Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Electrochemistry question: n value in ΔG° =-nFE°cell

  1. Jan 13, 2016 #1
    If I want to find the standard ΔG for a reaction in an electrochemical cell, I can use the equation ΔG° =-nFE° where n is the number of moles transferred, F is Faraday's constant, and E° is the standard emf of the cell. If I know the standard reduction potentials of the redox reactions happening in each half-cell, I can calculate the standard emf with the equation E°cell=E°reduction, anode-E°reduction, cathode. Standard reduction potentials are calculated under standard conditions, so all concentrations are at 1 molar. Now, consider the redox reaction:
    2Fe3+ + 2Cl- ---> 2Fe2+ + Cl2
    The two half reactions are:
    2Fe3+ + 2e- ---> 2Fe2+
    2Cl----> Cl2 + 2e-

    So if I calculate E°cell and want to use it to find ΔG°, I just have to determine the value of n, the number of moles of electrons transferred, and plug the values into ΔG° =-nFE°cell. Two electrons are transferred in the balanced equation above, so I could understand using n=2. However, this is the number of moles of electrons transferred for every 2 moles of Fe3+ and Cl- that react, as you can see from the equation. I am trying to calculate the standard ΔG, in which all concentrations are 1 molar, so n=1, correct? 1 mole of electrons is transferred for every one mole of Fe3+ and Cl- that react, and since we are dealing with concentrations of 1 molar, the "concentration" of electrons being transferred is 1 molar (I put concentration in quotes because the electrons themselves are dissolved in solution, but I refer to the number of moles in terms of concentration just to make my point clear.

    So does anyone know if n should be 1 or 2? Thanks!
  2. jcsd
  3. Jan 14, 2016 #2

    James Pelezo

    User Avatar
    Gold Member

    n = number moles electrons involved in 'balanced' oxidation-reduction process. Example ...
    Cr3+ + 3e- => Cro; Eo = - 0.74v (Oxidation Rxn)
    3Ag+ + 3e- => 3Ago; Eo = + 0.80v (Reduction Rxn)
    Cro => Cr3+ + 3e-
    3Ag+ + 3e- => 3Ago
    Cro + 3Ag+ => Cr3+ + 3Ago (Net Oxidation-Reduction Rxn)

    ∆Go = -nFEo
    n = 3 moles e- for balanced redox equation
    F = 96,500 Coulombs
    Eo = EoRedn - EoOxdn = (+0.80v) - (-0.74v) = +1.54v
    ∆Go = - (3)(96,500 C)(1.54v) = -445,830 volt-Coulombs = -445,430 joules = -446 Kj (3 sig. figs)
    For your problem, n = 2.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Threads - Electrochemistry question value Date
Electrochemistry question Aug 27, 2016
Electrochemistry question Apr 20, 2015
Electrochemistry cathode question Nov 19, 2012
Detailed electrochemistry question Mar 6, 2012
Electrochemistry question Jan 16, 2012