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12.3 Types of Intermolecular Forces (
I
F)
•IFs are based on electrostatic interactions
between opposite charges, but they are much
weaker than chemical bonds for two reasons:
– Smaller el. charges are involved
– The distances between the el. charges are longer
•Covalent radius – ½ the distance between two
identical bonded atoms
•van der Waals radius – ½ the distance between two
adjacent, identical (nonbonded) atoms from different
molecules →The van der Waals radius of an atom is
always longer than its covalent radius
–IFs are also called van der Waals forces
– Stronger IFs lead to higher Tband Tmvalues
•Ion-Dipole forces – IFs between an ion and a
nearby polar molecule (ionic compounds like
NaCl dissolved in polar solvents like H2O)
•Dipole-Dipole forces – IFs between two polar
molecules (δ+of one molecule attracts δ-of
the other)
– Increase with increasing the dipole moment
Example:
CH3CH2CH3and CH3CN have almost the
same molecular weight and size. Since
CH3CN is much more polar, its dipole-dipole
forces are much stronger and its boiling and
melting points, Tband Tm, are much higher.
•Hydrogen Bonding –IF between molecules
containing H-atoms connected to highly
electronegative small atoms with lone pairs
like F, Oand N
– Due to the very high polarity of the H-F, H-O and
H-N bonds (the δ+of His attracted to the δ-of
the lone pair of F, Oor N)
Example:
CH3OH has an O-H
bond ⇒H-bonding
•Charge-Induced Dipole forces
–Induced dipole – a dipole induced in a particle
through a distortion of its electron cloud by an
electric field such as a nearby charge or another
dipole
–Polarizability – the ease with which the electron
cloud of a particle can be distorted (increases with
increasing the size of the electron cloud and
therefore with the molar mass of the particle)
¾Ion-Induced Dipole forces – IFs between an ion
and a dipole it induces in a nearby particle
¾Dipole-Induced Dipole forces – IFs between a
dipole and a dipole it induces in a nearby particle
