Hydogen bonding is of an electrostatic origin and can be considered a special type of dipole-dipole interaction. When hydrogen is covalently bonded to an electronegative atom such as oxygen or nitrogen it has a net partial positive charge. The positive partial charge on hydrogen can attract another electronegative atom such as oxygen or nitrogen. A simple example is the water dimer shown below.
What is the length of a hydrogen bond? How does the length of a hydrogen bond compare to that of a covalent bond?
Measure the distance of the covalent bond between oxygen and hydrogen. (You may have to zoom in first --left mouse button and shift key--).
1) click the right mouse button ---> Select --> Mouse Click Action ---> Toggle Distance Monitor ---> Click
2) Click on the two atoms e.g., oxygen covalently bound to hydrogen, or oxygen and hydrogen in a hydrogen bond.
Check your values:
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What is the distance of a hydrogen bond formed between oxygen and hydrogen
in a water dimer?
|Hydrogen bonds are much weaker than covalent bonds. However, when a large number of hydrogen bonds act in unison they will make a strong contributory effect. This is the case in water. In water hydrogen bonds are constantly being formed and broken.Click here for a movie that demonstrates this. The time scale is about 10 picoseconds.|
|When the temperature of liquid water is lowered the speed of molecules slows increasing the time that hydrogen bonds last. The results in the rigid lattice structure known as ice.|
In liquid water each molecule is hydrogen bonded to approximately 3.4 other water molecules. In ice each each molecule is hydrogen bonded to 4 other molecules.
Compare the two structures below. Notice the empty spaces within the ice structure.
In ice Ih, each water forms four hydrogen bonds with O---O distances of 2.76 Angstroms to the nearest oxygen neighbor. The O-O-O angles are 109 degrees, typical of a tetrahedrally coordinated lattice structure. The density of ice Ih is 0.931 gm/cubic cm. This compares with a density of 1.00 gm/cubic cm. for water.
The hydrogen bonds form a tetrahedral array in ice crystals.
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