Types of Reactions: Double Displacement Reactions

🔄 Quick Recap

We've learned about displacement reactions where a more reactive element replaces a less reactive element in a compound. Now, let's explore double displacement reactions, where two compounds exchange their ions to form two new compounds.

📚 What are Double Displacement Reactions?

A double displacement reaction (also called double replacement reaction) occurs when the ions of two compounds exchange places to form two new compounds.

The general form is:

AB + CD → AD + CB

It's like a dance where both partners switch at the same time!

🧪 Activity Time! Observing a Double Displacement Reaction

Activity: Formation of Barium Sulphate

Materials Needed:

• Sodium sulphate solution
• Barium chloride solution
• Two test tubes
• Test tube stand

Procedure:

1. Take about 3 mL of sodium sulphate solution in a test tube
2. Take about 3 mL of barium chloride solution in another test tube
3. Mix the two solutions
4. Observe what happens

Observations:

• A white solid forms immediately in the mixture
• This solid does not dissolve and settles at the bottom of the test tube

What Happened?

This is a classic double displacement reaction. The sodium and barium ions exchange places:

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

The white solid is barium sulphate (BaSO₄), which is insoluble in water and forms a precipitate.

🔍 Precipitation Reactions

One of the most common types of double displacement reactions is a precipitation reaction.

A precipitation reaction occurs when two solutions mix and an insoluble solid (precipitate) forms. The precipitate forms when certain positive and negative ions combine.

For example, when lead nitrate solution is mixed with potassium iodide solution:

Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) + 2KNO₃(aq)

A yellow precipitate of lead iodide (PbI₂) forms.

🌍 Real-Life Applications of Double Displacement Reactions

1. Water Treatment

Hard water contains calcium and magnesium ions that can be removed through double displacement reactions. When soap (sodium stearate) reacts with hard water, insoluble calcium or magnesium stearate precipitates:

2C₁₇H₃₅COONa(aq) + Ca²⁺(aq) → (C₁₇H₃₅COO)₂Ca(s) + 2Na⁺(aq)

2. Medical Tests

Many diagnostic tests rely on precipitation reactions to detect substances in blood or urine. For example, testing for chloride ions in urine using silver nitrate:

Cl⁻(aq) + AgNO₃(aq) → AgCl(s) + NO₃⁻(aq)

3. Soil Analysis

Double displacement reactions are used to test soil pH and mineral content.

4. Photography

Traditional photography uses silver halide precipitation reactions to create images.

5. Antacids

Antacids work through double displacement reactions to neutralize excess stomach acid:

NaHCO₃(aq) + HCl(aq) → NaCl(aq) + H₂O(l) + CO₂(g)

🧮 Mathematical Corner: Predicting Precipitates

To predict whether a precipitate will form in a double displacement reaction, you need to know the solubility rules for common ionic compounds:

General Solubility Rules:

1. Most sodium, potassium, and ammonium compounds are soluble
2. Most nitrates and acetates are soluble
3. Most chlorides are soluble (except AgCl, PbCl₂, and HgCl₂)
4. Most sulfates are soluble (except BaSO₄, PbSO₄, and CaSO₄)
5. Most carbonates, phosphates, and sulfides are insoluble
6. Most hydroxides are insoluble (except NaOH and KOH)

Example:

Will a precipitate form when solutions of calcium chloride and sodium carbonate are mixed?

Step 1: Write the equation

CaCl₂(aq) + Na₂CO₃(aq) → CaCO₃(?) + 2NaCl(?)

Step 2: Apply solubility rules

• NaCl is soluble (rule 1 and 3)
• CaCO₃ is insoluble (rule 5)

Step 3: Identify the precipitate

CaCl₂(aq) + Na₂CO₃(aq) → CaCO₃(s) + 2NaCl(aq)

Conclusion: Yes, calcium carbonate will precipitate.

✅ Solved Examples

Example 1:

Write the balanced chemical equation for the reaction between sodium hydroxide and hydrochloric acid, and identify the type of reaction.

Solution:

• Sodium hydroxide: NaOH
• Hydrochloric acid: HCl
• The reaction: NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)
• Type: Double displacement reaction (sodium and hydrogen exchange places)
• This is also a neutralization reaction (acid + base → salt + water)

Example 2:

Predict whether a precipitate will form when solutions of silver nitrate and sodium chloride are mixed. Write the balanced chemical equation.

Solution:

• Silver nitrate: AgNO₃
• Sodium chloride: NaCl
• The reaction: AgNO₃(aq) + NaCl(aq) → AgCl(?) + NaNO₃(?)
• Checking solubility: AgCl is insoluble (rule 3), NaNO₃ is soluble (rules 1 and 2)
• Balanced equation: AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq)
• A white precipitate of silver chloride will form

⚖️ Quick Comparison: Double Displacement vs. Single Displacement

Aspect	Double Displacement	Single Displacement
Number of new compounds	Two	One
Reactants	Two compounds	One element and one compound
Process	Exchange of ions	Replacement of one element
Trigger	Often solubility differences	Difference in reactivity
Common examples	Precipitation, neutralization	Metal displacement

⚠️ Common Misconceptions

1. Misconception: All double displacement reactions form precipitates. Truth: Some form soluble products or gases instead of precipitates.

2. Misconception: Double displacement only occurs with ionic compounds. Truth: While most common with ionic compounds, it can also occur with covalent compounds.

🧠 Memory Trick

Remember "SWAP" for double displacement reactions:

• Solutions mix
• Witness the exchange
• Atoms rearrange
• Products form (often with precipitation)

💡 Key Points to Remember

• Double displacement reactions involve the exchange of ions between two compounds
• The general form is AB + CD → AD + CB
• Precipitation reactions are common types of double displacement reactions
• These reactions are important in water treatment, medical tests, and many industrial processes
• Solubility rules help predict whether a precipitate will form

🤔 Think About It!

1. Why do some double displacement reactions form precipitates while others don't?
2. How might you use a double displacement reaction to test for the presence of specific ions in an unknown solution?
3. Can you think of any double displacement reactions that occur in your kitchen or bathroom?

🔜 What Next?

Next, we'll explore oxidation-reduction reactions, which involve the transfer of electrons between substances!