[sn1sn2e1e2]

ΔG°=27.12 kJ/mol
K (25°C)= 1.77e-5
ΔH°= -2.51 kJ/mol

What is the K value at 35 degrees C?

You use the equation Ln(k2/k1)= (ΔH°/R)(T₁^-1 minus T₂^-1)
and get 1.71e-5

However, when you use the ΔG°=-RTLnK equation, you get something different: 2.51e-5.
Does this mean you cannot use both equations and that one of the equations cannot be fit into this situation? I thought both equations represented at equilibriums values!!

[mjc123]

ΔG varies with temperature. The value given applies to 25°C (at least, it corresponds to -RTlnK(25°C)). To get the value at 35°C you would have to use the ΔH given to calculate ΔS, and recalculate ΔG at 35°C. The first way you did it is simpler (and correct).

[sn1sn2e1e2]

@mjc123,


Just to clarify: You can't directly use ΔG°=-RTLnK since this formula only applies to equilibrium at 273k?

For example, you cannot use this formula to calculate delta G at 1000k. But I can't see why you cannot use this formula to calculate the K at a higher temperature since you are GIVEN "ΔG°" and "the desired temperature". Why can't you just plug in the values and simply get the desired K value of the higher temperature?

ΔG°=-RTLnK

(27.12 kJ/mol)=(-8.3145 J/Kmol)*(kJ/1000J)*(308k)Ln(K)

You get your K at the higher temperature (308k), but I still can't see how this method is wrong. 

[mjc123]

You can use the formula at any temperature, but you cannot use the ΔG° value from one temperature to calculate K at a different temperature, because ΔG° varies with temperature, according to ΔG° = ΔH° - TΔS°. ΔH° and ΔS° are, to a first approximation, invariant with temperature, but obviously ΔG° is not. You must calculate it at the desired temperature in order to use the equation for K.
To clarify (in case this was the problem): ΔG° refers to standard states at a specified temperature, which may be anything - not some universal "standard temperature" such as 273K. So ΔG°(298K) means standard states at 298 K, ΔG°(308K) means standard states at 308 K, etc. - and their values will be different. You need to use the value appropriate to the temperature for which you want to determine K. 27.12 kJ/mol is the value of ΔG°(298K) (not 273K) and you cannot use it to calculate K at 308K.