The answers to following question seem wrong to me. The books has plainly told correct answer is (3), but it seems incorrect to me. Please help.

  1. The chemical reaction $\ce{2AgCl(s) + H2(g) -> 2 HCl(aq) + 2 Ag(s)}$ taking place in a galvanic cell (under standard condition) is represented by the notation $$\ce{Pt(s) | H2(g), \pu{1 bar} | 1 M KCl(aq) | AgCl(s) | Ag(s)}\tag{1}$$ $$\ce{Pt(s) | H2(g), \pu{1 bar} | 1 M HCl(aq) | 1 M Ag+(aq) | Ag(s)}\tag{2}$$ $$\ce{Pt(s) | H2(g), \pu{1 bar} || 1 M HCl(aq) | AgCl(s) | Ag(s)}\tag{3}$$ $$\ce{Pt(s) | H2(g), \pu{1 bar} | 1 M HCl(aq) | Ag(s) | AgCl(s)}\tag{4}$$

Photo of problem: https://i.stack.imgur.com/Oflyg.jpg

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    $\begingroup$ 3 seems blatantly wrong. Don’t think you would even need a salt bridge in this reaction. $\endgroup$ Apr 14 '20 at 9:23
  • $\begingroup$ It would be interesting to hear why it seems wrong to you. If there were a correct answer choice, this question would have been closed as homework. $\endgroup$ Apr 14 '20 at 14:29
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    $\begingroup$ You have developed not very welcome custom to post photos instead of elaborated text of the task question and your explicitly done effort. It may be considered as rather lazy part on your side, putting the burden on shoulders of responders to explicitly type the relevant text referring to you question. Additionally, the photo content cannot be indexed for site searching. It increases chances for the question to be closed for lack of the OP effort. $\endgroup$
    – Poutnik
    Apr 15 '20 at 13:01

The chemical reaction is:

$$\ce{2AgCl(s) + H2(g) -> 2 HCl(aq) + 2 Ag(s)}$$

If we split this into half reactions, we get an oxidation half reaction at the anode of:

$$\ce{H2(g) -> 2 H+(aq) + 2 e-}$$

And a reduction half reaction at the cathode of:

$$\ce{2AgCl(s) + 2e- -> 2 Ag(s) + 2 Cl-(aq)}$$

If you want all the species at standard state, you need a partial pressure of $\pu{1 bar}$ of hydrogen gas, a $\mathrm{pH}$ of about 0, and a chloride concentration of $\pu{1 mol/L}$.

In cells where the reagents can't react directly, you don't need a salt bridge. In the reaction we are looking at, $\ce{AgCl}$ is confined to the cathode because it sticks to it, and hydrogen gas is confined to the anode because that's where we are releasing it, and it has low solubility in water.

The cell notation starts with the conductor of the anode half reaction, and ends with the conductor of the cathode half reaction. Different phases are separated by vertical lines. If there is more than one species in a phase, they are separated by commas.

$$\ce{Pt(s) | H2(g, \pu{1 bar}) | H+(aq, \pu{1 mol/L}) , Cl- (aq, \pu{1 mol/L}) | AgCl(s) | Ag(s)}$$

The books has plainly told 3. to be correct.

The answer is (5) none of the above. Using the format of the answer key, the best answer would be:

$$\ce{Pt(s) | H2(g), \pu{1 bar} | 1 M HCl(aq) | AgCl(s) | Ag(s)}$$

You get this answer from answer 3 by changing the salt-bridge boundary ("||") to a phase boundary ("|"). Having a salt bridge between the hydrogen gas and the aqueous hydrogen ions would mean that you are generating hydrogen ions at the electrode where they are not in contact with $\pu{1 M}$ hydrogen ions.

  • $\begingroup$ Just one more doubt. If I represent it as : Pt(s) | 1M HCL(aq) |H2(g)| Agcl(s)|Ag(s) would it be right? $\endgroup$ Apr 14 '20 at 14:25
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    $\begingroup$ @Adityasuresh No, because the hydrogen gas is not in contact with the silver chloride at the other electrode. The hydrochloric acid, on the other hand, is in contact with both electrodes. $\endgroup$ Apr 14 '20 at 14:28

This question is very similar to the question I nave answered recently: Is this galvanic cell representation right?. I like the answer given by Karsten Theis elsewhere, but it seems OP still did not understand the line notation for electrochemical cells. Thus, I like to give the same description here to clear out OP's doubts.

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, the line notation for the given Galvanic cell (under stranded conditions) for the reaction, $\ce{2AgCl(s) + H2(g) -> 2 HCl(aq) + 2 Ag(s)}$ (as correctly given by Karsten Theis) is:

$$\ce{Pt(s) | H2(g, \pu{1 bar}) | H+(aq, \pu{1 mol/L}) , Cl- (aq, \pu{1 mol/L}) | AgCl(s) | Ag(s)}$$

Any changes to this notation other than combining $\ce{H+ (aq)}$ and $\ce{Cl- (aq)}$ to $\ce{HCl (aq)}$ would be given you a incorrect notation for the given cell reaction.

Note: Basically, all 4 answers are wrong in this measure.


  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).
  • 2
    $\begingroup$ So 1 atm as standard state is incorrect, and 1 bar is correct? Oh, I just looked it up, it changed from 1 atm to 1 bar in 1982 according to Wikipedia... $\endgroup$ Apr 14 '20 at 19:26
  • $\begingroup$ I'm sorry for my sloppyness. It should be $\pu{1 atm}$, according to the book I referenced (which is 2001 book).. I also confused sometimes on the difference between $\pu{atm}$ and $\pu{bar}$ because it is only 0.01! :-) I believe correct standard condition is $\pu{1 atm}$. $\endgroup$ Apr 14 '20 at 19:34
  • $\begingroup$ I think the book is behind the times. The update was from atm to bar, so they should have bar, at least in the second edition. $\endgroup$ Apr 14 '20 at 19:48
  • $\begingroup$ It was the second edition. Still have atm. $\endgroup$ Apr 14 '20 at 19:51
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    $\begingroup$ Clarification: Your first comment is correct. In 1982 it has been changed to $\pu{10^5 Pa}$ ($\pu{1 Bar}$) by IUPAC. However, the difference is so small that most scientist seemingly have ignored the change. For example, according to the same Wikipedia article, NIST uses a temperature of $\pu{20 ^\circ C}$ ($\pu{293.15 K}$ or $\pu{68 ^\circ F}$) and an absolute pressure of $\pu{1 atm}$. Yet, $\pu{1 bar}$ is IUPAC accepted STP condition internationally. $\endgroup$ Apr 14 '20 at 20:25

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