TOPIC: Intermolecular Forces Part 2: Dipole- Dipole and Hydrogen Bonding Do Now: List the 4 categories of Nonpolar Molecules – all of these have DISPERSION FORCES Noble Gas –group 18:  He, Ne, Ar, Kr, Xe, Rn 7 diatomic elements  H 2, N 2, O 2, Cl 2, F 2, I 2, Br 2 Pure Hydrocarbons – molecules with only C and H  General formula C x H y : examples = CH 4, C 2 H 6, C 3 H 8 these 3 small symmetrical molecule  CO 2, CF 4, CCl 4

All molecules have Dispersion forces (the regents calls these Van der Waals) 2 other types of forces (IMF): 1. Dipole-Dipole forces 2. Hydrogen bonds -if one of these are present, they are more important.

2. Dipole-dipole forces 2. Dipole-dipole forces: Stronger then dispersion forces occur between polar (asymmetrical) molecules (they have a partial charge at each pole – one is typically much larger than the other) Click here for animation (slide 3 of 13) Click here for animation

Dipole-dipole Forces & Polar Molecules Polar Molecule shows permanent separation of charge; has poles: one end partially (-) & one end partially (+); Asymmetrical

3. Hydrogen bonds 3. Hydrogen bonds: strongest IMF occur between molecules that have an : H-F H-O or H-N bonds ONLY Strongest Intermolecular Force Hydrogen Bonding Dipole-Dipole Dispersion

Hydrogen Bonding H-O N-H Occurs between molecules with H-F, H-O, or H-N bonds

Hydrogen Bonding Hydrogen bonding is extreme case of dipole-dipole bonding F, O, and N are all small and electronegative  strong electrons attraction  H has only 1 electron, so if being pulled away H proton is almost “naked” H end is always positive & F, O, or N end is always negative

Strength of Hydrogen Bonding Fluorine most electronegative element, so  H-F bonds are most polar and exhibit strongest hydrogen bonding, so strongest IMF  H-F is stronger than H-O which is stronger than H-N (H-bonding…sound like FON to me!!!)

O H H O H H H-Bonding = strongest IMF much harder to “pull” molecules apart

C Dispersion Forces= weakest IMF much easier to “pull” molecules apart C H H H H H H H H

Hydrogen bonding: strongest IMF influences physical props a great deal H-F > H-O > H-N IMF vs Physical Properties If IMF  then:  Boiling point   Melting point   Heat of Fusion   Heat of Vaporization  while:  Evaporation Rate  Change from solid to liquid w/o changing temp Change from liquid to gas w/o changing temp Rate at which conc. will go from liquid to gas

Why do some substances exist as gases, some as liquids, and some as solids at room temp? #1 reason = IMF

 If IMF are strong, substance will be solid or liquid at room temp  Particles want to clump together  If IMF are weak, substance will be gas at room temp  Particles free to spread apart

Why do some substances exist as gases, some as liquids, and some as solids at room temp? #1 reason = IMF #2 reason = temperature (avg. KE)

Temp = average KE If we change T we change KE Increase KE will help “pull” molecules apart (overcome IMF)

Indicate type of IMF for each molecule: NH 3 Ar N 2 HCl HF Ne O 2 HBr CH 3 NH 2 Hydrogen bonding Dispersion forces Dipole-dipole forces Hydrogen bonding Dispersion Dipole-dipole Hydrogen bonding