Question

If standard reduction potential \((E^0)\) of \((\text{Al}_{(aq)}^{+3}|\text{Al}_{(s)})\), \((\text{Fe}_{(aq)}^{+2}|\text{Fe}_{(s)})\), \((\text{Cu}_{(aq)}^{+2}|\text{Cu}_{(s)})\) and \((\text{Ag}_{(aq)}^{+1}|\text{Ag}_{(s)})\) are \(-1\cdot 66\) V, \(-0\cdot 44\) V, \(+0\cdot 34\) V and \(+0\cdot 79\) V respectively. Which of the following reaction is non spontaneous ?

• A. \(2\text{Ag}_{(s)}+\text{Fe}_{(aq)}^{+2}\rightarrow 2\text{Ag}_{(aq)}^{+1}+\text{Fe}_{(s)}\)
• B. \(2\text{Al}_{(s)}+3\text{Cu}_{(aq)}^{+2}\rightarrow 2\text{Al}_{(aq)}^{+3}+3\text{Cu}_{(s)}\)
• C. \(\text{Fe}_{(s)}+\text{Cu}_{(aq)}^{+2}\rightarrow \text{Fe}_{(aq)}^{+2}+\text{Cu}_{(s)}\)
• D. \(2\text{Al}_{(s)}+3\text{Fe}_{(aq)}^{+2}\rightarrow 2\text{Al}_{(aq)}^{+3}+3\text{Fe}_{(s)}\)

Hint:

A reaction is spontaneous when the cell EMF, E(cathode) minus E(anode), is positive.

Answer 1

The Correct Option is A

Step 1: Use the activity series:
A metal with a more negative reduction potential displaces a metal with a less negative (more positive) one from solution. The order is Al, Fe, Cu, Ag.

Step 2: Test each reaction:
(B), (C) and (D) have a more reactive metal (Al, Fe, Al) displacing a less reactive ion (Cu, Cu, Fe), so they go forward.
In (A) silver, the least reactive, would have to displace iron, which cannot happen. So (A) is non spontaneous.

Final Answer:
Reaction (A). \[ \boxed{\text{(A)}} \]