Hydrogen Bonding

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Hydrogen Bonding

Hydrogen bonding is a special type of dipole-dipole attraction between molecules, not a covalent bond to a hydrogen atom. It results from the attractive force between a hydrogen atom covalently bonded to a very electronegative atom such as a N, O, or F atom and another very electronegative atom. Hydrogen bond strengths range from 4 kJ to 50 kJ per mole of hydrogen bonds.

  • In molecules containing N-H, O-H or F-H bonds, the large difference in electronegativity between the H atom and the N, O or F atom leads to a highly polar covalent bond (i.e., a bond dipole). The electronegativities are listed below.

element electronegativity value
H 2.1
N 3.0
O 3.5
F 4.1
  • Because of the difference in electronegativity, the H atom bears a large partial positive charge and the N, O or F atom bears a large partial negative charge.

  • A H atom in one molecule is electrostatically attracted to the N, O, or F atom in another molecule.

= O

= N

= H

Hydrogen bonding between two water (H2O) molecules. Note that the O atom in one molecule is attracted to a H atom in the second molecule. Hydrogen bonding between a water molecule and an ammonia (NH3) molecule. Note that the N atom in the NH3 molecule is attracted to a H atom in the H2O molecule.

Physical Consequences of Hydrogen Bonding

  • At 25oC, nitrosyl fluoride (ONF) is a gas whereas water is a liquid. Why?
  • ONF and water have about the same shape.
  • ONF has a higher molecular weight (49 amu) than water (18 amu).
    • Conclusion: London dispersion forces not responsible for the difference between these two compounds.
  • ONF and water have the same dipole moment.
    • Conclusion: dipole-dipole forces not responsible for the difference between these two compounds.
  • ONF cannot form hydrogen bonds; water can form hydrogen bonds.

= O

= F

= N

= H

The structure of ONF. The structure of H2O.
microscopic view of ONF gas molecules microscopic view of liquid water
Microscopic view of ONF at 25oC. Microscopic view of H2O at 25oC.

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