molar Gibbs free energy

[vpena_1I]

For the change in molar Gibbs free energy of NH3(l) --> NH3(g), how is it that you can plug in any temperature for ΔG=ΔH-TΔS when the ΔH and ΔS are the change in enthalpy/entropy of vaporization? I thought you would need to use the specific temperature at which NH3 vaporizes to be able to use ΔHvap and ΔSvap.

[Kevin Xu 4F]

I think when you are referring to calculating the standard change in gibbs free energy, that is G not, the delta H and delta S in this case are the delta H not and delta G not at standard temperature and pressure, 25 degrees C and 1 atm. Thus, the delta H and delta S remain constant while the temperature is changing, and the delta G calculated is delta G not. Delta G reaction however, would have varying values for delta H and delta S based on the reaction temperature.

[vpena_1I]

I think when you are referring to calculating the standard change in gibbs free energy, that is G not, the delta H and delta S in this case are the delta H not and delta G not at standard temperature and pressure, 25 degrees C and 1 atm. Thus, the delta H and delta S remain constant while the temperature is changing, and the delta G calculated is delta G not. Delta G reaction however, would have varying values for delta H and delta S based on the reaction temperature.

The problem I am referring to doesn't specify, but I thought it was talking about ΔGrxn.
It reads
"4J.3 Calculate the change in molar Gibbs free energy for the process NH3(l) --> NH3(g) at 1 atm and (a) -15C; (b) -45C (see Tables 4C.1 and 4F.1). In each case, indicate whether vaporization would be spontaneous."

In the solutions manual, it says to use the formula
ΔG=ΔHvap-TΔSvap

Like you said, I thought ΔHvap and ΔSvap would be specific to only one temperature, being the temperature at which NH3 vaporizes. That's why I am confused as to why the solution to this problem would be just to plug in the different temperatures into the same formula.