INTERMOLECULAR FORCES, INSTANTANEOUS (TEMPORARY) DIPOLE INDUCED DIPOLE FORCE
(2)
Dipole- dipole forces (permanent dipole
forces)
(3)
Hydrogen-bonding
The increasing strength order of van der waals forces describes that hydrogen bonding is the strongest of all. Temporary dipole forces < Permanent dipole forces < Hydrogen bonding.
Energy is
required to convert liquids into gas hence enthalpy change of vaporization or
boiling points increases from small to large molecules. e.g; Among halogens
fluorine has the least boiling point and iodine has the highest boiling point.
Alkines
also show similar treated in their enthalpy energies i.e; increases with the
increasing (number of atoms per molecule). For different isotopes straight
chain compound have high boiling point and branched chain have low boiling
point.
Shape Molecule Boiling point
CH3-CH2-CH2-CH2-CH3 360C
CH3-CH-CH2-CH3 280C
CH3
CH3
CH3-C-CH3 100C
CH3
CH3
CH3CH2CH2CH2CH3
> CH3-CH-CH2-CH3 > CH3-C-CH3
CH3 CH3
360C 280C 100C
Increased
branching gives the molecules an almost spherical shape and unbranched gives
have must more chance of dispersion than other molecules consequently
dispersion forces decreases with increased branching and the boiling points
decreases. PTFE (Teflon) is a non stick plastic dispersion attractive forces
are weak in plastic and oil or grease whereas these forces are strong in oil or
grease itself. This difference gives the polymer a non-stick properties.
Another example
is low density and high density poly (ethane) low density polymer have high
degree of branching whereas high density
plastic have less degree of branching.
It is observed that branching decreases intermolecular forces hence, low
density plastic due to high degree of
branching is less strong and have low boiling point whereas high density
plastic due to less degree of branching is more opaque to light and have
greater strength.
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