Molecules with hydrogen atoms bonded to electronegative atoms such as O, N, and F (and to a much lesser extent, Cl and S) tend to exhibit unusually strong intermolecular interactions. These result in much higher boiling points than are observed for substances in which London dispersion forces dominate, as illustrated for the covalent hydrides of elements of groups 1417 in Figure \(\PageIndex<5>\). This is the expected trend in nonpolar molecules, for which London dispersion forces are the exclusive intermolecular forces. In contrast, the hydrides of the lightest members of groups 1517 have boiling points that are more than 100°C greater than predicted on the basis of their molar masses. The effect is most dramatic for water: if we extend the straight line connecting the points for H2Te and H2Se to the line for Chico best hookup apps period 2, we obtain an estimated boiling point of ?1step 30°C for water! Imagine the implications for life on Earth if water boiled at ?130°C rather than 100°C.
Figure \(\PageIndex<5>\): The Effects of Hydrogen Bonding on Boiling Points. 3, and H2O) are anomalously high for compounds with such low molecular masses.
These types of plots of one’s boiling hot situations of the covalent hydrides of the current weather out-of communities 1417 demonstrate that new boiling hot products out-of the newest lightest people in for every single collection where hydrogen connecting try it is possible to (HF, NH
Why do strong intermolecular forces produce such anomalously high boiling points and other unusual properties, such as high enthalpies of vaporization and high melting points? The answer lies in the highly polar nature of the bonds between hydrogen and very electronegative elements such as O, N, and F. The large difference in electronegativity results in a large partial positive charge on hydrogen and a correspondingly large partial negative charge on the O, N, or F atom. Consequently, HO, HN, and HF bonds have very large bond dipoles that can interact strongly with one another. Because a hydrogen atom is so small, these dipoles can also approach one another more closely than most other dipoles. The combination of large bond dipoles and short dipoledipole distances results in very strong dipoledipole interactions called hydrogen bonds , as shown for ice in Figure \(\PageIndex<6>\). A hydrogen bond is usually indicated by a dotted line between the hydrogen atom attached to O, N, or F (the hydrogen bond donor) and the atom that has the lone pair of electrons (the hydrogen bond acceptor). Because each water molecule contains two hydrogen atoms and two lone pairs, a tetrahedral arrangement maximizes the number of hydrogen bonds that can be formed. In the structure of ice, each oxygen atom is surrounded by a distorted tetrahedron of hydrogen atoms that form bridges to the oxygen atoms of adjacent water molecules. Instead, each hydrogen atom is 101 pm from one oxygen and 174 pm from the other. In contrast, each oxygen atom is bonded to two H atoms at the shorter distance and two at the longer distance, corresponding to two OH covalent bonds and two O???H hydrogen bonds from adjacent water molecules, respectively. The resulting open, cagelike structure of ice means that the solid is actually slightly less dense than the liquid, which explains why ice floats on water, rather than sinks.
For every single liquid molecule welcomes a couple of hydrogen ties regarding several other liquids particles and you can donates two hydrogen atoms to form hydrogen securities having a couple a great deal more drinking water molecules, generating an unbarred, cagelike construction. The structure away from drinking water liquid is extremely comparable, but in this new water, the fresh new hydrogen bonds are constantly busted and formed because of rapid molecular action.