Attractive Forces Between Molecules

Which phase has particles more strongly attracted? Which phase has particles more strongly attracted? Solid, Liquid, Gas? Solid, Liquid, Gas?

Intermolecular Forces of Attraction Forces of attraction between molecules. Called “Van der Waals Forces” They arise from weak “electrostatic attractions” between particles.

Shows electrostatic attractions between opposite charged areas on molecules

Forces Between Polar Molecules Dipole-Dipole Attractions: Dipole-Dipole Attractions: Occurs between polar molecules Occurs between polar molecules Polar Molecule = “Dipole” Polar Molecule = “Dipole” asymmetrical molecule with polar bonds asymmetrical molecule with polar bonds

Forces Between Polar Molecules Hydrogen Bonding Hydrogen Bonding Dipole-Dipole Attraction but stronger Dipole-Dipole Attraction but stronger Occurs between molecules that have hydrogen atoms bonded to very small, highly electronegative atoms like F, O or N Occurs between molecules that have hydrogen atoms bonded to very small, highly electronegative atoms like F, O or N Creates a very polar bond and an “extra polar” molecule. Creates a very polar bond and an “extra polar” molecule. Results in unusually high MP/BP temps. Results in unusually high MP/BP temps.

Remember: Remember: Hydrogen Bonding is “FON”!! Hydrogen Bonding is “FON”!!

Hydrogen bonds are responsible for: Hydrogen bonds are responsible for: Ice Floating on Water Ice Floating on Water Ice is ordered with an open structure to optimize H-bonding. Ice is ordered with an open structure to optimize H-bonding. Therefore, ice is less dense than water. Therefore, ice is less dense than water.

Also it is responsible for water’s Also it is responsible for water’s Surface tension Surface tension How it beads How it beads

Forces Between Nonpolar Molecules Dispersion Force (“weak force”) Dispersion Force (“weak force”) Only attractive forces between nonpolar molecules and noble gas atoms. Only attractive forces between nonpolar molecules and noble gas atoms. Weakest of all intermolecular forces. Weakest of all intermolecular forces.

Instantaneous or “momentary” dipoles are formed as electrons move around the atoms. Instantaneous or “momentary” dipoles are formed as electrons move around the atoms. Also called an “induced dipole” force. Also called an “induced dipole” force.

For nonpolar gases to become liquids: For nonpolar gases to become liquids: Gas particles are so far apart, weak dispersion forces cannot be felt. Gas particles are so far apart, weak dispersion forces cannot be felt. Molecules must be close and moving slowly so that the momentary dipoles can be created Molecules must be close and moving slowly so that the momentary dipoles can be created High pressure/low temp. conditions High pressure/low temp. conditions

Dispersion Forces have different strengths. Dispersion Forces have different strengths. The more total electrons in a molecule, the greater the force can get. The more total electrons in a molecule, the greater the force can get. Ex: Cl 2 has 34 electrons (it’s a gas) Ex: Cl 2 has 34 electrons (it’s a gas) Br 2 has 70 electrons (it’s a liquid) I 2 has 106 electrons (it’s a solid)

Why Do Ionics Dissolve in Water? Molecule-Ion Attractions: Molecule-Ion Attractions: Attraction between polar solvent molecules and ions in an ionic crystal. Attraction between polar solvent molecules and ions in an ionic crystal. Allows polar solvent (like water) to “pull” the ions into solution, creating “hydrated” ions. Allows polar solvent (like water) to “pull” the ions into solution, creating “hydrated” ions. Remember which side of water is negative!

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What Effects do these Forces Have? Melting and Boiling Point Temperature: Stronger forces make it harder to separate molecules and undergo a phase change. Stronger forces make it harder to separate molecules and undergo a phase change. H-Bonding = highest MP/BP H-Bonding = highest MP/BP Dispersion Forces = lowest MP/BP Dispersion Forces = lowest MP/BP

Solubility: solutes must form attractions to solvent to dissolve in it. Polar solvents attract polar solutes Polar solvents attract polar solutes Nonpolar solutes only dissolve in nonpolar solvents. Nonpolar solutes only dissolve in nonpolar solvents. Polar solvents can attract ions Polar solvents can attract ions

Viscosity : Viscosity : a measure of resistance of a fluid to flow. Stronger IMF = more viscosity Since there are cohesive forces between the molecules of liquid, like intermolecular forces, these forces create an "internal friction" which reduces the rate of flow of that fluid, so when a substance has greater IMF, these frictional forces are stronger. Therefore, it has more resistance toward moving

Vapor Pressure: Pressure exerted by collisions of vapor particles above a liquid Pressure exerted by collisions of vapor particles above a liquid The weaker the attractions The weaker the attractions between molecules in a liquid, the easier to become a gas. Therefore: Nonpolars with weaker attractions have higher VP than polars Nonpolars with weaker attractions have higher VP than polars

Vapor Pressure Measured in a closed system at a specific temperature. Measured in a closed system at a specific temperature.

Vapor Pressure If the temperature of a liquid increases, what happens to VP? If the temperature of a liquid increases, what happens to VP? es/propOfSoln/vp3.html

Vapor Pressure Boiling Point: Boiling Point: occurs when VP equals the atmospheric pressure pushing down on the liquid. occurs when VP equals the atmospheric pressure pushing down on the liquid.

Normal Boiling Point: Normal Boiling Point: The temp. a liquid boils at standard pressure The temp. a liquid boils at standard pressure 1 atm 1 atm KPa KPa 760 mmHg (torr) 760 mmHg (torr) For water it is 100°C.

Can you boil water at different Temperatures? Can you boil water at different Temperatures? Yup! Yup! It depends more on the amount of air pressure pushing down on the liquid. It depends more on the amount of air pressure pushing down on the liquid.

Using Reference Table H

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