deltaGnot = -RTlnkeq and deltag = RTln(q/keq)

[mcgill2012]

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im having trouble with the concept of delta G and delta Gnot. what exactly is the difference, and what allows me to determine which of the two equations i should be using? will i actually need to apply these equations, or is it enough to understand what they are telling me conceptually (ie: that free energy increases with distance from the equilibrium position, when keq is fractional delta Gnot is positive, etc)

 

[cwfergus]

im having trouble with the concept of delta G and delta Gnot. what exactly is the difference, and what allows me to determine which of the two equations i should be using? will i actually need to apply these equations, or is it enough to understand what they are telling me conceptually (ie: that free energy increases with distance from the equilibrium position, when keq is fractional delta Gnot is positive, etc)

1. delta Gnot is the Gibbs free energy in standard conditions. Those are the conditions where you use the formula -RTlnKeq. delta G on the other hand is the gibbs free energy at any given time, given the current ratio of products to reactants. Thats when you use the other formula.

2. It will be obvious which one you use. If they tell you it's at standard conditions, use the first one. If it is not, and you want to calculate the deltaG at a different concentration of reactants/products, you use the second one. The second formula basically just shows that in your body, even though a Gibbs free energy for a reaction is positive, it could be negative if the concentrations are varying and therefore spontaneous.

3. I'd learn how to use them as well. You never know what they'll throw at you

 

[methylethyl88]

delta G and delta Gnot are really one of the same things:

For Gnot, you are in standard conditions so atmospheric pressure is 1ATM, temperature is 25C, and most importantly, all solute species are at 1M.

If deltaG = RTln(q/keq) then deltaG = RTln(q) - RT(keq)

If all solute species are at 1M, then q = [products] / [reactants] = 1. The ln(1) is 0 so then deltaG = -RT(keq) which is the same formula for deltaGnot

 

[mcgill2012]

thanks! so delta Gnot is the delta G for a reaction that hasnt started yet, when all reactants are 1 molar?

 

[pmahesh107]

delta G and delta Gnot are really one of the same things:

For Gnot, you are in standard conditions so atmospheric pressure is 1ATM, temperature is 25C, and most importantly, all solute species are at 1M.

If deltaG = RTln(q/keq) then deltaG = RTln(q) - RT(keq)

If all solute species are at 1M, then q = [products] / [reactants] = 1. The ln(1) is 0 so then deltaG = -RT(keq) which is the same formula for deltaGnot

I don't think you're correct when you say all solute species are at 1 M. If they were all at one molar, then K would also be zero and hence delta Gnaught would be zero. The conditions for delta Gnaught are STP and that the reaction is at equilibrium.

@McGill - the only difference between Gnaught and G is the conditions - ie temperature and concentration of reactants/products.

 

[Rabolisk]

I think he is correct. Your reasoning is flawed..
dG = RTln(Q/Keq).
If Q was 1, then dG = RTln(1/Keq) = -RTlnKeq = dGknot.

 

[pmahesh107]

Oh you're right, since delta G at equilibrium is zero. Sorry!

 

[SKation]

1. delta Gnot is the Gibbs free energy in standard conditions. Those are the conditions where you use the formula -RTlnKeq. delta G on the other hand is the gibbs free energy at any given time, given the current ratio of products to reactants. Thats when you use the other formula.

2. It will be obvious which one you use. If they tell you it's at standard conditions, use the first one. If it is not, and you want to calculate the deltaG at a different concentration of reactants/products, you use the second one. The second formula basically just shows that in your body, even though a Gibbs free energy for a reaction is positive, it could be negative if the concentrations are varying and therefore spontaneous.

3. I'd learn how to use them as well. You never know what they'll throw at you

Hey,

Do u know when we would use the deltaG=-nFE equation. Im using the EK book and im confused on how to differentiate between the two deltaG equations.

Thanks

 

[SKation]

Oh you're right, since delta G at equilibrium is zero. Sorry!

Hey,

im confused on the deltaG=-RTln(K), if we ar ein standard state conditions wouldnt this equal 0?

Thanks!