Standard Free Energy

[anavistas]

So for standard condition ∆G0 the temperature(25 C) is not required? In TBR it says it is one of the conditions but in my textbook it calculates it for any temperature.

Thanks in advance,

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[Isoprop]

Strictly speaking, standard conditions don't have a particular temperature. However, when you look up standard Gibbs free energies (Delta G^o) in tables, they are usually given at 25 degrees Celcius. So unless otherwise stated, assume that the temperature is at 25 degrees Celcius, but keep in mind that standard Gibbs free energies can hypothetically be calculated using any temperature. I've seen a table of standard Gibbs energies at 0 degrees Celsius for example.

 

[eric51]

It depends on what area of chemistry you are in. In organic chem, standard conditions are usually 25C, in physical it's often 0C, and in biochem its often 37C.

 

[anavistas]

Thanks everybody,

I was thinking the same, probably TBR was not precise.

 

[anavistas]

Pobably it's a silly question but my brain doesn't work!
deltaG0 is zero for an element in its standard state and so is deltaH0, but delta S0 is not zero for temperatures over 0K, and deltaqG0= deltaH0 -Tdelta S0, I'm making a mistake somewhere but I can't find it.

 

[eric51]

Pobably it's a silly question but my brain doesn't work!
deltaG0 is zero for an element in its standard state and so is deltaH0, but delta S0 is not zero for temperatures over 0K, and deltaqG0= deltaH0 -Tdelta S0, I'm making a mistake somewhere but I can't find it.

That equation only holds true for reactions, and it really doesn't make any sense to say that deltaG0 for an element in its std state is 0. deltaG0 has to be in comparison to something else. If you want to look at just an element, you use the equation G=H-TS, and for a free element, G is not necessarily 0.

 

[Isoprop]

Also...

The zero is a reference point because we really don't know what the absolute values of G and H. We set the elements in their "natural" states as zero, and the delta G and H for them are obviously zero.

So when we say that the delta G of formation of an element is zero, we don't mean that we decmopose an molecule into elements, the delta g of that reaction is 0. We compare the delta G of formation of the products and the reactants to predict the delta G of that reaction.

 

[Isoprop]

That equation only holds true for reactions, and it really doesn't make any sense to say that deltaG0 for an element in its std state is 0. deltaG0 has to be in comparison to something else. If you want to look at just an element, you use the equation G=H-TS, and for a free element, G is not necessarily 0.

I think he's talking about delta G of formation.

 

[anavistas]

Thanks it makes sense now