3.9 Hydrogen Bonding
Hydrogen Bonding
Because of water’s polarity, individual water molecules are attracted to one another. You can see this in theFigure
3.16. The positively charged hydrogen side of one water molecule is attracted to the negatively charged oxygen side
of a nearby water molecule. This force of attraction is called ahydrogen bond. You can watch hydrogen bonds
forming between water molecules in the animation at this URL: http://www.northland.cc.mn.us/biology/biology111
1/animations/hydrogenbonds.html.
FIGURE 3.16
Hydrogen bonds are intermolecular (“between-molecule”) bonds, rather than intramolecular (“within-molecule”)
bonds. They occur not only in water but in other polar molecules in which positive hydrogen atoms are attracted to
negative atoms in nearby molecules. Hydrogen bonds are relatively weak as chemical bonds go. For example, they
are much weaker than the bonds holding atoms together within molecules of covalent compounds. To learn more
about hydrogen bonding and when it occurs, see the video at this URL:
http://www.youtube.com/watch?v=lkl5cbfqFRM
MEDIA
Click image to the left for use the URL below.
URL: http://www.ck12.org/flx/render/embeddedobject/5084Hydrogen Bonds and Changes of State
Changes of state from solid to liquid and from liquid to gas occur when matter gains energy. The energy allows
individual molecules to separate and move apart from one another. It takes more energy to bring about these changes
of state for polar molecules. Although hydrogen bonds are weak, they add to the energy needed for molecules to
move apart from one another, so it takes higher temperatures for these changes of state to occur in polar compounds.
This explains why polar compounds have relatively high melting and boiling points. TheTable3.5 compares melting
and boiling points for some polar and nonpolar covalent compounds.