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Example 17: Calculate the electrode potential of given electrode $$\ce{Pt,Cl2 (\pu{1.5 bar}) | 2 Cl- (\pu{0.01 M})}\,; \quad E^\circ_\ce{Cl2/2Cl-} = \pu{1.36 V}$$

Solution: The reaction of electrode is $$\ce{\underset{\pu{1.5 bar}}{Cl2 (g)} + 2 e- -> \underset{\pu{0.01 M}}{2 Cl-}}$$ $$E = E^\circ - \frac{0.0591}{n}\log{\frac{[\ce{Cl-}]^2}{P_\ce{Cl2}}} = 1.36 - \frac{0.0591}{2}\log{\frac{(0.01)^2}{1.5}} = \pu{1.483 V}$$

In the solution, the author has directly used the pressure (which is in bars) in the Nernst equation without first converting it to its SI units. How can this be right?

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  • $\begingroup$ I would recommend to properly rotate the image, or, even better, type the stuff using MathJax. Most people won't bother trying to decipher what's been photocopied and will just down-vote. Also, Nernst is a name of scientist and should be capitalized, whereas bar is a unit and shouldn't be capitalized. $\endgroup$ – andselisk Jan 3 '18 at 11:19
  • $\begingroup$ @andselisk Sure, I'll keep that in mind. $\endgroup$ – Senthil Arihant Jan 3 '18 at 12:04
  • $\begingroup$ I edited the question for you, feel free to check out whether it's correct; also you can visit this page, this page and this one on how to format your future posts better with MathJax and Markdown. $\endgroup$ – andselisk Jan 3 '18 at 12:06
  • $\begingroup$ No prob at all, just spare some free time and get to know these formatting features; otherwise, as I said, questions are often getting closed and downvoted because of that. $\endgroup$ – andselisk Jan 3 '18 at 12:09
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Just as you are supposed to use activity instead of concentration for dissolved components when applying Nernst equation, you are also supposed to use fugacity instead of the partial pressure for the gaseous components. The reason for that will become transparent if you follow the derivation of Nernst equation using chemical potential.

Fugacity is defined with a reference to the standard state with the pressure of $\pu{10^5 Pa}$ or $\pu{1 bar}$. At ordinary pressures fugacity is numerically approximately equal to the pressure expressed in bars or atmospheres ($\pu{1 bar} \approx \pu{0.987 atm}$), so the author of the book is right.

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  • $\begingroup$ Oh…I didn't know that. We didn't drive Nernst equation…I'm in class 12, and so I guess that's above my level... Well, thanks man! Also, what's the difference between activity and concentration? $\endgroup$ – Senthil Arihant Jan 3 '18 at 12:02
  • $\begingroup$ @SenthilArihant No prob; I'd suggest to read en.wikipedia.org/wiki/Thermodynamic_activity $\endgroup$ – andselisk Jan 3 '18 at 12:07

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