Intermolecular ForcesTopic 4.3between

Intra vs. Inter molecular forces Intra-

› strong forces (ionic or covalent) 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, reacting, solubility, conductivity, volatility

3 main “types” of intermolecular forces1. temporary/induced/

instantaneous dipole-dipole forces called Van der Waals’

2. permanent dipole-dipole forces (polar molecules)

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

strength increases

1. van der Waals’ forces

van der Waals’ YouTube (:20) also known as London Dispersion Forces even nonpolar molecules have forces

that hold them together the distribution of electrons around an

individual atom, at a given instant in time, may not be perfectly symmetrical› this can produce temporary/instantaneous

dipoles (polar molecule)› this can then induce a nearby molecule to

be polar and therefore a very weak attraction between the two molecules

› the more electrons in an atom, the greater the Van der Waals’ forces

Sticky secret. Tiny hairs on geckos' feet help maximize

contact with surfaces, allowing van der Waals forces to go to work.

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 van der Waals’ forces

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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 Van der Waals’ the hydrogen (H) in a molecule is

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

H-NOF

Intermolecular forces affect on boiling point (4.3.2)

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 to each other

BP is 100°C

In H2S, H is 2.1 and Cl is 2.5, only a difference of 0.4 H2S can only dipole-dipole bonding to each other

BP is -60°C

H-NOF ?

H-NOF ?yes

H-NOF ?no

CH3OCH3 vs. CH3CH2OH ?

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

H-NOF ?

NH3 vs. PH3 ?

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

CH3CH2 CH3 vs. CH3CHO vs. CH3CH2OH ?

only van der Waals’low BP

dipole-dipole medium BP

hydrogen bonding

highest BP