Intermolecular Attractions

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Intermolecular Attractions
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What is the difference?

• What is the difference between
• Inter-molecular?
• Intra-molecular?
• More solid
• Moves less
• These are the forces that hold our world together!

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Intermolecular Forces

• Macro-covalent
• Ionic
• Metallic
• Hydrogen
• Dipole-Dipole
• Dispersion

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Macrocovalent

• The strongest of them all.
• Diamond (all Carbon atoms)
• Rocks (Silicon and Oxygen)
• See Toy Examples

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• Covalent
• Lattice points occupied by atoms
• Held together by covalent bonds
• Hard, high melting point
• Poor conductor of heat and electricity
• Examples

graphite
diamond
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Non-crystalline quartz glass
Crystalline quartz (SiO2)
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Ionic

• Very strong
• Permanent strong charge interactions
• Usually crystal in formation
• Very high melting points
• See toy example.
• Example of Lattice

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Metallic Bonding

• Solids at room temperature
• This means STRONG Intermolecular Forces
• Metal Atoms give up electrons
• This creates many dipoles throughout the atoms,
  they SHIFT but are always there
• SEA of electrons

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Hydrogen Bonding

• A strong intermolecular attractions
• Caused by very strong dipole-dipole attraction
  between molecules with N-H,
• O-H, and F-H bonds.
• Responsible for many of waters special
  properties.

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Hydrogen Bonding
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Hydrogen Bond
The hydrogen bond is a special dipole-dipole
interaction between the hydrogen atom in a polar
N-H, O-H, or F-H bond and an electronegative O,
N, or F atom. IT IS NOT A BOND.
A B are N, O, or F
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Dipole-Dipole (can you think of an example of
this?)
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Dipole-Dipole Attraction

• Caused by attraction of d end of one polar
  molecule to d- of another polar molecule.
• For a molecule to have dipoles it must be polar
• A. Asymmetrical in shape
• and
• B. With polar bonds

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Molecular Polarity

• In order for a molecule to be polar it must have
  polar bonds and be asymmetrical in shape.
• Lone pairs always lead to asymmetry.
• Linear, trigonal planar, and tetrahedral are
  symmetrical shapes and dipoles will cancel if all
  bonds are equal.

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Dipole-Dipole Forces
Attractive forces between polar molecules
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Dispersion Forces (non polar molecules)

• Moving the Fat Around
• Caused by momentary (temporary) dipoles (pg. 444)
• Get stronger with increase in mass
• Only attractive force between non-polar molecules
  (symmetrical or those with only non-polar bonds)
  or single atoms.

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London Dispersion Forces
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Momentary Dipoles
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Boiling point is a true measure of the strength
of intermolecular attractions. Why? Lets look
at the effect of size on nonpolar molecules.
Melting Points and Boiling Points of Similar Substances with Increasing Formula Weights Melting Points and Boiling Points of Similar Substances with Increasing Formula Weights Melting Points and Boiling Points of Similar Substances with Increasing Formula Weights Melting Points and Boiling Points of Similar Substances with Increasing Formula Weights
Substance FW (g/mol) mp (C) bp (C)
F2 38 -220 -188
Cl2 71 -100.98 -34.6
Br2 160 -7.2 58.78
I2 254 113.5 184.35
Conclusion?
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Boiling Point of Different Hydrocarbons
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Lets compare strength of intermolecular
attractions in molecules of the same size.
Melting Points and Boiling Points of Substances with Similar Formula Weights Melting Points and Boiling Points of Substances with Similar Formula Weights Melting Points and Boiling Points of Substances with Similar Formula Weights Melting Points and Boiling Points of Substances with Similar Formula Weights
Substance FW (g/mol) mp (C) bp (C)
F2 38 -220 -188
NO 30 -164 -152
CH3OH 32 -94 65
Ca 40 893 1484
NaF 42 993 1695
F2 nonpolar, NO dipole-dipole, CH3OH
hydrogen bonding, Ca metal, NaF ionic. What
does that say about the relative strength of
these attractions?
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Practice Label each with Phase and type of IMF

• SiO2
• Fe
• CCl4
• NO
• SO2
• C2H6
• C10H22
• Hg

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• O3
• CO2
• N2
• C(diamond)
• C2H5OH
• C2H21OH
• SO3
• CI4

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Boiling Point of Different Hydrides
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Boiling Points of Halogen Hydrides
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Boiling Point Differences
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Volatility
Affects of Intermolecular Forces

• Ability to evaporate
• High when attractions are weak. Why?
• Compare volatility of alcohol and water.
• Capillarity
• Ability to climb up a tube or surface
• High when liquid molecules are attracted to the
  tube or surface.
• Ex. Meniscus, paper towels, blood test

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Capillary Action
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Incredibly Tacky Household Products
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Surface Tension

• A skin develops on the surface of liquids.
• Waters is very strong.
• High when molecules are attracted to each other?
  cohesion.
• Ex. Paper clip floating
• Water striders

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Basilisk Lizard
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More Surface Tension Examples

• Dont touch the tent
• Rainx

• Surfactants? Soaps and detergents

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Surfactants

• Molecules that act to disrupt a liquids surface
  tension? wetting agent
• Structure-long non-polar hydrocarbon tail and a
  polar or ionic head

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How do they work?

• Disrupt surface tension by acting like a wedge

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Surfactants as Cleaning Agents

• Non-polar hydrophobic (water-hating) tails and
  polar or ionic hydrophilic (water-loving) heads
  form micelles.

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• Soap dissolves grease by taking grease molecules
  into the non-polar interior of the micelle.

Micelles are carried away during rinsing.
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Shampoo/Conditioners
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Changes of State and Attractive Forces
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Heating Curves
Water
Why is the plateau for boiling so much longer
than for melting?
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Solubility

• What dissolves in what?
• Ionic substances dissolve in water
• Covalent compounds
• A. Non-polar dissolves in non-polar solvents.
• B. Polar solutes dissolve in polar solvents.
• C. Partially polar (only polar in a small part
  of the molecule) solutes dissolve in partially
  polar solvents.
• Like Dissolves Like!

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The solute and the solvent have to be attracted
to each other in order to dissolve.
Ionic solid dissolving in water
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