Question

Consider the following standard reduction potentials :-

$Fe^{2\oplus} + 2e^{\ominus} \rightleftharpoons Fe$; E° = - 0.41 V

$Ag^{\oplus} + e^{\ominus} \rightleftharpoons Ag$; E° = 0.80 V

$O_2 + 2H_2O + 4e^{\ominus} \longrightarrow 4OH^{\ominus}$; E° = 0.40 V

What would happen if a block of silver metal is connected to a buried iron pipe via a wire:-

• A. The silver metal would corrode, a current would be produced in the wire, and $O_2$ would be reduced on the surface of the iron pipe.
• B. The silver metal would corrode, a current would be produced in the wire, and $Fe^{2\oplus}$ would be reduced on the surface of the iron pipe.
• C. The iron pipe would corrode, a current would be produced in the wire, and $Ag^{\oplus}$ would be reduced on the surface of the silver metal.
• D. The iron pipe would corrode, no current would be produced in the wire, and $O_2$ would be reduced on the surface of the iron pipe.

Answer 1

Answer: (C)

The iron pipe would corrode, a current would be produced in the wire, and $Ag^{\oplus}$ would be reduced on the surface of the silver metal.

Answer 2

The problem describes an electrochemical cell formed by connecting an iron pipe and a silver block in a buried environment (implying the presence of moisture and oxygen). We need to determine which metal corrodes, if a current is produced, and what cathodic reaction occurs.

1. Determine which metal corrodes (acts as anode): We compare the standard reduction potentials of iron and silver: $Fe^{2\oplus} + 2e^{\ominus} \rightleftharpoons Fe$; E° = - 0.41 V $Ag^{\oplus} + e^{\ominus} \rightleftharpoons Ag$; E° = 0.80 V

   The metal with a lower (more negative) standard reduction potential has a greater tendency to be oxidized (corrode). Since E°(Fe²⁺/Fe) (-0.41 V) is much lower than E°(Ag⁺/Ag) (0.80 V), iron will be oxidized and corrode. Anodic reaction (oxidation): $Fe \longrightarrow Fe^{2\oplus} + 2e^{\ominus}$

2. Determine if a current is produced: Since a spontaneous redox reaction occurs (iron corrodes), electrons will flow from the anode (iron) through the wire to the cathode (silver). This flow of electrons constitutes an electric current.

3. Determine the cathodic reaction (reduction): The silver metal acts as the cathode. At the cathode, reduction occurs. In a buried environment with moisture and oxygen, the most common cathodic reaction for corrosion is the reduction of oxygen: $O_2 + 2H_2O + 4e^{\ominus} \longrightarrow 4OH^{\ominus}$; E° = 0.40 V

   Comparing this with the reduction of Ag⁺: $Ag^{\oplus} + e^{\ominus} \rightleftharpoons Ag$; E° = 0.80 V

   While Ag⁺ reduction has a higher potential, there are no Ag⁺ ions present in the soil environment to be reduced. Therefore, the reduction of oxygen is the most plausible cathodic reaction.

Now let's evaluate the given options based on these conclusions:

• (A) The silver metal would corrode, a current would be produced in the wire, and $O_2$ would be reduced on the surface of the iron pipe.

  • "Silver metal would corrode": Incorrect. Iron corrodes.
  • "O₂ would be reduced on the surface of the iron pipe": Incorrect. Reduction occurs on the cathode (silver).

• (B) The silver metal would corrode, a current would be produced in the wire, and $Fe^{2\oplus}$ would be reduced on the surface of the iron pipe.

  • "Silver metal would corrode": Incorrect. Iron corrodes.
  • "$Fe^{2\oplus}$ would be reduced on the surface of the iron pipe": Incorrect. Iron is oxidized to $Fe^{2\oplus}$.

• (C) The iron pipe would corrode, a current would be produced in the wire, and $Ag^{\oplus}$ would be reduced on the surface of the silver metal.

  • "The iron pipe would corrode": Correct.
  • "a current would be produced in the wire": Correct.
  • "$Ag^{\oplus}$ would be reduced on the surface of the silver metal": This part is problematic. As discussed, there are no Ag⁺ ions readily available in the environment for reduction. The most likely cathodic reaction is oxygen reduction. However, given the options, this is the only option that correctly identifies the corroding metal and the production of current. If we strictly consider the reduction potentials and assume hypothetical availability of Ag⁺, its reduction is more favorable than O₂ reduction. In multiple-choice questions, sometimes the "best fit" option must be chosen. The first two parts are definitively correct. The third part, while chemically less probable in a natural buried environment, might be presented to test the knowledge of reduction potentials if Ag⁺ were available.

• (D) The iron pipe would corrode, no current would be produced in the wire, and $O_2$ would be reduced on the surface of the iron pipe.

  • "no current would be produced": Incorrect. A spontaneous reaction produces current.
  • "$O_2$ would be reduced on the surface of the iron pipe": Incorrect. Reduction occurs on the cathode (silver).

Comparing the options, option (C) is the only one that correctly identifies that the iron pipe would corrode and a current would be produced. While the cathodic reaction specified in (C) is not the most realistic one for a buried pipe (O₂ reduction is), it is the only option that has the first two crucial parts correct. If one strictly adheres to reduction potentials, Ag⁺ reduction (E° = 0.80 V) is more favorable than O₂ reduction (E° = 0.40 V) if Ag⁺ were present. Given the choices, (C) is the most plausible answer, assuming the question implies a comparison of reduction potentials for available species, even if Ag⁺ isn't naturally abundant.