Chemical Forums
Chemistry Forums for Students => High School Chemistry Forum => Topic started by: lilmul123 on April 04, 2007, 07:17:46 PM
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It's been a while since I've done questions like these. We're currently in the review stage for the AP Chemistry exam, and my mind is so disorganized, I can't remember how to do these. Er anyway...
Consider the reaction below.
H2(g) + Cl2(g) 2 HCl(g)
(a) Calculate H°, S°, G°, and K (at 298 K) using data in Appendix 4.
I have H S and G: -184, 20, and -190 kJ/mol respectively. Now I need to find K. The only equation I have on an equation sheet is (delta)G = -RT ln(K). When I use this equation to find K, it's extremely large. So large that it gives me an overflow error on my calculator.
This last question, I have the faintest idea on how to do it.
Consider the following reaction.
H2O(g) + Cl2O(g) 2 HOCl(g) K298 = 0.090
Use the data below for Cl2O(g).
G°f = 97.9 kJ/mol
H°f = 80.3 kJ/mol
S° = 266.1 J/Kmol
d. Estimate H°f and S° for HOCl(g).
e. Estimate the value of K at 500. K.
f. Calculate G at 25°C when PH2O = 17 torr, PCl2O = 2.8 torr, and PHOCl = 0.10 torr.
Honestly, I'd love to share my method, but I honestly haven't a clue. since it's at equilibrium (doesnt show up on this page, but the equilibrium sign is there on my homework site), so I figured that i needed to find when G = 0. But...that didn't really help at all. Also, I don't remember how to calculate G at all using pressures. Sorry if I'm asking too much, but I really need to understand this. the AP chem exam is May 15th!
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I have H S and G: -184, 20, and -190 kJ/mol respectively. Now I need to find K. The only equation I have on an equation sheet is (delta)G = -RT ln(K). When I use this equation to find K, it's extremely large. So large that it gives me an overflow error on my calculator.
You are taking the ln of a negative number, no no. You want to take the antilog to solve for K. or e(?G/RT) = K
The other question I can not really make out what those superscripts are. Take it out of the code bracket, that just makes it hard to read. I could copy and paste, but that is too much work :)
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I don't remember how to calculate G at all using pressures.
If you recall ?G = ?G° + RT ln(K)
K constants can be defined in terms of pressure or concentration. This makes sense if you think back to the ideal gas law:
PV = nRT -> P = (n/V)RT = cRT
So pressure is proportional to concentration (at least in the gas phase). That is to say, the K constant for this reaction in terms of pressures would be the same form as it would in terms of concentrations:
products over reactants.