E and E°, G and G°

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Michael Lonsway 3O
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Joined: Wed Sep 21, 2016 2:57 pm

E and E°, G and G°

Postby Michael Lonsway 3O » Sun Feb 12, 2017 1:28 pm

What's the difference between E and Ecell° as well as G and deltaG°? I know that they're used in the same formulas but I'm wondering under what conditions they'd change. I'm also wondering the names for each of the symbols since I know that there is Gibbs free energy and cell potential but I don't know the names of the other two.

Jessica Chern 1H
Posts: 22
Joined: Sat Jul 09, 2016 3:00 am

Re: E and E°, G and G°

Postby Jessica Chern 1H » Sun Feb 12, 2017 1:57 pm

Well generally, ° means it's under standard conditions. So for example, E is different from E° when the concentrations of the products and reactants are not the same.

Desiree Martin 2A
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Joined: Fri Jul 22, 2016 3:00 am

Re: E and E°, G and G°

Postby Desiree Martin 2A » Sun Feb 12, 2017 2:07 pm

E Cell and E Cell°are different in that E Cell°is the standard cell potential, while E Cell merely represents the cell potential. The Standard Cell Potential is measured when all species taking part are in their standard states. You use the standard cell potentials given for half equations to figure out the total Standard Cell Potential=cathode-anode. The units will be in 1V for both E Cell and E Cell°. The value for E Cell will usually be given, while the value of E Cell° will have to be calculated using the formula given above(Standard Cell Potential=cathode-anode.) If E Cell° is greater than zero, then it usually means the reaction is spontaneous when you relate it to the equation deltaG°=-n*F*E Cell°. Now, deltaG represents the maximum non-expansion work that a reaction can do at constant pressure and temperature. This answer is usually in joules or kJ(C*V/1000). While deltaG° represents the Standard Reaction Gibbs Free Energy with the units also in joules or kJ(C*V/1000). The Standard Reaction Gibbs Free Energy and Gibbs Free Energy represent if a reaction is spontaneous or not. A reaction is spontaneous when the values of deltaG and deltaG°are negative.

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Re: E and E°, G and G°

Postby Esther_Choe_1K » Sun Feb 12, 2017 2:30 pm

I wanted to clarify/add to the explanation above.
The textbook has a table with the E node values of the REDUCTION reactions (aka standard reduction potentials) we need to know for this class. If you have the balanced reduction reaction, you can look up the E node of the reaction.
You can plug this E node value into the equation E = Enode - (RT/nF)*ln(P/R) to find the E of the reaction given the concentration of products and reactants.

So far in class and in the textbook problems, we've mostly considered delta G more than just G because we want to know the change in Gibbs free energy for reactions and phase changes since the sign of the change lets us know if the process is spontaneous or not. Do not worry about the G but rather focus on what delta G means and the sign of delta G.

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