Calculate the concentration ratio

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SUMMARY

The discussion centers on calculating the concentration ratio using Nernst's equation and understanding electrode potentials in electrochemical cells. Participants clarify that the total electrode potential is necessary to determine the Fe-ratio and emphasize that at equilibrium, while the cell potential (Ecell) is zero, the individual half-cell potentials are not. The conversation highlights the importance of correctly calculating cell potentials by simply subtracting the half-cell potentials without reversing the sign of one. This method simplifies the process and avoids common misconceptions.

PREREQUISITES
  • Nernst's equation for calculating electrode potentials
  • Understanding of half-cell reactions in electrochemistry
  • Concept of cell potential and equilibrium in electrochemical cells
  • Knowledge of oxidation and reduction reactions
NEXT STEPS
  • Study the application of Nernst's equation in various electrochemical scenarios
  • Learn about the calculation of standard electrode potentials for different half-reactions
  • Explore the concept of equilibrium in electrochemical cells and its implications
  • Investigate the differences between oxidation and reduction reactions in electrochemical contexts
USEFUL FOR

Chemistry students, electrochemists, and anyone involved in the study or application of electrochemical reactions and cell potentials.

Kqwert
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Homework Statement
Hello,

If we have the half-reactions:

Fe3++e- -> Fe2+
and
2H+ + 2e- -> H2

Calculate the ratio [Fe3+] / [Fe2+] at pH 2.
Relevant Equations
.
Any hint? I was immediately thinking standard electrode potentials as we have two half reactions, but not sure exactly how to go on..
 
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Not standard electrode potentials. Do you know any equation that says what the real potential is?
 
I think I can use Nernst's equation, but not sure how to calculate the real potential..
 
In general question is incomplete, as it doesn't say anything about a pressure of gaseous hydrogen. I would go for a standard 1 atm.
 
How do I go on and solve it? I assume I can calculate the electrode potential of the hydrogen reaction, but I need the total electrode potential in order to find the Fe-ratio, right? Any hints?
 
At equilibrium both cells have the same potential.
 
Ok, I thought Ecell was 0 at equilibrium??
 
Yes, but it doesn't mean potentials of both half cells are zero. It means difference between them is zero, that is something entirely different.
 
Thank you. When saying "difference between them are zero". Is that when the cell potential is calculated on reduction form? I.e. both half reactions are given as reduction reactions. I know another way of looking at it is writing one reaction as an oxidation reaction and the other as a reduction rx.
 
  • #10
Doesn't matter whether the reaction is reduction of oxidation, it occurs at exactly the same potential.

Your problem stem from the fact you were probably taught to calculate the cell potential by adding half cells but reversing the sign of one of them - this is completely unnecessary. Just subtract them and math will do the trick of reversing the sign if necessary (-(-1) is 1 after all).
 

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