CHE140 Lecture Notes 11.2.15 – 11.13.15 11.2.15 Hydrogen Bonds Strongest Dipole – Dipole Forces London Dispersion Forces Weakest Hydrogen Bonds  Any H bonded directly to O, H, or N are called bond donors  O, H, and N are bond acceptors Dispersion Forces (London Dispersion Forces)  Weakest intermolecular force  The only intermolecular force present in non-polar molecules  Always present – in all molecules  Caused by a temporary dipole o A temporary dipole is caused by a temporary upset due to an uneven distribution of electrons (happens very quickly) Can we predict the strength of dispersion forces  Strength is determined by size and number of electrons  Polarizability – the tendency of the electrons in an atom or molecule to be distorted, creating a temporary dipole o High polarizability  stronger dispersion forces  Larger molecules with more electrons o Low polarizability  weaker dispersion forces  Smaller molecules with less electrons  Hydrocarbons – molecules with C and H only o Non-polar How can we measure the strength of intermolecular forces  Boiling (and melting) points o o Molecule Melting Point ( C) Boiling Point ( C) CH 4 -187 161