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Types of Reactions: Displacement Reactions

🔄 Quick Recap​

So far, we've explored combination reactions (where substances join to form one product) and decomposition reactions (where one substance breaks into multiple products). Now, let's learn about displacement reactions - a fascinating type of chemical reaction where elements compete for partners!

📚 What are Displacement Reactions?​

A displacement reaction occurs when a more reactive element displaces (replaces) a less reactive element from its compound.

The general form is:

A + BC → AC + B

Where A is more reactive than B.

Displacement Reaction

Think of it as a stronger player taking the place of a weaker player in a team!

🧪 Activity Time! Observing a Displacement Reaction​

Activity: Iron Nails in Copper Sulphate Solution​

Materials Needed:​

  • 3 iron nails
  • Copper sulphate solution
  • 2 test tubes
  • Thread
  • Sandpaper

Procedure:​

  1. Clean the iron nails with sandpaper to remove any coating
  2. Fill two test tubes with copper sulphate solution
  3. Tie two nails with thread and immerse them in one test tube
  4. Keep one nail aside for comparison
  5. Wait for 20 minutes and observe the changes

Observations:​

  • The blue color of the copper sulphate solution fades
  • The iron nails develop a brownish coating
  • The solution turns pale green

What Happened?​

Iron is more reactive than copper. It displaces copper from copper sulphate solution:

Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)
  • The blue color fades because copper ions (Cu²⁺) are removed from the solution
  • The brown coating on the nails is copper metal (Cu)
  • The solution turns pale green due to the formation of iron(II) sulphate (FeSO₄)

🔍 The Reactivity Series​

To predict whether a displacement reaction will occur, we need to know the reactivity series of metals. The reactivity series arranges metals in order of their chemical reactivity, from most reactive to least reactive:

Most Reactive → K, Na, Ca, Mg, Al, Zn, Fe, Pb, H, Cu, Hg, Ag, Au ← Least Reactive

A metal can displace any metal below it in the reactivity series from its compounds.

Examples:​

  1. Zinc can displace copper from copper sulphate:

    Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s)

  2. Iron can displace copper from copper chloride:

    Fe(s) + CuCl₂(aq) → FeCl₂(aq) + Cu(s)

  3. But copper cannot displace iron from iron sulphate because copper is less reactive than iron:

    Cu(s) + FeSO₄(aq) → No reaction

🧮 Mathematical Corner: Predicting Displacement Reactions​

To predict whether a displacement reaction will occur, follow these steps:

  1. Identify the metals involved
  2. Check their positions in the reactivity series
  3. If the free metal (not in compound form) is higher in the series than the metal in the compound, displacement will occur
  4. If the free metal is lower in the series, no reaction will occur

Example:​

Will zinc metal displace silver from silver nitrate solution?

Step 1: The metals are zinc (Zn) and silver (Ag) Step 2: In the reactivity series: Zn is higher than Ag Step 3: Since Zn is more reactive than Ag, displacement will occur:

Zn(s) + 2AgNO₃(aq) → Zn(NO₃)₂(aq) + 2Ag(s)

🌍 Real-Life Applications of Displacement Reactions​

1. Metal Extraction​

Less reactive metals like copper and silver are often extracted from their ores through displacement reactions with more reactive metals.

2. Metal Plating​

Electroplating involves displacement reactions to coat one metal with another. For example, iron objects can be coated with zinc (galvanizing) to prevent rusting.

3. Metal Purification​

Some metals are purified through displacement reactions, where impurities are removed by more reactive metals.

4. Batteries​

Many batteries work on the principle of displacement reactions, where one metal displaces ions of another to generate electricity.

✅ Solved Examples​

Example 1:​

What happens when a zinc rod is placed in a copper sulphate solution?

Solution:

  • Zinc is more reactive than copper (higher in the reactivity series)
  • Zinc will displace copper from copper sulphate
  • The reaction is: Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s)
  • Observations: The blue color of CuSO₄ solution fades, and reddish-brown copper deposits on the zinc rod

Example 2:​

Will copper metal displace zinc from zinc sulphate solution? Write the balanced equation if the reaction occurs.

Solution:

  • Copper is less reactive than zinc (lower in the reactivity series)
  • Copper cannot displace zinc from zinc sulphate
  • No reaction will occur
  • The equation would be: Cu(s) + ZnSO₄(aq) → No reaction

⚠️ Common Misconceptions​

  1. Misconception: Any metal can displace any other metal from its compounds. Truth: Only a more reactive metal can displace a less reactive metal.

  2. Misconception: The reactivity of a metal is fixed under all conditions. Truth: Temperature, concentration, and other factors can influence the outcome of displacement reactions in some cases.

🧠 Memory Trick​

To remember the concept of displacement reactions, think of "KARMA":

  • Kick out the less reactive
  • Always the stronger wins
  • Reactivity determines the victor
  • Metals follow the series
  • Activity defines the order

💡 Key Points to Remember​

  • Displacement reactions involve a more reactive element replacing a less reactive element
  • The reactivity series helps predict whether displacement will occur
  • The general form is: A + BC → AC + B (where A is more reactive than B)
  • Common signs include color changes in solution and deposits of the displaced metal
  • These reactions are important in metallurgy, electroplating, and battery technology

🤔 Think About It!​

  1. Why do copper vessels develop a green coating when exposed to moist air for a long time?
  2. How could you use displacement reactions to arrange metals in order of their reactivity?
  3. Why are displacement reactions important in the extraction of metals from their ores?

🔜 What Next?​

Next, we'll explore double displacement reactions, where ions exchange partners - like a chemical dance where everyone switches partners!