lecture 8.4c- intermolecular forces
DESCRIPTION
Section 8.4 lecture (part C) for Honors & Prep ChemistryTRANSCRIPT
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Bellwork- Polar ammonia
1)Draw a 3D sketch of ammonia (NH3)
2)What is the shape of this molecule?
3)Why does the molecule have this shape?
4)Show the bond and molecule polarity on the sketch.
5)Would this molecule be polar if it was planar?
Know your pet molecules!
CH4 NH3 H2O CO2 CH2OI may replace H’s with Cl or F to make polar bonds!
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Intermolecular forces are forces between two or more molecules
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Intermolecular forces
Intermolecular attractions are weaker than either ionic or covalent bonds.
These attractions determine whether a molecular compound is a gas, a liquid, or a solid at a given temperature.
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Intermolecular Forces
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dipole-dipole interactions occurbetween polar molecules
London Dispersion Forces occur between all molecules (polar and non-polar)
There are two types of intermolecular forces
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The partial charges in polar molecules cause attractions and repulsions between separate polar molecules
A hydrogen bond is a specific type of dipole-dipole interaction that is extra
strong
DIPOLE-DIPOLE INTERACTIONS
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HYDROGEN BOND
A molecule can H-bond if it contains hydrogen bound to an atom with high electronegativity (N, O, or F)
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IN THIS CASE- The especially strong partial charge (δ+) on hydrogen makes it “stick” to anything negative.
Hydrogen bonding
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N O F
No one in the corner grabs electrons like us! – N, O, & F
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London Dispersion Forces are the weakest of all IMFs. They occur
between all molecules.
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London Dispersion Forces
A covalent molecule consists of atoms bound by overlapping electron clouds.
Sometimes these electron clouds have temporary “thin or thick spots” causing temporary partial charges.
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These temporary partial charges can cause temporary partial charges on other molecules or atoms.
London Dispersion Forces
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Temporary partial charges experience attractions and repulsions just like permanent partial charges and ion charges.
London Dispersion Forces
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The larger the molecule the larger the London Dispersion
Forces
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What is the difference between the
partial charges on a polar molecule
(which cause dipole-dipole interactions)
and the partial charges
that cause London Dispersion attractions?
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Getting weak
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The stronger the intermolecular forces the more likely the compound
will be a solid at room temperature.
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Compounds with weak intermolecular forces have low boiling and melting points.
Strong intermolecular forces require a lot of energy to overcome, so compounds with strong intermolecular forces have high
melting and boiling points.
Liquids with strong intermolecular forces will hold on to their molecules preventing
them from escaping as a vapor.
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Molecular Solid- Strong intermolecular forces hold molecules in a rigid & regular pattern.
Molecular Liquid- Some intermolecular forces are overcome and molecules can slip past each other.
Molecular gas- Weak intermolecular forces cant hold molecules together, so each molecule is free and independent.
states of matter
Gases have weak intermolecular forces that were easy to break free of!
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Intermolecular forces
Weakest lower melting point
Dispersion forces
Dipole-dipole interactions
Hydrogen bonds
STRONGEST higher melting point
Ionic compounds have even higher melting points because the ionic bond must be
overcome to free the ions. Ionic bonds are much stronger than any intermolecular force.
Intermolecular Forces
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Network solid (Network crystal)- atoms are held together in a ordered pattern by strong covalent bonds.
–Melting a network solid would require breaking covalent bonds throughout the solid.
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Diamond is a network solid.
Diamond does not melt.
It vaporizes to a gas at 3500°C or above.
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TYPES OF SOLIDS
ionic solids – made of ions held together by ionic bonds
molecular solids- made of molecules held together by intermolecular forces
metallic solids- atoms held together by metallic bonds(sea of electrons model)
network solids- atoms held together by covalent bonds
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ClassworkFill in the table
Intermolecular force
Molecules that experience it
Relative strength
dispersion forces
dipole-dipole interactions
hydrogen bonds
-N-H» F-H»
-O-H»
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ClassworkFill in the table
Intermolecular force
Molecules that experience it
Relative strength
dispersion forces Non-polar molecules
weak
dipole-dipole interactions
Polar molecules stronger
hydrogen bonds
-N-H» F-H»
-O-H»
Polar molecules with N, O or F bound to H
Strongest of all intermolecular forces
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Would you expect the following solids to have a relatively high or low melting point?
Metal
Ionic solid
Non-polar molecular solid
Polar molecular solid
Atomic solid of a noble gas
Network solid