IntermolecularIntermolecular ForcesForces

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

molecules: ◦ Intramolecular forces ◦ Intermolecular forces

Intermolecular Forces – the forces that bond molecules to each other

Intramolecular 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?

Intermolecular forces are weak relative to covalent bonds

Intramolecular 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) ForcesDispersion (London) ForcesWeakest intermolecular force that

act between non-polar molecules

Shared pairs of e- can temporarily occupy positions that create momentary, uneven distributions of charge

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)

PracticeSheet on polar molecules is for

homework!

On this sheet please add this question:

Based on what we know about intermolecular forces predict the solubility of this molecule in a)

water and b) acetone