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-Determine if a molecule is polar or nonpolar
-Evaluate the type of intermolecular forces
that occurs between covalent molecules based
on the structural features of the molecules
-Evaluate how the intermolecular forces affect
the boiling points of substances
TUTORIAL :
http://www.youtube.com/watch?v=uYtwU0uRK7o
A polar molecule
 contains polar bonds
 has a separation of positive and negative charge
called a dipole, indicated with + and –
 has dipoles that do not cancel
+ –
H–Cl
dipole
••
H—N—H
H
dipoles do not cancel
3
Ammonia has a
trigonal pyramidal
shape. The one
unshared pair of
electrons give that
side of the
molecule a partial
negative charge
and the other side
a partial positive
side.
p. 380
A nonpolar molecule
 contains nonpolar bonds
Cl–Cl
H–H
 or has a symmetrical arrangement of polar bonds
5
Boron trifluoride has a
trigonal planar shape
with each bond having a
120o angle and no
unshared electrons so it
is nonpolar. If the shape
of any molecule is
trigonal planar and the
central atom is
surrounded by the same
three atoms, then it will
always be nonpolar
p. 380
Determine the polarity of the H2O molecule.
Solution: The four electron groups of oxygen are
bonded to two H atoms. Thus the H2O molecule has
a net dipole, which makes it a polar molecule.
7
Carbon dioxide is
linear and has
symmetry so it is
nonpolar
Water has a bent shape
with two unshared pair
of electron on oxygen
so it is polar
Methane (CH4) is nonpolar because the carbon is surrounded by the same
atom on all sides. However for CH3Cl, CH2Cl2 & CHCl3 have the chlorine and
hydrogen attached to the carbon in various ratios, they are polar. Even
though they are all have a tetrahedral shape, they are not all nonpolar.
However the rule is if the molecule is tetrahedral where the central atom is
surrounded by the same for atoms, it will be nonpolar.
TUTORIAL ON INTERMOLECULAR FORCES:
http://www.youtube.com/watch?v=90q7xl3ndJ8&safe=active
between
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Intrastrong forces (ionic or covalent) that hold the
atoms in a molecule together
 takes 464 kJ/mol to break the H-O bonds
within a water molecule
 responsible for chemical properties
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Interweak forces that holds molecules to one
another
 takes only 19 kJ/mol to break the bonds
between water molecules
 the strength of the intermolecular forces
determines the physical properties of the
substance
 melting, boiling, reacting, solubility,
conductivity, volatility
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3 main “types” of intermolecular
forces
1.
2.
3.
temporary/induced/instantaneous
dipole-dipole forces called Van der
Waals’
permanent dipole-dipole forces (polar
molecules)
a stronger type of dipole-dipole
bonding called hydrogen bonding
strength
increases
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VAN DER WAAL FORCES
also known as London Dispersion Forces
even nonpolar molecules have forces that
hold them together
the distribution of electrons around an
individual atom, at a given instant in time,
may not be perfectly symmetrical
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this can produce temporary/instantaneous dipoles
(polar molecule)
this can then induce a nearby molecule to be polar
and therefore a very weak attraction between the
two molecules
the more electrons in an atom, the greater the Van
der Waals’ forces
Sticky secret. Tiny hairs on
geckos' feet help maximize
contact with surfaces, allowing
van der Waals forces to go to
work.
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attractive forces between the positive end of
one polar molecule and the negative end of
another polar molecule
must be in close proximity for the dipoledipole forces to be significant
the more polar the molecule, the greater the
dipole-dipole force
stronger than van der Waals’ forces
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HYDROGEN BONDING
a specific type of dipole-dipole type
interactions
stronger than other dipole-dipole and Van der
Waals’
the hydrogen (H) in a molecule is bonded to a
small, highly electronegative element (usually
an N, O or F atom) on another molecule
H-NOF
TUTORIAL:
http://www.youtube.com/watch?v=S4-lAq2IH-8&safe=active
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the greater polarity of a molecule, the higher
the boiling point
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In HF, H is 2.1 and F is 4.0, difference of 1.9
In HCl, H is 2.1 and Cl is 3.0, difference of 0.9
BP is 20°C
BP is -85°C
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H-NOF ?
H2O vs. H2S?
 In H2O, H is 2.1 and O is 3.5, difference of 1.4
 water molecules can hydrogen bond to each other
 BP is 100°C
 In H2S, H is 2.1 and Cl is 2.5, only a difference
of 0.4
 H2S can only dipole-dipole bonding to each other
 BP is -60°C
H-NOF ?
no
H-NOF ?
yes
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CH3OCH3 vs. CH3CH2OH ?
-24°C
only dipole – dipole
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H-NOF ?
78°C
has hydrogen bonding
NH3 vs. PH3 ?
-33°C
-87°C
has hydrogen bonding
only dipole – dipole
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CH3CH2 CH3 vs. CH3CHO vs. CH3CH2OH ?
only van der Waals’
low BP
dipole-dipole
medium BP
hydrogen bonding
highest BP