Intermolecular Forces

•Section 4.3

Introduction There are ionic, giant covalent, and simple

molecular covalent bonds between atoms If there are no attractive forces between

molecules, then all substances would be gases There must be some force that attracts

molecules to other molecules that does not create a real bond

These are known collectively as intermolecular forces

Types of Intermolecular Forces In order of increasing strength: van der Waals' forces (AKA London forces or

dispersion forces) Dipole-dipole forces Hydrogen bonding

van der Waals' Forces

Exist between all species Is the only intermolecular force between

non-polar species Result of temporary (instantaneous) dipoles

when one side of the molecule becomes partially negative because of the random motion of electrons

Van der Waals Continued This results in a weak dipole moment that

then attracts other molecules Strength of this force increases with molar

mass as more electrons are available for temporary dipoles

Effect of van der Waals' Forces on Boiling Point

Higher boiling point means more energy is needed to break the intermolecular forces

As the molar mass increases, the boiling point increases

As the surface area increases, the boiling point increases (again, more electrons available)

More elongated the molecule, the stronger the van der Waals' forces, so the higher the boiling point

Continued Sometimes the van der Waals' force can be

quite strong as in the case of some polymers that have high mass and are very long molecules

Dipole-dipole forces Dipole moment: a measure of the polarity of a

molecule Arrows are used to represent the polarity of

the bond (heading toward the partially negative part)

The permanent dipoles formed cause electrostatic attraction between molecules that have the permanent dipoles

Stronger than van der Waals' forces in molecules of similar size

Effect of Dipole-dipole Forces on Boiling Point

Compared to molecules of similar mass with only van der Waals' forces, much higher boiling point

Polar molecules have van der Waals' forces and dipole-dipole forces

Stronger intermolecular force: higher boiling point

Hydrogen Bonding Not a “true” bond Occurs when hydrogen is bonded to highly

electronegative, small atoms, like N, O, or F Creates a very high dipole moment as the more

electronegative atom attracts the electrons, leaving the hydrogen very partially positive

Can be thought of as part way between a dipole-dipole force and a dative covalent bond

Continued For maximum strength, the 2 atoms and the

hydrogen should be in a straight line When X and Y are N, O, or F: Xδ-: hydrogen bond Hδ+ - Yδ-

Usually much stronger than other intermolecular forces

Effect of H-bonding on Boiling Point

Consider HF and HCl: HF has van der Waals' forces, dipole-dipole

forces, and H-bonding HCl has van der Waals' forces and dipole-

dipole forces Boiling point of HF is higher than HCl More energy is needed to break the

intermolecular forces

Consider H2O and H2S

H-bonding present in H2O, but not in H2S Boiling point of water is higher than H2S H-bonding allows water to form tetrahedral

shapes when it is a liquid H-bonding allows water to form hexagons when

it freezes, so ice is less dense than liquid water H-bonding in water also forms temporary

hexagon arrays on the surface of water, giving it a high surface tension

Consider NH3 and PH3

NH3 can hydrogen bond with itself and with the water when the ammonia is aqueous

PH3 can only hydrogen bond with water, not with itself

Ammonia has the higher boiling point

More to Consider

Considering CH3OCH3 and CH3CH2OH CH3OCH3 has the hydrogens bonded to the

carbons, not the oxygen, so no hydrogen bonding occurs

CH3CH2OH has the hydrogen bonded to the oxygen, so H-bonding occurs

CH3CH2OH has a higher boiling point than CH3OCH3

Another Consideration CH3CH2CH3, CH3CHO, and CH3CH2OH All have about the same mass CH3CH2CH3 cannot hydrogen bond with itself or with

water CH3CHO cannot hydrogen bond with itself, but can

with water CH3CH2OH can hydrogen bond with itself and water b. p. trend: CH3CH2CH3 < CH3CHO < CH3CH2OH

Biological Importance The weak bond between the nitrogenous

bases in nucleic acids is a H-bond Occurs between thymine and adenine as well

as between cytosine and guanine Easily broken by enzymes