Posted by DeeDee on Saturday, November 17, 2012 at 6:38pm.
It would be so much easier to find what you're doing wrong if you posted how you did one of them (with numbers).
dHrxn = (n*dHf products) - (n*dHf reactants)
Watch the number of significant figures.
BTW I posted a response to your NH4NO3 calculation.
I saw, and it was SO helpful! Thank you!!
Okay, here is how I did (b):
(4)x(-285.8) - (1)(11.1)=-1150
My table has 9.16 for N2O4 but I've assume you looked this up correctly in your table.
4(-285.8)-(11.1) = -1154.3 is what I obtained. Also, note the question tells you to enter to the tenths place. You've rounded to 1150 AND didn't enter to the tenths.
I noticed that right after I wrote you, and typed in -1154.3 thinking I had solved it, but the program rejected it. I don't know any other possible answer. I did the equations the same way for all three reactions, and the third one was counted correct while the others are not, no matter how many times or different ways I try to write it. All the enthalpy numbers I got were provided by the program too.
Thank you so much for helping and clarifying!! It was very helpful. I will keep this page bookmarked as to show my prof.
Thanks for sharing!
If you care to type your three solutions, including the one that was counted right, I might pick out the problem. I'm thinking it is a significant figure problem. If we multiply -285.8 x 4 we get -1143.2 but we are allowed only 4 places so this would round to -1143. Then we add in -11.1 for -1154.1 as the answer. Does that make sense to you?
Ah, yes it does!
Here are the three complete:
(a)
2 H2S(g) + 3 O2(g) -> 2 H2O(l) + 2 SO2(g)
[2(-285.8)+2(-296.8)] - [2(-20.6)] =
(-571.6 + -593.6) - (-41.2) = -1124 (-1124.0?)
(b)
N2O4(g) + 4 H2(g) -> N2(g) + 4 H2O(g)
[4(-285.8)] - [1(11.1)]= -1154.3
(c)
SO2(g) + 1/2 O2(g) -> SO3(g)
-395.7 - -296.8 = -98.9
(c) -98.9 was correct, but the others were not, even though I did the exact same way, to a T. Also, I tripled checked all my numbers and they are consistent with the numbers provided on the site for the problem.
This question is due tonight at 11:30 and I have used up all my free tries.
Thank you so much for your continued help!
I think a is 1124. Putting the zero on makes it too many s.f.
I think b is 1154.1 as suggested earlier.
c works out to be -98.90 but the problem says to the tenth; therefore the 0 should not be there. Hope this takes care of the problem.