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Electrochemical cells: Is gibbs free energy dependent upon concentration? 
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#1
Mar2411, 11:02 AM

P: 1,672

Is the gibbs free energy of some reaction in an electrochemical cell dependent upon reactant concentration? The Nernst equation clearly states that open circuit potential is dependent upon concentration, and basic thermodynamics states that open circuit potential is directly proportional to gibbs free energy, so my intuition says yes. However, some sources (not the most credible) I have found say no. My electrochem text doesn't seem to give a definitive answer.
Approaching the question from another angle, I would think that entropy would be higher with a system with a lower concentration of reactants so given a constant temperature the gibbs free energy should be less via [tex]\Delta[/tex]G = [tex]\Delta[/tex]H  T[tex]\Delta[/tex]S. 


#2
Mar2411, 01:57 PM

P: 2

Please reorient yourself to see the forest through the trees. Take time to review and reflect on the fundamentals. Ask for help. If you are indeed truly overwhelmed, then perhaps you should orient your studies elsewhere. Only you know the answer to that... 


#3
Mar2411, 02:01 PM

P: 2

looking at this another way. If as you mention delta T = 0 then delta H = 0. Therefore, delta G = (1)* T * delta S, subsequently if concentration increases delta S is negative. Negative times negative is positive therefore delta G is positive!!!!! I hope that simplifies the concept and brings clarity for you. :)



#4
Mar2411, 02:49 PM

P: 1,672

Electrochemical cells: Is gibbs free energy dependent upon concentration?



#5
Mar2411, 09:02 PM

HW Helper
P: 1,987

Your intuition is right. E.g. eliminating complicating factors of different materials, imagine a 'concentration cell', so same electrode material both sides, same ions but 2 different concentrations. If the concentrations on both sides are equal there will be no current nor potential, so that answers you question, it must depend on concentration. For an electric current to flow is a way towards equalising the concentrations. So that will happen unless stopped by an opposing potential. Magnitude RT ln (c_{1}/c_{2}). (Ignoring junction potentials.) You worry about the sign! (However in any given setup this will be clear, so this is just about conventions.) Same damned RT ln c that you get in every damned chemical thermodynamic thing. I would not completely agree that anyone whose grasp of thermodynamics is shaky, unconfident, unstable should get out of the field because that would be a lot of people, also in any other field at all nearby they will still need the thermodynamics. 


#6
Mar2411, 11:07 PM

P: 1,672

I thought it would be a well understood concept given the popular equation RTln(K) = [tex]\Delta[/tex]G, but when everyone tells you "no" you start to question your intuition. That darn equation shows up in all of my electrochem, combustion, and stat thermo books. 


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