IntermolecularIntermolecular ForcesForcesChemistry 11

How are molecules held How are molecules held together?together?There are two types of attraction in

molecules: ◦ Intra-molecular forces ◦ Intermolecular forces

Inter-molecular Forces – the forces that bond molecules to each other

Intra-molecular Forces – the forces that bond the atoms to each other within a molecule

Until now, we have been discussing intramolecular forces (ionic, polar, non-polar bonds)

How are molecules held How are molecules held together?together?

Inter-molecular forces are weak relative to covalent bonds

Intra-molecular forces are strong covalent bonds

We can conclude that it does not take very much energy to break the bonds that hold molecules to each other (intermolecular forces)

These forces are also referred to as van der Waals forces after the physicist who studied them

We will be discussing 4 types of intermolecular forces◦Dipole-Dipole Forces◦Ion-Dipole Forces◦Dispersion (London) Forces◦Hydrogen Bonding

Dipole-Dipole ForcesDipole-Dipole ForcesDipole is a polar

moleculeThese molecules will

orient themselves so that oppositely charged ends of molecules are near to one another

The electrostatic attractions between these ends are called dipole-dipole forces

H Cl

+ –

+–

+ –

+ –

+ –

Lets see it!

Result of Dipole-Dipole Result of Dipole-Dipole ForcesForcesPolar molecules will tend to attract

one another more than similarly sized non-polar molecules

Therefore the energy required to separate polar molecules from one another is greater than that needed to separate non-polar molecules

Ion-Dipole ForcesIon-Dipole ForcesThe force of attraction between an

ion and a polar molecule (a dipole)

Example: NaCl breaks up because the ion dipole with water is stronger than the attraction of Na+ to Cl-

Let’s See it!

Dispersion (London) Forces Dispersion (London) Forces (LDF)(LDF)Weakest intermolecular force that act

between non-polar moleculesShared pairs of e- can temporarily

occupy positions that create momentary, uneven distributions of charge

Molecules are constantly vibrating and this causes momentary, uneven distributions of charges

This causes non-polar molecules to become slightly polar for just an instant

At this instant it is capable of inducing a dipole in a nearby molecule resulting in an intermolecular force of attraction

This is called a dispersion force

Hydrogen BondingHydrogen BondingAn attraction between the

hydrogen atom, bonded to a highly electronegative atom, and the negative end of a dipole nearby

Like Dissolves LikeLike Dissolves LikeIonic solutes dissolve in polar

solvents ◦(ex: NaCl and H2O)

Non polar solutes dissolve in non polar solvents◦(ex: solid I2 and liquid Br2)

In order of strength:1. Hydrogen Bonding:

• Hydrogen bonded to highly EN atom (O, N, F)

2. Dipole-Dipole Forces:• The larger the EN, the stronger the

dipole-dipole force.• More energy is required to

separate polar molecules from each other than non-polar molecules of similar molar mass.

3. London Dispersion Forces:Larger molecules, with many

electrons, or with electrons far removed from the atomic nuclei, are more easily polarised than small molecules.

As the molar mass increases, LDF becomes stronger and the boiling points of non-polar molecules substances increases.

Try These 1. Which of the following

molecules have a permanent dipole moment?a. H2Ob. CO2c. CH4d. N2e. COf. NH3

Solution CO2, CH4 and N2 are

symmetrical, and hence they have no permanent dipole moments.

A molecule with polar bonds unsymmetrically arranged will possess a permanent dipole.