When Intermolecular Forces Rule CHM 101 Sinex When Intermolecular Forces Rule PGCC CHM 2000 Sinex

Comparison of properties for the states of matter Property Solid Liquid Gas Volume definite indefinite Shape Relative density high ~1 g/mL very low Compressibility low-none very high Fluidity/mobility no yes Particle packing close far apart Diffusion rate very slow medium fast Motion of particles vibrations only translation rotation Forces between particles IMF’s or bonding IMF’s none if ideal gas

The liquid state of matter Molecules are closely packed but free to move about randomly. There must be a force of attraction between the molecules.

As molecules start to vaporize Molecules are great distances apart in gaseous state but free to move about randomly. What about in solids? Click here for a simulation of the states of matter

Force-distance relationship For the Fattraction proportional to 1/distance molecules in contact with each other

Force-distance relationship 1

Force-distance relationship 2

Force-distance relationship 3

Force-distance relationship 4

Force-distance relationship 5 Less than 20% the strength at 5 molecular distances

Inverse functions of distance to various powers The strength of IMF’s H-bonding (fixed distance ~200 pm) Ion-ion (1/r) Ion-dipole (1/r2) Dipole-dipole (1/r3) Ion-induced dipole (1/r4) Dipole-induced dipole (1/r6) Induced dipole-induced dipole (1/r6) Inverse functions of distance to various powers Increasing strengths of IMF How do the other functions behave compared to 1/r? Click here for an interactive Excel spreadsheet to explore

Hydrogen bonding in water O-H 96 pm covalent bond O H 186 pm …

Base Pairs in DNA: H-bonding AT pair                            GC pair Why not here?

What holds the iodine molecules together in solid iodine? 272 pm for I2 covalent bond 427 pm between two molecules in crystalline structure About half an iodine atom space between molecules

weak intermolecular forces between layers Graphite weak intermolecular forces between layers (van der Waals forces) C-C bond distance: 142 pm C to C distance between layers – 336 pm graphene