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Question

13. In the galvanic cell:

$$\ce{Pt(s) | I2(g) | I-(aq) || Fe^3+(aq) | Fe^2+(aq) | Pt(s)}$$

  1. Representation of anode is incorrect and cell will not work.
  2. $[\ce{Fe^3+}] = [\ce{Fe^2+}] = [\ce{I-}] = \pu{1 M}$ is sufficient for $E_\mathrm{cell} = E_\mathrm{cell}^\circ.$
  3. $\ce{I-}$ gets oxidized to $\ce{I2}$ and $\ce{Fe^3+}$ gets reduced to $\ce{Fe^2+}.$
  4. None of these.

Given Answer

3. $\ce{I-}$ gets oxidized to $\ce{I2}$ and $\ce{Fe^3+}$ gets reduced to $\ce{Fe^2+}.$

My Thoughts

The representation of anodic half confuses me. I am unsure whether it is right representation. According to me, it should be $\ce{I-} | \ce{I2}$ (anodic half). It seems to me that answer should be 1.

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  • $\begingroup$ Both 1 and 3 seems correct.. $\endgroup$
    – Zenix
    Apr 14, 2020 at 10:25
  • $\begingroup$ But wouldn't both answers collide with each other? $\endgroup$ Apr 14, 2020 at 12:21

1 Answer 1

1
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The chapter 1 of the reference 1 describes the Line Notation for Galvanic Cells as follows:

A shorthand way of completely describing an electrochemical cell such as Galvanic cell is called line notation. It is a shorthand way of expressing the redox reaction progressing in the electrochemical cell. In this notation:

  1. A slash (/) or a vertical line (|) represents a phase boundary, and a comma separates two components in the same phase. A double slash (//) or a double vertical line (||) represents a phase boundary whose potential is regarded as a negligible component of the overall cell potential (e.g., salt bridge).
  2. When a gaseous phase is involved, it is written adjacent to its corresponding conducting element.
  3. The constituents of the cathode on the right and the constituents of the anode on the left.
  4. The phases of all reactive species are listed and their concentrations or pressures are given if those species are not in their standard states (i.e. $\pu{1 bar}$ for gasses and $\pu{1M}$ for solutions).

Thus, in the given cathode representation, both $\ce{Fe^3+ (aq)}$ and $\ce{Fe^2+ (aq)}$ are separated by a vertical line (|), which is an incorrect representation, according to the description (1) above. They should be actually separated by a comma (as $\ce{Fe^3+ (aq)}$, $\ce{Fe^2+ (aq)}$) because there is no phase separation between them (both are in same phase).

Also, according to the description (2) above, $\ce{I2 (g)}$ cannot be introduced into $\ce{Pt (s)}$ anode (as indicated in the representation) because $\ce{I2 (g)}$ is the product of the reaction, not a reactant.

Based on these, given answers (1) and (3) should be correct. Also, OP's representation of 'anode half' is also correct since there is phase separation.

References:

  1. Allen J. Bard, Larry R. Faulkner, In Electrochemical Methods: Fundamentals and Applications; Second Edition, John Wiley & Sons, Inc.: New York, NY, 2001 (ISBN 0-471-04372-9).
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