Gibbs Free Energy help

1. Mar 7, 2009

Swerting

1. The problem statement, all variables and given/known data
The reaction is $$NH_{4}Cl(s)\rightarrow NH_{3}(g)+HCl(g)$$
$$\Delta H^{o}=+176 kJ and \Delta G^{o}=+91.2 kJ$$ at 298 K
What is the value of $$\Delta G$$ at 1000 K?

2. Relevant equations
$$\Delta G=\Delta H-T\Delta S$$
The same applies if all 'deltas' are standard

3. The attempt at a solution
Well, I solved for standard change of entropy ($$\Delta S^{o}$$) and came up with .284 kJ/K, which is the same when using a table of standard entropies. My problem is, I'm not sure where to go from this to find $$\Delta G$$, or even a way to link standard values to normal values for these. I tried plugging in the values for standard delta H and delta S with 1000K to find delta G, but something tells me that this is incorrect. Any help would be greatly appreciated!

-Swerting

2. Mar 7, 2009

Mapes

Why do you think your first approach is incorrect?

3. Mar 7, 2009

Swerting

I believe that my first approach is incorrect since standard values are at 298.15K, and this is asking for delta G at 1000K, implying that delta H and delta S have changed as well.

4. Mar 7, 2009

Mapes

Do you know how to calculate changes in $\Delta H$ and $\Delta S$ with temperature? (Hint: it involves the heat capacity.) Alternatively, you could look up these values at 1000 K.

5. Mar 7, 2009

Swerting

I can calculate $$\Delta H$$ from changes in temperature, but unfortuneately, not $$\Delta S$$, nor could I find a table of entropies at 1000K.

6. Mar 7, 2009

Swerting

Alright, well, in the depths of the internet I finally found the answer explained, and apparently my using standard delta H and S were correct. The answers from the company that make the questions say that delta S doesn't change (thought says nothing about delta H) and so I just plug in the values and solve for delta G. Ah well, who would've guessed that values so dependant on temperature don't really change over an actual temperature change. :P
Thank you very much for your help!

-Swerting