Transcript CHEMICAL BONDING Set 6 - Welcome to Westford Academy

CHEMICAL
BONDING
Set 6
Cocaine
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Credits
• Thank you to Mr. Neil
Rapp who provided the
bulk of this powerpoint
on his website
www.chemistrygeek.co
m
• Other information
comes from Zumdahl, Steven,
and Susan Zumdahl. Chemistry.
Boston: Houghton Mifflin, 2003.
Bond Polarity
HCl is POLAR because it
has a positive end and a
negative end. (difference
in electronegativity)
+d -d
••
••
H Cl
••
Cl has a greater share in
bonding electrons than
does H.
Cl has slight negative charge (-d) and H has slight
positive charge (+ d)
Molecular Polarity
• REMEMBER: Polarity refers to a separation of
charges.
• Entire molecules can be polar or nonpolar,
too!
Polar
Nonpolar
Determining Molecular Polarity
1) Draw the Lewis dot structure of the
molecule.
2) Find the polarity of each bond within the
molecule.
3) If ALL the bonds are nonpolar, the molecule
is nonpolar.
4) If one or more of the bonds is polar,
determine the symmetry of the molecule.
Molecular Symmetry
• Molecular symmetry is NOT the same as
geometrical symmetry!
• Molecular symmetry occurs when all items
(bonds and/or lone pairs) on the central atom
are identical.
• NOTE: It matters what is attached, not how it
is attached (single, double, triple bonds count
the same!)
Symmetry
• If a molecule is symmetrical, the molecule is
NONPOLAR regardless of what types of bonds
it contains.
• If a molecule is not symmetrical AND it has at
least one polar bond, the molecule is POLAR.
Lone pairs ≠ H atoms (not symmetrical)
All atoms are F (symmetrical)
Intermolecular Forces
• Intermolecular forces: forces of attraction
between molecules (like international =
between countries)
• Intramolecular forces: forces of attraction
within a molecule, i.e. a bond
Types of Intermolecular Forces (IMFS)
• Dipole-dipole forces: occurs between polar
molecules – partial positive end of molecule A
attracts partial negative end of molecule B
Types of IMFs
• Hydrogen “bonding”: a special subset of
dipole-dipole interactions – occurs between H
atom of molecule A and F,O,N or S atom of
Molecule B
Types of IMFs
• Dipole-induced dipole forces: attraction
between a polar molecule and nonpolar
molecule – the polar molecule creates a
temporary dipole in the nonpolar molecule.
Types of IMFs
• London dispersion forces (or dipsersion forces
or van der Waal’s forces or induced dipoleinduced dipole forces): attractions between
two nonpolar molecules
Effects of IMFs
• “Like Dissolves Like”
– Polar dissolves Polar
– Nonpolar dissolves
Nonpolar
• Other properties
affected: melting
points, boiling points
Properties of Three Molecular Compounds
Compound
Molecular Formula
Molar Mass (g)
Boiling Point (°C)
Water
H2O
18.0
100
Methane
CH4
16.0
-33.4
Ammonia
NH3
17.0
-164
•Hydrogen bonds explain why water is a liquid at room temperature, while
compounds of comparable mass are gases.
•The difference between methane and water is easy to explain – because
methane is nonpolar, the only forces holding the molecules together are
relatively weak dispersion forces.
•Ammonia and water is not as obvious – molecules of both can form
hydrogen bonds, but the ammonia is a gas which indicates its IMFs are
not as strong. This is because the EN difference between N-H is a lot less
than the EN difference between O-H.
IMFs and Dissolving
• This is why oil and water will not mix! Oil is
nonpolar, and water is polar.
• The two will repel each other, and so you
can not dissolve one in the other
HOMEWORK
1) Explain what determines a substance’s state
at a given temperature.
2) Compare and contrast intermolecular forces
and describe intramolecular forces.
3) Evaluate which of the molecules listed below
can form hydrogen bonds. For which of the
molecules would dispersion forces be the
only intermolecular force? Give reasons for
you answers.
a) H2
b) H2S
c) HCl
d) HF
MORE HOMEWORK
4) In a methane molecule (CH4), there are four
single covalent bonds. In an octane molecule
(C8H18), there are 25 single covalent bonds.
How does the number of bonds affect the
dispersion forces in samples of methane and
octane? Which compound is a gas at room
temperature? Which is a liquid?