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Old 03-29-06, 08:32 PM   #1
EJ123
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Jschen, please help me on Chem:]

Im doing a worksheet and the book is all confusing lol.
Im going to take an educated guess on them first.

The amount of heat released when one mole of vapor condenses is called its _____. (that I have no idea lol)

These are either: Always, Never, or Sometimes:

1. Melting and vaporization are exothermic processes. (I think Never true)
2. In order to convert 1 mole of H20(L) to 1 mol of H2O(g), 40.7 kj must be supplied. ( Always true?)
3. When ice metls the temperatures of the ice increases until the entire sample becomes liquid. (Sometimes true?)
4.When ammonium nitrate dissovles in water, the solution gets cold. This is an example of an exothermic reaction. (Always true?) Yeah yeah these are newsbish easy questions I should know, but the book is confusing,(and the class is really boring)

Jschen you should be our teacher and do experiments with food inside the bomb calorimeter=D
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Old 03-29-06, 08:43 PM   #2
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Originally Posted by EJ123
.

The amount of heat released when one mole of vapor condenses is called its _____. (that I have no idea lol)
molar heat of condensation
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Old 03-29-06, 08:44 PM   #3
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Quote:
Originally Posted by EJ123
The amount of heat released when one mole of vapor condenses is called its _____. (that I have no idea lol)
That's just a matter of definition. I'll let you look it up. [e] Or look at the above response.

Quote:
Originally Posted by EJ123
These are either: Always, Never, or Sometimes:

1. Melting and vaporization are exothermic processes. (I think Never true)
2. In order to convert 1 mole of H20(L) to 1 mol of H2O(g), 40.7 kj must be supplied. ( Always true?)
3. When ice metls the temperatures of the ice increases until the entire sample becomes liquid. (Sometimes true?)
4.When ammonium nitrate dissovles in water, the solution gets cold. This is an example of an exothermic reaction. (Always true?)
Correct on the first and second counts. Wrong on the third and fourth.

(1) You must supply heat to melt or vaporize something. Therefore, it's an endothermic process. So it's most definitely not exothermic.
(2) Follows from (1).
(3) At what temperature does ice melt? Given your answer, what's the only answer to (3) that's consistent with that answer? (As for why this is the case, that's a bit more complex, and we can discuss it further if you want.)
(4) Review your definitions of exothermic and endothermic. Hint: "therm" has to do with heat, "exo" means out, and "endo" means in.
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Old 03-29-06, 08:47 PM   #4
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1. never (endothermic)
2. always
3 . never true, the ice will stay at the same temp until all the ice melts, then the temp of the water will continue to rise until the water boils... the points at which the temp starts to rise is called the critical point i believe
4. i donno, im too tired to think.. i hate chem
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Old 03-29-06, 08:50 PM   #5
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Oh yeah 4 is endo.
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Old 03-29-06, 08:51 PM   #6
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I have chem first period so im half asleep lol.
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Old 03-29-06, 08:53 PM   #7
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Quote:
Originally Posted by EJ123
I have chem first period so im half asleep lol.
read your book, and make sure you know all your definitions. Chem makes 100 times more sense if you know your definitions (ie. endo/exo-thermic reactions...)
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Old 03-29-06, 08:55 PM   #8
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I know what those mean lol.
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Old 03-29-06, 09:02 PM   #9
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I have chem first period so im half asleep lol.
I never thought I'd hear that from you. Just bring a few Monsters to class. That'll keep ya going for a few hours.
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Old 03-29-06, 09:08 PM   #10
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In fact I do haha. I have one saved for tomorrow
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Old 03-29-06, 09:25 PM   #11
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Quote:
Originally Posted by EJ123
3. When ice metls the temperatures of the ice increases until the entire sample becomes liquid. (Sometimes true?)
hey EJ, here's a simple graph that'll explain this question... it should be in your chem book..
Attached Images
File Type: jpg ej123.JPG (40.8 KB, 8 views)
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Old 03-29-06, 11:02 PM   #12
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EJ123, stop PM'ing me with chemistry questions! Just post here, so others can answer, too!
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Old 03-29-06, 11:06 PM   #13
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so others can answer, too!
hahaha, thats all i have to contribute to this thread, and any other chem related thread, i never did so hot in my chem classes at university.
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Old 03-29-06, 11:08 PM   #14
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EJ123, stop PM'ing me with chemistry questions! Just post here, so others can answer, too!
Haha ok=p
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Old 03-30-06, 08:50 AM   #15
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Wow, amazingly I actually half remembered the answers to a couple of those. Haven't seen anything resembling Chem since sophmore year in college (now finishing 1st year Grad school after a 5 year Undergrad). Maybe I didn't sleep through as many of those classes as I thought. (Chem was at 8:30am!)
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Old 03-30-06, 11:47 PM   #16
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Hey Jschen i just need something cleared up. (REALLY basic)

If the molecules are Polar then they are held together by dipole-dipole attraction or hydrogen bonding?
and..
If the molecules are non-polar they are sometimes hold toghether by dispersion forces?

Sometimes I get these mixed around! ^ Thats 'correct' isn't it? ISN"T IT?
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Old 03-31-06, 12:03 AM   #17
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blue_neon, you're correct.

Some common interactions that are meaningful on the atomic level are the following, especially the ones in bold:

induced dipole/induced dipole, aka van der Waals, aka London dispersion: weakest, but always present
dipole/induced dipole: still very weak, requires one molecule to have a dipole, and thus be polar
charge/induced dipole: can rival dipole/dipole interactions under optimal conditions
dipole/dipole: starting to be "meaningful" to consider since it's not omnipresent
charge/dipole: significant
charge/charge: very strong
weak bonds: hydrogen bonds and the like, generally overwhelm the above forces

Generally if just looking at the interaction between two small molecules, as you go down the list, the strength is at least an order of magnitude stronger, and so it tends to overwhelm any other forces. So if there's hydrogen bonding, you can basically ignore other forces. If not, but there's charge/charge interactions, those will dominate. However, this is not true when discussing bulk properties or macromolecules (DNA, proteins, etc) since in those cases, the sheer numbers of weak interactions make them no longer insignificant.
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Old 03-31-06, 12:25 AM   #18
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Thanks jschen. Thinking about it now...how can you can a dipole-dipole attraction if the atom is non polar! (to my basic knowlege)
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Old 03-31-06, 12:38 AM   #19
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Well, you ask another interesting question. Atoms in their neutral state are never polar (though they can be polarizable), so I'll assume you mean molecule. It depends on what we mean by a polar molecule (and therefore a nonpolar molecule). If a polar molecule is defined as one with a net dipole, then it's quite possible for a nonpolar molecule to have dipoles in it. It just has to have enough symmetry with dipoles pointing in the correct directions for the dipoles to cancel out. If a polar molecule is defined as one with any meaningful dipole, then well, nonpolar molecules by definition cannot have meaningful dipole-dipole interactions.

This dual definition of polarity results in two commonly used polarity scales that are not very well in sync with each other, so this is a very real issue. As a very simple example, carbon dioxide by the first definition is nonpolar. But by the second definition, it's most definitely polar since the C=O bonds individually have strong dipoles. (But since they point 180 degrees away from each other, the dipoles cancel, so there is no net dipole.) And depending on what you're using to determine whether something is "polar", carbon dioxide can act as something very polar or as something nonpolar.
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Old 03-31-06, 01:41 AM   #20
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Chem makes more since when you read the book... and attend lecture!
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