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Intermolecular Forces (London (Dispersion) forces (The London dispersion…
Intermolecular Forces
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Dipole-Dipole forces
Dipole-dipole forces are attractive forces between the positive end of one polar molecule and the negative end of another polar molecule. Dipole-dipole forces have strengths that range from 5 kJ to 20 kJ per mole. They are much weaker than ionic or covalent bonds and have a significant effect only when the molecules involved are close together (touching or almost touching).
- Polar molecules have a partial negative end and a partial positive end.
- The partially positive end of a polar molecule is attracted to the partially negative end of another.
- In a ICl molecule the more electronegative chlorine atom bears the partial negative charge; the less electronegative iodine atom bears the partial positive charge.
- The partially positive iodine end of one ICl molecule is attracted to the partially negative chlorine end of another ICl molecule.
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The relative strengths of these interactions are London (dispersion) forces < dipole-dipole forces < hydrogen bonds.
Hydrogen Bonds
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.
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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
Intermolecular forces are largely responsible for the bulk properties of matter, that is, its physical such as melting point and boiling point
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Intermolecular forces of attraction are much weaker than covalent bonds. For example, the standard enthalpy change of vaporization of water, is 44.02 kJ/mol whereas 926 kJ/mol is required to break the two O-H polar covalent bonds in molecule of water.
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- 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.
