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Author Topic: Why cant chlorine make H-bonds?  (Read 14173 times)

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Gargamel

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Why cant chlorine make H-bonds?
« on: May 31, 2009, 10:35:43 AM »

Hello

I wonder why chlorine can't make hydrogen bonds while fluorine, oxygen and nitrogen can? The electronegativities are:

F 4,0
O 3,5
N 3,0
Cl 3,0

The question is why can NH3 and HF make hydrogenbonds while HCl cannot? The weird thing is especially why can NH3 while HCl cant if we look at the electronegativities?

I think it has something to do with Cl having 3 shells, but i can't understand how it should matters because the electronegativity is the same as nitrogen and Cl has lonepairs as well.

Please help
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Gargamel

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Re: Why cant chlorine make H-bonds?
« Reply #1 on: June 02, 2009, 08:51:12 AM »

I think i found the explanation. In another book i found some boiling points:


F2 -188oC
Cl2 -34oC
Br2  59oC
I2  184oC

Which means the intermolecular forces (London) decrease when the molecules gets bigger (bp increase). The valence electrons in a large molecule is far away from the nucleus and these electrons is therefore easier to move. This means that the valence electrons in fluorine is hold tight to the nucleus and hard to move.

With this in mind, I think the partial charge at a given time is greater in Nitrogen compared to Chlorine (beacuse the electrons is hard to move in Nitrogen and therefore not so dependant on the surrounding charges), and this is why N can make H-bonds while Cl cant. Please confirm if my explanation is correct, because i havent found it all in book. (some parts are from my head, and sometimes its wrong :) )

Thanks
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AWK

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Re: Why cant chlorine make H-bonds?
« Reply #2 on: June 03, 2009, 02:13:01 AM »

Quote
The question is why can NH3 and HF make hydrogenbonds while HCl cannot? The weird thing is especially why can NH3 while HCl cant if we look at the electronegativities?
Both NH4F and NH4Cl exist
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AWK

Gargamel

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Re: Why cant chlorine make H-bonds?
« Reply #3 on: June 03, 2009, 07:39:11 AM »

I am asking about intermolecular forces, not intra.

I mean why can HF connect to another HF molecule. Like why can one H2O connect to another H2O while HCl cannot connect with H-bonds. My books says that HCl can only make London-bonds between eachother and not hydrogenbonds. (London forces/Van der Waalsk forces).

Example:

H-F  - - - H-F - - - H-F
+ -          + -         + -

The partial positive charged H make a H-bond to the partial negative charged neighbor F (dotted line)

Apoligize if my question wasnt precise enough.

So the question is, why Cl not make H-bonds. It is very electronegative.


(im from Denmark, so the text may seem weird :))
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408

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Re: Why cant chlorine make H-bonds?
« Reply #4 on: June 03, 2009, 08:10:30 AM »

H-bonds are by definition only between OH, NH, FH.  It is simply a subcategory of the more general category of dipole-dipole interactions.  So while HCl can form strong dipole dipole interactions, it cannot form a hydrogenbond because H-bonds are defined as being between only OH NH FH.
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Gargamel

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Re: Why cant chlorine make H-bonds?
« Reply #5 on: June 03, 2009, 09:36:08 AM »

Oh i dont think its as simple as that? :)

The H-bonds between NH3 molecules is alot stronger compared to the dipol-dipol bonds between HCl molecules. The boiling points is around (looked up on a graf in my book, not exact numbers)

NH3 -40oC
HCl -85oC

What im trying to say is that there must be more hidden in the strenght of H-bonds than you can see from the electronegativities, N and Cl  are both 3. It must have something to do with the size of Cl (or some other thing) but i need the final hint to understand it.
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typhoon2028

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Re: Why cant chlorine make H-bonds?
« Reply #6 on: June 03, 2009, 09:42:45 AM »

Consider the structures of ammonia and water.

The "hydrogen bond" is made between the slightly positive hydrogen and the lone electron pair.
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Gargamel

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Re: Why cant chlorine make H-bonds?
« Reply #7 on: June 03, 2009, 09:49:06 AM »

Ah offcourse! In water O gets surrounded by 2 H in the neighbour watermolecule and in ammonia N gets surround by 3 H in the neighbour molecule.

My brain relax now :) Thanks for help.
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