Why do the standard emf and calculated emf values differ in electrochemistry?

[DieCommie]

If E(cell) = E(cathode)-E(anode)

and

E(cell) = (RT/nF)ln K

Then shouldn't E(cathode)-E(anode)=(RT/nF)ln k ?

I have a question asking to find the voltage of the cell and I am given a equilibrium constant... which means i should use the second equation with "k" in it, and i get the right answer. But why does this value differ from the first equation's E(cell)?

I hope this explains my question, Thx!

 

[r3dxP]

If E(cell) = E(cathode)-E(anode)

and

E(cell) = (RT/nF)ln K

Then shouldn't E(cathode)-E(anode)=(RT/nF)ln k ?

I have a question asking to find the voltage of the cell and I am given a equilibrium constant... which means i should use the second equation with "k" in it, and i get the right answer. But why does this value differ from the first equation's E(cell)?

I hope this explains my question, Thx!

i thought E(cell) = E(anode) + E(cathode)

 

[DrMark]

Welcome to the wonderful world of electrochemistry, perhaps the most confusing area of analytical chemistry. First off the cell potential can be expressed as:

E_cell =E⁰_red + E⁰_ox

or

E_cell =E⁰_red - E⁰_red

As to E_cell=RT/nF ln(K) This is incorrect. Think about it where are the standard redox potentials ? This is actually part of the nernst equation and used to calculate half cell effective potentials (i.e. concentration dependence).

for:

ox + ne^- -> red

E=E⁰+RT/nF ln(K) .^.. K=[ox]/[red]

So then:

E_cell=E_red - E_ox

 

[DieCommie]

As to E_cell=RT/nF ln(K) This is incorrect. Think about it where are the standard redox potentials ? This is actually part of the nernst equation and used to calculate half cell effective potentials (i.e. concentration dependence).

Hmm... my text explicitly says E_cell⁰=(RT/nF)(ln k). The description is "Relating the standard emf of the cell to the equilibrium constant". This is why i thought it should equal the emf calculated from E_red+E_ox. Where are the standard redox potentials? I don't know...seems like they should be there. I suppose i understand less than i thought Thanks for trying to help though...

 

[DrMark]

Hmm... my text explicitly says E_cell⁰=(RT/nF)(ln k). The description is "Relating the standard emf of the cell to the equilibrium constant". This is why i thought it should equal the emf calculated from E_red+E_ox. Where are the standard redox potentials? I don't know...seems like they should be there. I suppose i understand less than i thought Thanks for trying to help though...

Sorry, some confusion here on my part. What your talking about is the standard emf, that is when all activities for products and reactants are 1. This then leads to E⁰=RT/nF ln(K) for a give redox reaction. The value of E⁰_red - E⁰_red should give the same result. Edit: Edit: Are both reduction potentials using the same reference electrode.