# Gibbs Free Energy help

## Homework Statement

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?

## Homework Equations

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

## 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

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Mapes
Homework Helper
Gold Member
Why do you think your first approach is incorrect?

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.

Mapes
Homework Helper
Gold Member
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.

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

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