Dipoles and Interactions
Name: Dani M.
Would you please explain what dipoles are, and what
dipole dipole interactions. Especially in regards to water and its properties.
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.
Click here to return to the General Topics Archives
Update: June 2012