Q.Arrange the following metals in the order in which they displace each other from the solution of their salts:
Al, Cu, Fe, Mg and Zn.

The key idea is the reactivity series of metals — a more reactive metal will displace a less reactive metal from the solution of its salt.

Reasoning:

1. Recall the standard reactivity series (most to least reactive): Mg > Al > Zn > Fe > Cu.
2. A metal higher in the series can displace any metal below it from its salt solution.
3. Therefore, the order of displacement ability is the same as the reactivity series.

Why This Works — The Concept of Galvanic Corrosion (Displacement)

When a metal is placed in a solution of a salt of another metal, a displacement reaction occurs if the more reactive metal (the one that loses electrons more easily) is the solid. The more reactive metal acts as the anode (oxidised), while the ions of the less reactive metal in solution get reduced and plate out as the solid metal.

Think of it as a tug-of-war for electrons. The metal with a more negative standard reduction potential (or a lower position in the electrochemical series) holds its electrons less tightly — it is more willing to give them away. So it will "kick out" any metal ion that has a less negative (more positive) reduction potential.

The standard reduction potentials (at 25°C) for the half-reactions involved are:

The more negative the $E^\circ$, the stronger the reducing agent (the metal is more reactive). So the reactivity order from most reactive to least is: Mg > Al > Zn > Fe > Cu.

Step-by-Step Reasoning

1. Identify the most reactive metal.

From the list, Mg has the most negative $E^\circ$ ($-2.37$ V). It will displace every other metal from their salt solutions. For example:

and so on.

2. Identify the next most reactive.

Al ($-1.66$ V) is less reactive than Mg but more reactive than Zn, Fe, and Cu. So Al will displace Zn, Fe, and Cu from their salts, but not Mg. For instance:

but Al cannot displace Mg from $\text{Mg}^{2+}$ because Mg is more reactive.

3. Continue down the series.

Zn ($-0.76$ V) will displace Fe and Cu, but not Mg or Al.

Fe ($-0.44$ V) will displace only Cu, but not Mg, Al, or Zn.

Cu ($+0.34$ V) is the least reactive — it cannot displace any of the others. In fact, Cu itself will be displaced by all the others.

4. Arrange the order of displacement.

The metal that displaces the most others is placed first. So the order in which they displace each other (from most displacing to least displacing) is exactly the reactivity series:

Method: Electrochemical Series (Reactivity Series) Approach

This method uses the standard reduction potential (or reactivity) of metals to predict displacement reactions. A more reactive metal (higher on the reactivity series) will displace a less reactive metal from its salt solution.

Steps:

1. Recall the standard reactivity series for the given metals (from most reactive to least reactive):

   • Mg (most reactive)
   • Al
   • Zn
   • Fe
   • Cu (least reactive)

2. Understand the displacement rule:

   A metal can displace any metal below it in the series from its salt solution.

   For example, Mg can displace Al, Zn, Fe, and Cu from their salts.

3. Arrange the metals in decreasing order of reactivity (this is the order of displacement ability):

   • Mg → Al → Zn → Fe → Cu

4. Write the final order (from most displacing to least displacing):

   • Mg > Al > Zn > Fe > Cu

Key Concept (Why this works):

• More reactive metals have a greater tendency to lose electrons (oxidise) and thus reduce the ions of less reactive metals.
• In the electrochemical series, metals with more negative reduction potentials are stronger reducing agents and will displace those with less negative (or positive) potentials.

Final Answer:

Here are the common mistakes students make when tackling this galvanic series / displacement question, along with how to avoid each.

Mistake 1: Confusing the Reactivity Series Order

The Error: Students often misplace Al and Zn, or put Fe before Mg. A common wrong order is: Mg > Al > Zn > Fe > Cu (correct) vs. Mg > Zn > Al > Fe > Cu (incorrect — Al is actually more reactive than Zn in the electrochemical series).

Why it happens: In many everyday contexts (like the reactivity series taught in Class 10), Zn appears more reactive than Al because Al forms a protective oxide layer. However, in displacement reactions from salt solutions (aqueous medium), the standard electrode potential ($E^\circ$) is the true guide.

How to avoid:

• Memorise the electrochemical series for these five metals using their standard reduction potentials ($E^\circ$):
  • Mg: $-2.37\ \text{V}$ (most reactive)
  • Al: $-1.66\ \text{V}$
  • Zn: $-0.76\ \text{V}$
  • Fe: $-0.44\ \text{V}$
  • Cu: $+0.34\ \text{V}$ (least reactive)
• Trick: Remember the mnemonic "Mighty Al Zebras Fight Copper" (Mg > Al > Zn > Fe > Cu).
• Key rule: A metal with a more negative $E^\circ$ will displace a metal with a less negative (or positive) $E^\circ$ from its salt solution.

Mistake 2: Forgetting the Displacement Direction

The Error: Students write the order as Cu > Fe > Zn > Al > Mg (the reverse of the correct order).

Why it happens: They confuse "displaces" with "is displaced by." The question asks: "in the order in which they displace each other" — meaning the most reactive metal (which displaces all others) comes first.

How to avoid:

• Read carefully: "Displace each other" means metal A displaces metal B from B's salt. The most reactive metal displaces the most others.
• Use a simple test: If you put Mg metal into a solution of CuSO$_4$, Mg displaces Cu. So Mg is more reactive than Cu. Therefore, Mg comes before Cu in the list.
• Correct order: Mg > Al > Zn > Fe > Cu (most reactive to least reactive).

Mistake 3: Ignoring the Role of the Oxide Layer on Aluminium

The Error: Students think Al is less reactive than Zn because Al doesn't visibly react with water or dilute acids in the lab.

Why it happens: Al has a tough, adherent oxide layer ($\text{Al}_2\text{O}_3$) that passivates it. In displacement reactions from salt solutions, this layer can slow down the reaction, but thermodynamically, Al is still more reactive than Zn.

How to avoid:

• Distinguish between kinetics and thermodynamics: The oxide layer affects rate (how fast), not spontaneity (whether it happens). In a salt solution, the oxide layer may dissolve or be breached, and Al will eventually displace Zn$^{2+}$.
• Always use the standard electrode potential for the order, not your lab observation of a single reaction.

Mistake 4: Writing the Order as a List Without Justification

The Error: Students just write Mg, Al, Zn, Fe, Cu without showing the reasoning or the displacement reactions.

Why it happens: They memorise the order but don't understand the underlying concept.

How to avoid:

• Show at least one displacement reaction for each pair. For example:
  • Mg displaces Al: $3\text{Mg} + 2\text{Al}^{3+} \rightarrow 3\text{Mg}^{2+} + 2\text{Al}$
  • Al displaces Zn: $2\text{Al} + 3\text{Zn}^{2+} \rightarrow 2\text{Al}^{3+} + 3\text{Zn}$
  • Zn displaces Fe: $\text{Zn} + \text{Fe}^{2+} \rightarrow \text{Zn}^{2+} + \text{Fe}$
  • Fe displaces Cu: $\text{Fe} + \text{Cu}^{2+} \rightarrow \text{Fe}^{2+} + \text{Cu}$
• Conclusion: Since Mg displaces all others, it is the most reactive. Cu is displaced by all, so it is the least reactive.

Final Correct Answer (Exam-Ready Format)

Correct order: $\boxed{\text{Mg} > \text{Al} > \text{Zn} > \text{Fe} > \text{Cu}}$

Reasoning: Based on standard electrode potentials ($E^\circ$):

• Mg ($-2.37\ \text{V}$) has the most negative $E^\circ$, so it is the strongest reducing agent and displaces all others.
• Cu ($+0.34\ \text{V}$) has the most positive $E^\circ$, so it is the weakest reducing agent and is displaced by all others.

Common mistake to avoid: Do not reverse the order or misplace Al and Zn. Use the electrochemical series, not memory tricks that ignore the oxide layer.