Intermolecular Forces

Intramolecular Forces: The attractive forces between atoms and ions within a molecule e.g ionic, covalent, metallic Strong Intermolecular Forces: The attractive forces between molecules E.g. Van der Waals forces (London dispersion forces, dipole-dipole forces), hydrogen bonds Weak (in comparison to intramolecular forces) I.e. much less energy to melt H2O (inter) than for it to decompose into H2 and O2 (intra)

Intermolecular Forces If covalent bonds were the only forces at work, most molecular compounds would be gases as there would be no attraction between molecules strong enough to group the molecules as liquids or solids

van der Waals Forces Dipole-dipole London Dispersion

Dipole-dipole Forces of attraction between oppositely charged ends of polar molecules. The strength of the dipole-dipole force is dependent on the polarity of the molecule

London Dispersion Attractive forces between all molecules, including nonpolar molecules Result of temporary displacements of the electron cloud around atoms in a molecule (extremely short-lived dipoles) Strength depends on number of electrons in a molecule Therefore weaker than dipole-dipole

London Dispersion Forces Strength depends on number of electrons, more electrons more chances for instantaneous dipoles molecular shape bp = 36˚C 28˚C 9.5˚C Tetris Analogy

Intermolecular Forces and Boiling Point The stronger the intermolecular forces the higher the boiling point. The more polar the molecule, the higher the boiling point More electrons = more LDF = higher boiling point

Boiling Points Unexpectedly high boiling points of hydrogen compounds containing nitrogen, oxygen, and fluorine Additional forces must exist

Hydrogen Bonding Strong dipole-dipole force between the positive H atom and the lone pair of electrons H has no electrons other than the bonding electrons, it is a “bare-naked” nucleus

Structure Dictates Melting/Boiling Point Explain Compound Molar Mass (g/mol) Number of Electrons Melting Point (ºC) Boiling Point (ºC) water ammonia -77.7 -33.3 methane -182.5 -161.6

Practice P. 260 # 1-4 P. 264 # 9, 11, 12 P. 266 #1-7