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Name: Dani M.
Status: student
Age: 17
Location: N/A
Country: N/A
Date: 6/30/2004


Question:
Would you please explain what dipoles are, and what dipole dipole interactions. Especially in regards to water and its properties.


Replies:
Dear Dani,

In physics, a "dipole moment" refers to a separation of two charges along a line, such as the following:

(+)-----(-)

Note that the total charge of the system in this example is zero. However, this system will be affected by an external electric field. If the dipole is otherwise free to rotate, it will do so in such a way as to minimize the interaction energy between the dipole and the field. If two dipoles are oriented like this

(+)-----(-)         (-)-----(+)

and the distance between their centers is fixed, then they will rapidly rotate towards an orientation like this

(+)-----(-)         (+)-----(-)

thereby minimizing repulsions and maximizing attractions. Therefore, dipole-dipole interactions are attractive since the dipole can always rotate in our example away from repulsive orientations.

In chemistry, "dipole" is short for "electric dipole moment of a molecule." Molecules either have overall dipole moments or they do not, depending on their formula and shape. In addition, small molecules can rotate easily. Water happens to have a very large overall electric dipole moment because of its angular shape and the fact that it's made of hydrogen and oxygen. This tends to create strong, attractive interactions between water molecules. In addition, water molecules can form "hydrogen bonds" to one another, which is an even stronger attractive interaction. This involves the attractive interaction between an H atom on one molecule and the O atom on another molecule (in water) The total of these two kinds of interactions tends to make water boil at a temperature which is much higher than other compounds with similar atomic weights. Water also has very high surface tension, and many other unique properties as well.

I hope this helps.

Prof. Topper



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