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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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