Calculating equilibrium constant

[littlebearrrr]

Homework Statement

Estimate the value of the equilibrium constant at 670K for the following reaction: 2NO2(g)⇌N2O4(g)

Homework Equations

ΔG°rxn = -RT*lnK

The Attempt at a Solution

This should be an easy problem. But for some reason, MasteringChem won't accept it, so I'm wondering if I did anything wrong. Here's what I did:

1) Using tabulated free energies of formation, I obtained ΔG°rxn=-2.8 kJ=-2.8x10^3 J.
2) I then divided this by 8.314 J/molK multiplied by 670K (multiplied by -1, which cancels).
3) I took e^ of both sides. K=1.65

Anyone know what I'm doing wrong? Just a small hint will suffice. Thank you in advance :)

 

[DrClaude]

1) Using tabulated free energies of formation, I obtained ΔG°rxn=-2.8 kJ=-2.8x10^3 J.

I get the same answer.

2) I then divided this by 8.314 J/molK multiplied by 670K (multiplied by -1, which cancels).

I would've used 8.315 J K^-1 mol^-1, but I don't think that's the problem here.

3) I took e^ of both sides. K=1.65

This is where things go bad! Could you show your calculation?

 

[Chestermiller]

In your table, what temperature is that ΔG^o at? I bet it's not at 670 K.

Chet

 

[littlebearrrr]

@ DrClaude and Chestermiller:

Sorry for the late response. Thank you for reviewing my post. To DrClaude: I forgot to account for the temperature in my calculations, as noted by Chestermiller. I calculated the std. change in enthalpy and entropy, and used the values to find ΔG^o at 670K. Then I plugged it into the equation from my first post to get a new answer of K=1.34e-5 (which turned out to be correct). Thank you both once again for looking at my post, and Chestermiller for offering that helpful bit of advice. :)

 

[DeeNos]

Your approach looks correct, so it is possible that there is an issue with the values you used for the free energies of formation. Double check that you are using the values for the correct temperature of 670K. It is also possible that there is a rounding error or a small mistake in your calculations. Review your steps and make sure all units are consistent. If you still cannot find the issue, try using a different source for the free energies of formation or ask your instructor for assistance.