): A soluble salt, dissociates into Na⁺ and SO₄²⁻ ions in water.
• Barium Chloride (BaCl₂): A soluble salt, dissociates into Ba²⁺ and Cl⁻ ions in water.

Chemical Equation:

Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq)

Observations and Products:

1. Upon mixing the two clear solutions, a white, insoluble solid immediately forms. This solid is Barium Sulphate (BaSO₄). Barium sulphate is highly insoluble in water, which causes it to precipitate out of the solution. This is the ‘precipitate’ in the reaction.
2. The other product formed is Sodium Chloride (NaCl), which remains dissolved in the solution as it is a soluble salt.

Ionic Equation:

2Na⁺(aq) + SO₄²⁻(aq) + Ba²⁺(aq) + 2Cl⁻(aq) → BaSO₄(s) + 2Na⁺(aq) + 2Cl⁻(aq)

Net Ionic Equation:

Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)

The net ionic equation clearly shows that barium ions and sulphate ions combine to form the insoluble barium sulphate precipitate, while sodium ions and chloride ions remain as spectator ions (they do not participate in the actual chemical change). The formation of the white precipitate is a visual indicator that a chemical reaction has occurred.

Applications and Importance:

• Qualitative Analysis: This reaction is used to test for the presence of sulphate ions (SO₄²⁻) or barium ions (Ba²⁺) in a solution.
• Industrial Processes: Used in various industrial processes, including the production of pigments and in wastewater treatment.
• Understanding Precipitation: Provides a clear example of how solubility rules govern the formation of precipitates in aqueous reactions.

Quick Revision Checklist:

• What is a double displacement reaction? (Ion exchange)
• What are the reactants for this specific reaction? (Sodium sulphate and barium chloride)
• What is the precipitate formed? (Barium sulphate)
• What is its color? (White)
• What is the other product? (Sodium chloride)
• Is it a redox reaction? (No)

Extra Practice Questions:

1. Which of the following would NOT typically result from a double displacement reaction?
   1. Formation of a precipitate
   2. Formation of a new element
   3. Formation of water
   4. Formation of a gas
2. When solutions of lead(II) nitrate and potassium iodide are mixed, a yellow precipitate forms. What type of reaction is this?
   1. Decomposition
   2. Combination
   3. Double displacement
   4. Single displacement
3. What are the spectator ions in the reaction between sodium hydroxide and hydrochloric acid?
   1. H⁺ and OH⁻
   2. Na⁺ and Cl⁻
   3. Na⁺ and H⁺
   4. Cl⁻ and OH⁻
4. Consider the reaction: CuSO₄(aq) + 2NaOH(aq) → Cu(OH)₂(s) + Na₂SO₄(aq). What is the color of the precipitate Cu(OH)₂?
   1. White
   2. Blue
   3. Green
   4. Reddish-brown
5. Which of the following is an example of a double displacement reaction that produces a gas?
   1. HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)
   2. AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq)
   3. Na₂CO₃(aq) + 2HCl(aq) → 2NaCl(aq) + H₂O(l) + CO₂(g)
   4. Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s)