Calculate the concentration ratio

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Discussion Overview

The discussion revolves around calculating the concentration ratio in an electrochemical cell, focusing on the application of electrode potentials and Nernst's equation. Participants explore various approaches and clarify concepts related to cell potential and half-reactions.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants suggest using standard electrode potentials, while others argue against this approach, seeking the real potential instead.
  • One participant proposes using Nernst's equation but expresses uncertainty about calculating the real potential.
  • Concerns are raised about the completeness of the question, particularly regarding the pressure of gaseous hydrogen, with a suggestion to assume standard conditions of 1 atm.
  • There is a discussion about the relationship between cell potential and equilibrium, with one participant stating that both cells have the same potential at equilibrium.
  • Another participant questions the notion that Ecell is zero at equilibrium, leading to clarification that while Ecell is zero, the potentials of the half-cells are not necessarily zero.
  • Participants discuss the implications of calculating cell potential based on reduction versus oxidation reactions, with one asserting that the potential remains the same regardless of the reaction direction.
  • A challenge is presented regarding the traditional method of calculating cell potential by adding half-cell potentials and reversing the sign of one, suggesting that simply subtracting them suffices.

Areas of Agreement / Disagreement

Participants express differing views on the appropriate method for calculating the concentration ratio and the implications of cell potential at equilibrium. There is no consensus on the best approach or the assumptions necessary for the calculations.

Contextual Notes

The discussion highlights potential limitations, such as the assumption of standard pressure for gaseous hydrogen and the incomplete nature of the original question, which may affect the calculations and interpretations of the results.

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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