Intermolecular Intermolecular ForcesForces

Pages 112-113; Pages 112-113; 167-169167-169

Intermolecular Intermolecular ForcesForces

Forces of attraction Forces of attraction between moleculesbetween molecules

Link molecules together Link molecules together instead of atomsinstead of atoms

Not very strongNot very strong

Three TypesThree Types

Dispersion Forces (AKA Dispersion Forces (AKA London Dispersion Forces)London Dispersion Forces)

Dipole InteractionsDipole InteractionsHydrogen BondingHydrogen Bonding

Dispersion ForcesDispersion Forces

Weakest of the three Weakest of the three intermolecular forcesintermolecular forces

Occur between nonpolar Occur between nonpolar moleculesmolecules

Caused by the movement of Caused by the movement of electrons around the electrons around the moleculemolecule

Dispersion ForcesDispersion Forces

H-H share electrons equally, but as electrons are zipping around the orbitals, they might both be found in the same area at the same time.

This would give the molecule a temporary or instantaneous dipole.

Dispersion ForcesDispersion Forces The larger the molecule (the more The larger the molecule (the more

electrons it contains), the stronger electrons it contains), the stronger the dispersion force becomes.the dispersion force becomes.

Nonpolar Nonpolar MoleculeMolecule

Boiling Boiling Point (°C, Point (°C,

1 atm)1 atm)

# # electronselectrons

ComparisComparison of on of

strengthstrength

HH22 -253-253

OO22 -183-183

ClCl22 -34-34

BrBr22 5959

2

16

34

70

weakest

strongest

Dipole InteractionsDipole Interactions

Also known as dipole-dipoleAlso known as dipole-dipole Occur when polar molecules Occur when polar molecules

are attracted to one anotherare attracted to one another Positive end of a polar Positive end of a polar

molecule attracts to the molecule attracts to the negative end of another polar negative end of another polar moleculemolecule

Dipole InteractionsDipole Interactions

Dipole InteractionsDipole Interactions

Dipoles can be Dipoles can be induced on induced on nonpolar nonpolar molecules as molecules as well.well.

Example: Example: Water and Water and OxygenOxygen

O

H H

O O

Dipole InteractionsDipole Interactions

O

H H

O O

Induced dipoles

Hydrogen BondingHydrogen Bonding

The intermolecular attraction The intermolecular attraction between a hydrogen atom between a hydrogen atom which is covalently bonded to which is covalently bonded to N, O or F AND an unshared N, O or F AND an unshared pair of electrons on another pair of electrons on another N, O, or F atomN, O, or F atom

H-FH-F H-O H-O H-N H-N

Hydrogen BondingHydrogen Bonding

Because H is so small, it can Because H is so small, it can approach the unshared pair approach the unshared pair of electrons very closely.of electrons very closely.

Dotted lines are used to show Dotted lines are used to show hydrogen bondinghydrogen bonding

Hydrogen bonding is a very Hydrogen bonding is a very strong IMF.strong IMF.

Hydrogen Bonding in Hydrogen Bonding in WaterWater

O

H H

O

H H

O

H H

O

H H

O

H H

Practice Problem 1Practice Problem 1

What type of intermolecular What type of intermolecular force will happen between the force will happen between the following molecules?following molecules? FF22 IClICl CClCCl44 HH22SS NHNH33 HFHF

Practice Problem 3Practice Problem 3

Water (HWater (H22O) and ammonia O) and ammonia (NH(NH33) are both relatively small ) are both relatively small polar molecules. Using IMF, polar molecules. Using IMF, explain why water’s boiling explain why water’s boiling point is 100°C while ammonia’s point is 100°C while ammonia’s is -33°C.is -33°C.

Practice Problem 2Practice Problem 2

Rank the molecules below in Rank the molecules below in order of lowest boiling point order of lowest boiling point to highest boiling point.to highest boiling point.HH22OO

CHCH44

HBrHBr