# Using the Nernst equation with a gaseous reactant/product

I have a question about using the Nernst equation.

$E = E^\varnothing + (0.059/n)(\log\frac{[Ox]}{[Red]})$

Specifically, I am interested in the reaction of $\ce{MnO4-}$ and $\ce{C2O4^2-}$ below:

$$\ce{2MnO4- + 5C2O4^2- + 16H+ = 2Mn^2+ + 10CO2 + 8H2O}$$

How can I write Nernst equation for $\ce{CO2/C2O4^2-}$ in order to determine the titration curve of this reaction?. I'm confused because $\ce{CO2}$ isn't in the solution, and therefore there isn't a concentration for it.

Is it right if I remove $\ce{CO2}$ from the equation? For example:

$$E = E^\varnothing + \frac{0.059}{2}\left(\log\frac{1}{[\ce{C2O4^{2-}}]}\right)$$

• Hello and welcome to Chemistry.SE! A good way to familiarize yourself with this site is by taking the short tour. Then you can always visit the help center if you have more questions. Regarding your question, this is not a "homework service" site, in that it is against policy to just give answers to homework-type of questions. This post gives a good discussion for how to ask a homework question. The main thing is that you need to show effort towards solving it yourself. – airhuff Mar 9 '17 at 20:28
• Thanks for reply my question. This is'n my homework. I'm having a experiment report in analytical-chemistry and I really need to solve this question. I've read many book and still can't find the right answer. All the document I've read just mention about the case that both Oxidizer and reducing are in the solid. – Sardin Mike Mar 9 '17 at 21:12
• @SardinMike have you ever heard about a reaction constant? How do you use pressure for it then? – ParaH2 Mar 9 '17 at 23:27

HINT

If we consider a reaction such that :

$$\mathrm{n^-\alpha^+Ox^+ + n^+\beta^-Red^- = n^+\alpha^-Ox^- + n^-\beta^+Red^+}$$

Where $\mathrm{Ox^+/Red^+}$ is the redox couple at the positive side of the battery and $\mathrm{Ox^-/Red^-}$ correspond to the negative side redox couple, then can show easely,

$$e=E^+-E^-=e^\circ+\frac{RT}{n^+n^-F}\ln\left(\frac{a_{Ox^+}^{n^-\alpha^+}\cdot a_{Red^-}^{n^+\beta^-}}{a_{Ox^-}^{n^+\alpha^-}\cdot a_{Red^+}^{n^-\beta^+}}\right)$$

Where $e^\circ$ is the stardart electromotrice force.

Now you just need to know what is the activity of a constituant when it is a gas or something else. In this answer you'll find some good informations.

Sorry in advance if there are typo but I think no.

• I looked at this several times. I don't understand the notation. – MaxW Mar 10 '17 at 20:30
• @MaxW which one(s) ? – ParaH2 Mar 10 '17 at 20:33