Intermolecular Forces

Intermolecular forces

are attractive forces

between

molecules.

(Example: water molecule to water molecule) Intramolecular forces

hold atoms together

within

in a molecule.

(Example: H to O bond within a water molecule).

• • Intermolecular vs Intramolecular 41 kJ to vaporize 1 mole of water (

inter

) 930 kJ to break all O-H bonds in 1 mole of water (

intra

) Generally,

inter

molecular forces are much weaker than

intra

molecular forces.

“Measure” of intermolecular force boiling point melting point D H vap D H fus D H sub 11.2

Intermolecular Forces

1. London Forces (a.k.a. Dispersion Forces)

Weakest

2. Dipole-Dipole Interactions 3. Ion-Dipole Interactions -(Salt dissolving in solution; Na+ and Cl- ) 4. Hydrogen Bonding (

STRONGEST)

London Dispersion Forces: Weakest

Occur between every compound and arise from the net attractive forces amount molecules which is produced from induced charge imbalances Figure 10-8 Olmsted Williams The larger the molecule the greater it’s Dispersion Forces are.

Animation: http://chemmovies.unl.edu/ChemAnim e/LONDOND/LONDOND.html

Figure 10-9 Olmsted Williams

The boiling point of long molecules increase with the length of the carbon chain.

Olmsted Williams Fig 10-10 Pg 437

How molecular shape affects the strength of the dispersion forces

The shapes of the molecules also matter. Long thin molecules can develop bigger temporary dipoles due to electron movement than short fat ones containing the same numbers of electrons.

Butane has a higher boiling point because the dispersion forces are greater. http://www.chemguide.co.uk/atoms/bonding/vdw.html

Polarizability

the ease with which the electron distribution in the atom or molecule can be distorted.

• • Polarizability increases with: greater number of electrons more diffuse electron cloud Dispersion forces usually increase with molar mass.

11.2

Is the Molecule Polar?

The more Electronegative atom will pull the electron density of the bond Closer to itself giving it a partial negative charge leaving the other Atom with a partially positive charge. This is a dipole moment.

CO 2 HCN

Molecules with 3 Atoms

Even though the C-O bond is polar, the bonds cancel each other out because the molecule is linear the dipole moments are equal and in opposite directions.Therefore CO 2 is non-polar.

The dipole moment between H-C points in the direction of C. The dipole moment points between C-N points in the direction of the N. Therefore the dipole vectors are additive and HCN is polar

SO 2

SO 2 is a polar molecule because the S-O dipole Moments don’t cancel each other out due to the angle

Molecules with 4 Atoms

CCl 4 is non-polar CHCl 3 is polar

How to Determine if a Molecule Is Polar

1. Draw Lewis Structure 2. If all of the regions of electron density are bound to the same thing (CCl molecule is non-polar 4 ; CO 2 ) than the 3. If the regions of electron density are not bound to the same

thing (atom)

than the molecule is polar (HCN; SO 2 )

Which of the following molecules are polar (have a dipole moment)?H

2 O, CO 2 , SO 2 , and CH 4 O dipole moment polar molecule O C O no dipole moment nonpolar molecule S dipole moment polar molecule H H C H H no dipole moment nonpolar molecule 10.2

Dipole-Dipole Forces

Attractive forces between

polar molecules

Orientation of Polar Molecules in a Solid Animation: http://chemmovies.unl.edu/ChemAnime/DIPOLED/DIPOLED.html

11.2

Dipole Forces occur between molecules containing a dipole moment. The positive end of the dipole moment on one mole is attracted to the Negative end of the dipole moment on a nearby molecule.

2-methyl propane (left) and acetone (right) Both compounds are about Equal in size and shape Figure 10-11 Olmsted Williams

Ion-Dipole Forces

Attractive forces between an

ion

and a

polar molecule

Ion-Dipole Interaction The larger the charge the stronger the force 11.2

Olmsted Williams

Fig 10-34

A molecular picture showing the ion-dipole Interaction that helps a solid ionic crystal dissolve in water. The arrows indicate ion-dipole interactions.

What type(s) of intermolecular forces exist between each of the following molecules?

HBr HBr is a polar molecule: dipole-dipole forces. There are also dispersion forces between HBr molecules.

CH 4 CH 4 is nonpolar: dispersion forces.

S SO 2 SO 2 is a polar molecule: dipole-dipole forces. There are also dispersion forces between SO 2 molecules.

11.2

The Hydrogen Bond

STRONGEST INTERMOLECULAR FORCE

http://chemmovies.unl.edu/ChemAnime/HYBOND/HYBONDD.html

Animation

The melting point, boiling point, heat of fusion and heat of vaporization of water are extremely high and do not fit the trend of properties relative to molar mass within Group VIA.

Water exhibits these unusual properties because of hydrogen bonding between water molecules.

• A hydrogen bond is an

intermolecular

bond.

• A hydrogen bond is formed between polar molecules that contain hydrogen covalently bonded to a small, highly electronegative atom: F, O, N.

F N —H

• it will be attracted to another F, O, or N, on another molecule.

• A dipole-dipole bond bond will be formed between the two molecules which is called a

hydrogen bond

.

hydrogen bond covalent bond covalent bond

13.8

Water in the liquid and solid states exists as groups in which the water molecules are linked together by hydrogen bonds.