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Hydrogen Bond
Name: Ria B.
Status: student
Grade: 9-12
Location:CA
Country: N/A
Date: 4/7/2005
Question:
What is meant by hydrogen bonding? I have studied a
little about Aqueous Solutions, and the term came up a few times.... but
sadly, I fail to know what it means. If you could help me to understand,
I would greatly appreciate it.
Replies:
Ria,
We know that water has the chemical formula H2O, and that the way they are
connected is H-O-H (that is, with the oxygen in the middle). Now, imagine
container wherein we have a bunch of water molecules, a bunch of H-O-H.
then, imagine a second container wherein we have another bunch of water
molecules, only this time, the hydrogens in these particular water
molecules have been marked - maybe something like this: H*-O-H* (never
mind how we can do this). Now suppose we poured all of the water in
container 2 into container 1. After a few seconds, we inspect the mixed
water molecules and what we find is that a large percentage of the water
has become H-O-H* (or H*-O-H). What can you conclude from this? Think
about it first before reading the next paragraph.
We must conclude that somehow, water exchanges hydrogens very easily and
very rapidly.
Now, we combine this idea with other information such as other molecules
which are about the same as water in mass, and size (and other chemical
properties) but we know do not exchange hydrogens the way water does. We
find that these other molecules tend to be gases at room-temperature -
which means that these molecules do not attract each other with the same
strength as water - which is a liquid at room-temperature. We therefore
have to conclude that this rapid exchange of hydrogens that water exhibits
must strengthen its attraction with other water molecules.
This then is hydrogen bonding, an explanation for why water molecules
exhibit a much stronger intermolecular attractive force with other water
molecules, and which comes from the fact that the hydrogen-exchange causes
a "bridging" effect between water molecules and holds them tightly to each
other - tighter than other molecules of the same size would.
Greg (Roberto Gregorius)
The element hydrogen forms a special type of bond unique to it -- hence
the name. Because hydrogen has only a single electron, when it forms a
bond with another element that is electronegative (that is, an element
that has a strong attraction for electrons such as F, Cl, O on the
right hand side of the periodic table) the positive charge of the proton of
the hydrogen is left partially "exposed" since there are no "inner"
electrons to shield the proton from surrounding electrons, and the single
electron from hydrogen is preferentially pulled toward the
electron-attracting element. This partial positive charge can then form
weak bonds by attraction from electrons in other molecules. While in
absolute terms of "bond strength" these hydrogen bonds is weak (about 1-3
kcal/mol compared to 10-30 kcal/mol for "regular" chemical bonds) they are
strong enough to have a large effect on the properties of hydrogen-bonded
molecules. In fact it is hydrogen bonds that hold the strands of DNA in
the double helix, and account for the unusual properties of water, such as
its high boiling point and its large solvency for ionic and polar solutes.
Vince Calder
Hi Ria!
In order to understand what is "hydrogen bonding"
you must go a little before at bonds in general.
I hope you know that covalent bonds and electrovalent
bonds strictly exist only in extreme cases. Actually
when atoms (and ions) relate to form some kind of
combination both types happens (in different
proportions, of course) in order that the minimum
energy state is achieved. Then the concept of bond
polarity is applied as justified by the molecular
bond theory.
Now the so called "polar bond" (mostly covalent
bond) have partial ionic character.
Ions interact with molecules about the same way
they interact with ions of opposite charge.
If the molecule is polar it is oriented in the ion
electric
field so that the dipole end of the opposite charge
is directed toward the ion. In any case, molecule is
polar or not, the ion presence forms a dipole moment.
in the molecule.
There are different ways that can happen . One is
called "permanent dipole-permanent dipole" and one of
them is what you want to know: the hydrogen bond.
Usually hydrogen has a valence number of 1- (minus 1)
Then it can share an electron pair forming a covalent
bond or can gain an electron an form an hidride ion.
When bonded covalently to a highly electronegative
element(F, N, O) the hydrogen atom carries some
high partial charge and due to its very low positive
nucleus (1 proton) it is possible for it to get close
to a second ion with partial negative charge.
Then there happen a formation of a strong
dipole-dipole bond.
In the water case it is represented as:
O--------H...............................O
So one hydrogen in the water system has a strong
covalent bond to one oxygen but is also attracted
to another oxygen from another water molecule.
I hope that helps you
Thanks for asking NEWTON!
Mabel
(Dr. Mabel Rodrigues)
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Update: February 2012
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