[scientific]

I had a problem that was as follows:

For
NH₄Cl(s)  NH₃ (g) + HCl (g)
ΔH° = + 176 kJ and ΔG° = +91.2 kJ at 298K. What is the value of ΔG at 1000K?

I was able to solve this by:
1.  ΔG° =  ΔH° - TΔS°
91.2 = 176 - (298)ΔS°
ΔS° = 0.285

2. ΔG = ΔH° - TΔS° (?)
ΔG = 176 - (1000)(0.285)
ΔG = -109

However, I thought enthalpy and entropy were affected by changes in temperature?  Why would I be able to use ΔH° and ΔS° for the second portion (in bold)? Do standard enthalpies/entropies have a standard temperature?

[Corribus]

It is generally assumed that enthalpy and entropy are invariant to temperature changes. It's not strictly true, but for small temperature changes it is not a bad approximation.

In reality, a change of temperature from 298K to 1000K would not usually be considered a small temperature change, and in real practice you'd want to take into consideration how enthalpy and entropy change over this range. But for the purposes of most general chemistry problems, it is commonly understood that entropy and enthalpy do not depend on temperature.

The standard enthalpy and entropy values are reference values determined for the reaction from the corresponding formation reactions, which in turn are standardized for 1 mole of substance formed at a standard pressure value. There is no standard temperature. As before, the standard reference values would change for different temperatures, but it is typical to approximate them as invariant over small temperature changes.