Describe relative positions and motions of particles in each of 3 phases

1. TOPIC: Intermolecular ForcesHow do particle diagrams of liquids & solids compare to those of gases?

2. Describe relative positions and motions of particles in each of 3 phases SOLID LIQUID GAS

3. Why do some substances exist as gases, some as liquids, and some as solids at room temp?

4. Part of answer has to do with forces between separate molecules (called intermolecular forces)

5. Intermolecular forces between molecules. They are weaker. Intramolecular forces are between individual atoms (we will learn this later) Intermolecular forces Intramolecular forces

6. Intermolecular Forces-IMF Inter means “between” or “among” Intermolecular forces = forces between neighbouring molecules

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

8. Most atoms don’t have a charge, unless they are ions, so we often refer to them as having partial charges and write them like this… This separation of Charge is responsible For the forces Between the molecules

9. 1. Dispersion Forces (van der waals): weakest IMF occur between nonpolar molecules Click here for animation (slide 4 of 13) • Nonpolar means no poles (+/-) • Can’t tell one end of molecule from other end • electrons are evenly distributed

10. instantaneous and momentary • fluctuate • results from motion of electrons if charge cloud not symmetrical will induce asymmetry in neighbor’s charge cloud!

11. 4 categories of Nonpolar Molecules(you need to memorize) Noble Gas molecules: He, Ne, Ar, Kr, Xe, Rn diatomics if both atoms are same: (7) H2, N2, O2, Cl2, F2, I2, Br2 Pure Hydrocarbons (CxHy): CH4, C2H6, C3H8 small symmetrical molecule CO2, CF4, CCl4

12. Dispersion Forces and Size Dispersion forces ↑ with molecule size larger the electron cloud, the greater the fluctuations in charge can be Rn > Xe > Kr > Ar > Ne > He I2 > Br2 > Cl2 > F2 C8H18 > C5H12 > C3H8 > CH4

13. 2. Dipole-dipole forces: • intermediate IMF • occur between polar 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)

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

16. Polar Molecules Polar means molecule has poles: (+) & (-) geometry and electron distribution are not symmetrical

17. 3. Hydrogen bonds: • strongest IMF • occur between molecules that have: • H-F H-O or H-N bonds ONLY

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

19. 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

20. Strength of Hydrogen Bonding Fluorine most electronegative element, so H-F bonds are most polar and exhibit strongest hydrogen bonding H-F > H-O > H-N (H-bonding…sound like FON to me!!!)

21. Hydrogen bonding: • strongest IMF • influences physical props a great deal • H-F > H-O > H-N

22. Strongest Intermolecular Force Hydrogen Bonding Dipole-Dipole Dispersion

23. Indicate type of IMF for each molecule: NH3 Ar N2 HCl HF Ne O2 HBr CH3NH2 Hydrogen bonding Dispersion forces Dispersion forces Dipole-dipole forces Hydrogen bonding Dispersion Dispersion Dipole-dipole Hydrogen bonding

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

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

26. DEMO… Gycerol Water Ethanol Acetone

27. IMF vs. Physical Properties If IMF  then: Boiling point  Melting point  Heat of Fusion  Heat of Vaporization  Surface tension  Viscosity  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

28. 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

29. 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)

30. IMF vs. Temp – it’s a balancing act IMF is weak = gas (YOU expect this) IMF is strong = solid/liquid (YOU expect this) IMF weak but Temp is low = molecules will move slower, having more opportunity to interact, may be a solid/liquid IMF is strong but Temp is high = molecules will move faster so ___________

31. Determining phase “Competition” between strength of IMF & average KE determines phase

32. Boiling point of N2 is 77 K (-196˚C) IMF are very weak dispersion forces

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