Between. Intra- › strong forces that hold the atoms in a molecule together takes 464 kJ/mol to break the H-O bonds within a water molecule › responsible.

Intermolecular ForcesTopic 4.4between

Intra vs. Inter molecular forces Intra-

› strong forces that hold the atoms in a molecule together takes 464 kJ/mol to break the H-O bonds within a water molecule

› responsible for chemical properties

Inter-› weak forces that holds molecules to

one another takes only 19 kJ/mol to break the bonds between water molecules

› the strength of the intermolecular forces determines the physical properties of the substance melting, boiling, solubility, conductivity, volatility

3 main “types” of intermolecular forces1. London forces (also called

dispersion forces or instantaneous induced dipole forces

2. dipole-dipole forces (polar molecules)

3. a stronger type of dipole-dipole bonding called hydrogen bonding

* Van der Waals and London forces are often used interchangeably– more later on this.

strength increases

1. London forces

van der Waals’ YouTube (:20) occurs in non-polar molecules the distribution of electrons

around an individual atom, at a given instant in time, may not be perfectly symmetrical› this can produce

temporary/instantaneous dipole (polar molecule)

› this can then induce a nearby molecule to be polar and therefore a very weak attraction between the two molecules

› magnitude of the force depends on…1. number of electrons and size of

the electron cloud with more electrons, valence electrons

are farther away from the nucleus and can be polarized more easily

2. shape of molecules molecules with shapes that have more

contact area have greater forces between them than those don’t

boiling point increases

this flat shape allows it to stick to one another

better

these round shapes do NOT allow

them to stick to one

another

Dipole-Dipole Forces attractive forces between the

positive end of one polar molecule and the negative end of another polar molecule

must be in close proximity for the dipole-dipole forces to be significant

the more polar the molecule, the greater the dipole-dipole force

stronger than London forces

Hydrogen Bonding

YouTube Hydrogen Bonding (1:40) YouTube Hydrogen Bonding Video (:

58) a specific type of dipole-dipole type

interactions stronger than other dipole-dipole

and London forces the hydrogen (H) in a molecule is

intermolecularly bonded to a small, highly electronegative element (usually an N, O or F atom) on another molecule

H-NOF

Van der Walls vs. London Forces

the term London forces is used for instantaneous induced dipole – induced dipole force in non-polar molecules

Van der Waals is a more inclusive term, for all intermolecular attractions

Physical Properties Related to Bonding Type

Melting point (mp) - solid to liquid Boiling point (bp) - liquid to gas Volatility - how easily it is converted

to gas Conductivity (conducts electricity)

› depends on whether the substance contains freely moving charged particles

Solubility - solute’s ability to dissolve in solvent

Intermolecular forces affect on boiling point

the greater polarity of a molecule, the higher the boiling point› In HF, H is 2.1 and F is 4.0, difference

of 1.9› In HCl, H is 2.1 and Cl is 3.0,

difference of 0.9BP is 20°C BP is -85°C

› H2O vs. H2S? In H2O, H is 2.1 and O is 3.5, difference of 1.4 water molecules can hydrogen bond (and London forces) to each other

BP is 100°C

In H2S, H is 2.1 and S is 2.5, only a difference of 0.4 H2S can dipole-dipole bond (and London forces) to each other

BP is -60°C

H-NOF ?

H-NOF ?yes

H-NOF ?no

CH3OCH3 vs. CH3CH2OH ?

-24°C 78°C dipole – dipole has hydrogen bonding

H-NOF ?

NH3 vs. PH3 ?

-33°C -87°C has hydrogen bonding dipole – dipole

CH3CH2 CH3 vs. CH3CHO vs. CH3CH2OH ?

only London forces (VDW) low BP

VDW & dipole-dipole medium BP

VDW, D-D, & hydrogen bonding

highest BP