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The Geniuses' Thread
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It's been a while since i've done chemistry so this may be wrongThe boiling point of water is much higher than that of ammonia, despite the fact that each ammonia molecule could form one more hydrogen bond than a water molecule could...because of the greater electronegativity of oxygen compared with nitrogen, thus the hydrogen bonds in water are stronger than those in ammonia.
The boiling point of water is also higher than the boiling point of hydrogen fluoride, but this time the greater number of possible hydrogen bonds for each water molecule is a more significant factor than the stronger hydrogen bonds in hydrogen fluoride brought about by fluorine's greater electronegativity.
OK, so I could learn this off and regurgitate it if I ever need to but I dislike that, so, I'm asking any Chemistry geniuses (or anyone with a passing knowledge, seeing as this is probably very simple)...why?? Why does the number of possible bonds work for the hydrogen fluoride example, but not for ammonia? The electronegativity difference between oxygen and nitrogen and oxygen and fluorine is almost the same, so that explains nothing, I think...
I haaaaaaate learning things off without having a clue what they mean, and I like when everything is all well-explained and clear in my mind, otherwise I just won't remember it. So errr yeah, anyone care to shed some light on this? Although I think I managed to confuse myself more by typing that out...
you could always ask in the Physics/Chemistry forum for help either
Bit wordy but it might give you an idea (if it's right :pac:)
It's the electronegativity difference between each of the elements (N, O, F) and Hydrogen that influences the bonds. The greater the difference, the stronger the partial charges that are set up (delta minus on the more electronegative element)
From wiki: electronegativities
N - 3.04 (0.86 difference with H)
O - 3.44 (1.24 difference...)
F - 3.98 (1.78 difference...)
I forget how this affects the whole Hydrogen bonding, i can remember the graph in the LC book but not the words beside it
someone who has that book handy will be able to tell ya though. I'm fairly certain there's an explanation where ever the H-bonding is mentioned about the higher boiling point for water compared to the other elements
Edit: here's a link that might help you as well - http://www.chemguide.co.uk/atoms/bonding/hbond.html0 -
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It's been a while since i've done chemistry so this may be wrong you could always ask in the Physics/Chemistry forum for help either
Bit wordy but it might give you an idea (if it's right :pac:)
It's the electronegativity difference between each of the elements (N, O, F) and Hydrogen that influences the bonds. The greater the difference, the stronger the partial charges that are set up (delta minus on the more electronegative element)
From wiki: electronegativities
N - 3.04 (0.86 difference with H)
O - 3.44 (1.24 difference...)
F - 3.98 (1.78 difference...)
I forget how this affects the whole Hydrogen bonding, i can remember the graph in the LC book but not the words beside it someone who has that book handy will be able to tell ya though. I'm fairly certain there's an explanation where ever the H-bonding is mentioned about the higher boiling point for water compared to the other elements
Edit: here's a link that might help you as well - http://www.chemguide.co.uk/atoms/bonding/hbond.html
Thanks so much, seriously! After reading this and much discussion, it all makes sense and I can skip off into the sunset, knowing that I understand. That stuff was all in my head...somewhere...just needed it explained a little bit.
Makes you think a bit though, doesn't it? Ah chemistry 
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shadowninty wrote: »Slow Show: Brainwave this morning - H20 has two lone pairs and two hydrogen ions - thus two hydrogen bonds per molecule. Ammonia only has one lone pair even if its has three hydrogen atoms, so only on hydrogen bond a molecule. Hydrogen Fluorine has far higher electronegativity then Nitrogen hence the higher boiling point of HF compared to Ammonia.
But what do lone pairs have to do with it?
All I know about lone pairs is that they have a greater repulsion than bonding pairs...and NH3 can form three hydrogen bonds, H2O two hydrogen bonds, and HF only one hydrogen bond...why would Ammonia only have one H-bond?
Like I get what you're saying at the end but I can't remember lone pairs having anything to do with the intermolecular forces...so if they do, teach me!. In unrelated news, words like 'intermolecular' add so much weight to any post... 0 -
This is my understanding of it. It is worth bearing in mind that when we did this in class, I didn't have the my textbook with me so I spent the 40 minutes messing with my calculator.But what do lone pairs have to do with it? All I know about lone pairs is that they have a greater repulsion than bonding pairs...and NH3 can form three hydrogen bonds, H2O two hydrogen bonds, and HF only one hydrogen bond...why would Ammonia only have one H-bond?
Like I get what you're saying at the end but I can't remember lone pairs having anything to do with the intermolecular forces...so if they do, teach me!. In unrelated news, words like 'intermolecular' add so much weight to any post...
EDIT: Wait, that's not really what you were asking.
So, from Roland's link:Water as a "perfect" example of hydrogen bonding
Notice that each water molecule can potentially form four hydrogen bonds with surrounding water molecules. There are exactly the right numbers of + hydrogens and lone pairs so that every one of them can be involved in hydrogen bonding.
This is why the boiling point of water is higher than that of ammonia or hydrogen fluoride. In the case of ammonia, the amount of hydrogen bonding is limited by the fact that each nitrogen only has one lone pair. In a group of ammonia molecules, there aren't enough lone pairs to go around to satisfy all the hydrogens.
In hydrogen fluoride, the problem is a shortage of hydrogens. In water, there are exactly the right number of each. Water could be considered as the "perfect" hydrogen bonded system.0 -
But what do lone pairs have to do with it? All I know about lone pairs is that they have a greater repulsion than bonding pairs...and NH3 can form three hydrogen bonds, H2O two hydrogen bonds, and HF only one hydrogen bond...why would Ammonia only have one H-bond?
Like I get what you're saying at the end but I can't remember lone pairs having anything to do with the intermolecular forces...so if they do, teach me!. In unrelated news, words like 'intermolecular' add so much weight to any post...
Ok, after turning to Wikipedia;
So two H-bonds every molecule for water (meaning a single Water Molecule can be part of 4 H-Bonds), while only one H-Bond for HF and Ammonia (again, double that), but HF has a smaller molecular mass thus Ammonia has a lower boiling point.Water is unique because its oxygen atom has two lone pairs and two hydrogen atoms, meaning that the total number of bonds of a water molecule is up to four. For example, hydrogen fluoride—which has three lone pairs on the F atom but only one H atom—can form only two bonds; (ammonia has the opposite problem: three hydrogen atoms but only one lone pair).0 -
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Has anyone here ever competed in the IrEUSO?
I got picked for it and I want to do the biology section, but how much am I expected to know? :eek:
I did several years ago, biology section at all, came second and I got through to the Irish team in the actual EUSO competition (where you get to do experiments and stuff!).. which was conveniently held in Ireland that year.
Amazing time though, and we did quite well overall too.
What year of school are you in? If you're in TY I'd recommend taking a look at the Leaving Cert textbook - know about cell structures (mitochondria etc) and their functions, about DNA.. There was almost certainly a lot more to it but I recall those both coming up a lot in the test. Have you any specific questions? I'll see if I can dig out anything I have from when I did it anyway. 0 -
I did several years ago, biology section at all, came second and I got through to the Irish team in the actual EUSO competition (where you get to do experiments and stuff!).. which was conveniently held in Ireland that year. Amazing time though, and we did quite well overall too.
What year of school are you in? If you're in TY I'd recommend taking a look at the Leaving Cert textbook - know about cell structures (mitochondria etc) and their functions, about DNA.. There was almost certainly a lot more to it but I recall those both coming up a lot in the test. Have you any specific questions? I'll see if I can dig out anything I have from when I did it anyway.
\o/
Firstly, wow, well done!
Im in TY, I planning on buying a textbook at the weekend, and then learning the fook out of it (at the moment im looking into DNA and RNA and all that jazz :P)
Theres one thing that worries me though, what level of knowledge is expected? Like uberdetailed knowledge of certain things, or a general knowledge of everything? It says on the website that OL LC level will suffice, but I talked to someone else who got gold last year and they said HL with some college stuff thrown in, so I dont have the faintest idea anymore
Im horribly worried that Im out of my depth 0 -
\o/
Firstly, wow, well done!
Im in TY, I planning on buying a textbook at the weekend, and then learning the fook out of it (at the moment im looking into DNA and RNA and all that jazz :P)
Theres one thing that worries me though, what level of knowledge is expected? Like uberdetailed knowledge of certain things, or a general knowledge of everything? It says on the website that OL LC level will suffice, but I talked to someone else who got gold last year and they said HL with some college stuff thrown in, so I dont have the faintest idea anymore
Im horribly worried that Im out of my depth
With these academic competitions almost all of the contestants are horribly out of their depth. They usually turn out to be great craic anyway. If I get a quarter of all the questions right in any intervarsity competition, I'm happy with myself!
You'll enjoy it on the day anyway even if you don't believe me now.
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The Great Chinholio wrote: »Ah yes. Thats why I dont come in here much. I always leave looking like derp
I always leave looking like0 -
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Truth. Mr. Hitchens was a hero of mine.Deleted User wrote: »Sad to see Christopher Hitchens go. He was worthy of the Genius Thread.
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