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Hydrogen bonding is a special type of dipole-dipole attraction betweenmolecules, not a covalent bond to a hydrogen atom. It results fromthe attractive force between a hydrogen atom covalently bonded toa very electronegative atom such as a N, O, or F atom and another veryelectronegative atom. Hydrogen bond strengths range from 4 kJ to50 kJ per mole of hydrogen bonds.
- In molecules containing N-H, O-H or F-H bonds, the large difference inelectronegativity between the H atom and the N, O or F atom leads to ahighly polar covalent bond (i.e., a bond dipole). The electronegativitiesare listed below.
element | |
H | 2.1 |
N | 3.0 |
O | 3.5 |
F | 4.1 |
- Because of the difference in electronegativity, the H atom bears a largepartial positive charge and the N, O or F atom bears a large partial negativecharge.
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- A H atom in one molecule is electrostatically attracted to the N, O, orF atom in another molecule.
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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 aliquid. 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 responsiblefor the difference between these two compounds.
- ONF and water have the same dipole moment.
- Conclusion: dipole-dipole forces not responsiblefor the difference between these two compounds.
- ONF cannot form hydrogen bonds; water can form hydrogen bonds.
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The structure of ONF. | The structure of H2O. | |
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Microscopic view of ONF at 25oC. | Microscopic view of H2O at 25oC. |
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