Question

Identify the two incorrect electrochemical reactions shown as taking place in the given cells at the respective electrodes.

• A. C & D
• B. A & C
• C. B & D
• D. A & B

Hint:

In electrochemical cells, always identify the electrode firstAt cathode reduction occurs and at anode oxidation occursThen compare the given half-cell reaction with the standard cell reaction.

Answer 1

The Correct Option is D

Step 1: Understand the question.
The question asks us to identify the two incorrect electrode reactions among the given electrochemical cells.
We need to compare each given reaction with the actual electrode reaction of the respective cell.

Step 2: Analyze reaction A for lead-storage cell.
In a lead-storage cell, the cathode reaction during discharge is:
\[ PbO_2 + SO_4^{2-} + 4H^+ + 2e^- \rightarrow PbSO_4 + 2H_2O \]
But the given reaction is:
\[ Pb + 2SO_4^{2-} + 4H^+ + 2e^- \rightarrow PbSO_4 + 2H_2O + SO_2 \]
This is not the correct cathode reaction of lead-storage cell.
So, reaction A is incorrect.

Step 3: Analyze reaction B for Leclanche cell.
In Leclanche cell, the cathode reaction involves reduction of \( MnO_2 \).
The correct cathodic reaction is generally represented as:
\[ 2MnO_2 + 2NH_4^+ + 2e^- \rightarrow Mn_2O_3 + 2NH_3 + H_2O \]
But the given reaction is:
\[ MnO_2 + NH_4^+ + 2e^- \rightarrow MnO + OH^- + NH_3 \]
This is not the correct cathode reaction for Leclanche cell.
So, reaction B is incorrect.

Step 4: Analyze reaction C for mercury cell.
In mercury cell, zinc amalgam acts as anode.
The anode reaction is:
\[ Zn(Hg) + 2OH^- \rightarrow ZnO(s) + H_2O + 2e^- \]
This matches the given reaction.
So, reaction C is correct.

Step 5: Analyze reaction D for fuel cell.
In alkaline hydrogen-oxygen fuel cell, oxygen is reduced at the cathode.
The cathode reaction is:
\[ O_2(g) + 2H_2O + 4e^- \rightarrow 4OH^- \]
This matches the given reaction.
So, reaction D is correct.

Step 6: Identify the incorrect reactions.
From the above analysis:
\[ A = \text{incorrect} \]
\[ B = \text{incorrect} \]
\[ C = \text{correct} \]
\[ D = \text{correct} \]

Step 7: Conclusion.
Therefore, the two incorrect reactions are A and B.
\[ \boxed{A \& B} \]