4.5 Intermolecular Forces

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  • Topic: Atom, Chemical bonding, Intermolecular force
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  • Published : February 25, 2013
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-many physical properties of compounds depend on the intermolecular forces: -wetting (water absorbed by matter)
-surface tension
-capillary action
-melting and boiling points
-cohesion (attraction between like molecules)
-adhesion (attraction between unlike molecules)
-intermolecular forces are much weaker than covalent bonds
ex. if covalent bonds are assigned a strength of 100, then intermolecular forces are between 0.001 to 15. -there are 3 types of intermolecular forces: Dipole-Dipole, London Dispersion Forces, and Hydrogen Bonding

Dipole-Dipole Forces
-is due to the simultaneous attraction between a dipole and the surrounding dipoles -the strength of the dipole-dipole force is dependent on the polarity of the molecule

London Dispersion Force
-exists in all molecules
-due to the simultaneous attraction of electrons of a molecule by the positive nuclei in another molecule -the strength of the force is dependent on the number of electrons in the molecule (table 1 p. 259)

Hydrogen Bonding
-seen in compounds where a hydrogen if bonded to highly electronegative atoms with lone pairs of electrons such as: N, O and F. The boiling points are much higher than other hydrogen compounds of group V, VI and VII elements. -the proton (H) is being shared between two pairs of electrons (similar to covalent bonds where one pair of electrons are shared between two nuclei)

Physical Properties of Liquids

-Many properties are determined by the intermediate forces: omelting points and boiling points
oSurface Tension: the attraction of molecules in a liquid for other molecules in the liquid. (called cohesion) oCapillary Action: liquid moving up a narrow tube due to the attraction of the molecules in the surface of the tube (called adhesion)
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