Gibbs Free Energy, How to find G with only Hstand and Gstand

[royblaze]

Homework Statement

For the decomposition of Ag2O(s), what is the Gibbs free energy change at 500K (assuming delta H and delta S do not vary with temparature?

delta standard Gformation: -61.0 kJ/mol
delta standard Hformation: -29.7 kJ/mol

Homework Equations

deltaG = deltaH - T(deltaS)

The Attempt at a Solution

I'm just not sure about how to do this. I had asked a TA how to do this question, he used the G = H - TS and solved for S, then fit it back in, but that doesn't DO anything other than prove his equation true... I looked it up on the internet, found some Gibbs HelmHoltz equation. But we've never learned that ANYWHERE: how am I supposed to solve this one WITHOUT the HelmHoltz equation?!

 

[PhaseShifter]

For the decomposition of Ag2O(s), what is the Gibbs free energy change at 500K (assuming delta H and delta S do not vary with temparature?

delta standard Gformation: -61.0 kJ/mol
delta standard Hformation: -29.7 kJ/mol

Homework Equations

deltaG = deltaH - T(deltaS)

You have delta G as a function of two constants and T...

 

[royblaze]

So you need delta S, no?

How do you do it then? I'm still lost. I tried solving for delta S and fitting it back into find a new delta G, but I am not getting the right answer.

 

[Mapes]

So you need delta S, no?

How do you do it then? I'm still lost. I tried solving for delta S and fitting it back into find a new delta G, but I am not getting the right answer.

What value of $\Delta S$ did you get? Perhaps you're using the wrong temperature for standard conditions?

 

[royblaze]

Okay so I have:

G = H - T*S

So for standard conditions, the T = 298.15K

-61 = -29.7 - T*S

S = (-61 + 29.7) / (-298.15) = .1049807144 KJ/mol-K

But S is in J/mol-k, so S = 104.9807144 J/mol-k

Then:

G = (-29.7) - (500)(104.9807144 J/mol-K)(1 kJ/1000J) = -82.1903572 kJ/mol

Is that right? The answer was +82.2 kJ/mol, though. Why is my answer negative?

 

[Mapes]

You found the Gibbs free energy of formation. The problem asks about decomposition.

 

[royblaze]

Ah, thank you so much.