Intermolecular Forces Topic 4.4 between

Intra vs. Inter molecular forces 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 forces London forces (also called dispersion forces or instantaneous induced dipole forces dipole-dipole forces (polar molecules) 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… 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 shape of molecules molecules with shapes that have more contact area have greater forces between them than those don’t

boiling point increases these round shapes do NOT allow them to stick to one another this flat shape allows it to stick to one another better boiling point increases

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 The hydrogen atom has no inner core of electrons, so the side of the atom facing away from the bond represents a virtually naked nucleus This positive charge is attracted to the negative charge of an electronegative atom in a nearby molecule Because the hydrogen atom in a polar bond is electron-deficient on one side (i.e. the side opposite from the covalent polar bond) this side of the hydrogen atom can get quite close to a neighboring electronegative atom (with a partial negative charge) and interact strongly with it (remember, the closer it can get, the stronger the electrostatic attraction)

H-NOF 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 0.9 BP is 20°C BP is -85°C

H-NOF ? 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 ? no H-NOF ? yes

H-NOF ? CH3OCH3 vs. CH3CH2OH ? -24°C 78°C dipole – dipole has hydrogen bonding 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