Homework Help: Physical Chemistry

Posted by Trixie on Friday, February 19, 2010 at 9:29pm.

Calculate the equilibrium constant at 25°C and at 100.°C for each of the following reactions, using data available in Appendix 2A. Remember that the organic molecules are in a separate section behind the organic molecules in Appendix 2A.
(a) HgO(s) Hg(l) + O2(g)
at 25°C
Found to be 5.47E-11 which is correct
at 100.°C

(b) propene (C3H6, g) cyclopropane (C3H6, g)
at 25°C
Found to be 5E-8 which is correct
at 100.°C

I used text to find the Gibbs of formation for each reactant & product to find the Gibbs of reaction. I then used G=-RTlnK to solve for K. I was successful in solving for K for the 25°C temps in both cases, but for some reason the 100.°C answers were incorrect. I thought I only had to plug in the new temp (as 373K) into the equation??

Physical Chemistry - DrBob222, Friday, February 19, 2010 at 9:45pm
I'm not positive I understand exactly; however, the delta G you are looking up is for 25 C. It is not for 100 C. I suspect that is your problem. Can you correct for that by using delta G = delta H - T*delta S.
Physical Chemistry - Trixie, Friday, February 19, 2010 at 9:59pm
I figured out what I was doing...I ended up using the Van't Hoff Equation and the enthalpies of formation to solve for K at 100 C. This stuff is so time consuming...

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