Condensed Phases and Intermolecular Forces

1. Condensed Phases and Intermolecular Forces

2. Let’s look at particle diagrams of liquids & solids and compare them to particle diagrams of gases

3. Describe & compare the relative positions and motions of particles in each of 3 phases:

4. The Question: Why do some substances exist as gases, some as liquids, and some as solids at room temp?

5. part of answer has to do with attractiveforces between separate but neighboring molecules

6. 2 broad categories ofattractiveforces come into play: • INTRAmolecular forces of attraction • INTERmolecular forces of attraction

7. Forces INTRAMOLECULAR INTERMOLECULAR Dispersion Covalent Dipole-Dipole Ionic Metallic Hydrogen Bonding

8. #1: Intramolecular Forces Intra means “within” Intramolecular attractive forces are chem bonding forces Definition: attractive forces that hold particles together in chemical bonds 3 types: ionic, covalent, or metallic bonds

9. # 2: Intermolecular Forces (IMF) (aka: van der Waals forces) Inter means “between” or “among” Intermolecular forces: attractive forces between neighboring molecules Intermolecular forces are weaker than Intramolecular forces

10. IMF: weaker than intramolecular (bonding) forces ≈ 5% to 15% of strength of intramolecular forces account for phase of matter at room temp strong IMF  condensed phase (solid/liquid) weak IMF  gas phase

11. IMF determine phase of matter phase is determined by: “competition” between strength of IMF and KE

12. If IMF are strong, substance will be solid or liquid at roomtemp • strong attractive forces keep particles • close together • If IMF are weak, substance will be gas at roomtemp • weak attractive forces allow particles to • spread far apart & be free to move

13. It’s all a balancing act! KE IMF [this substance = a gas at room temperature]

14. Intermolecular Forces vs. Kinetic Energy IMF KE [this substance = a condensed phase (solid/liquid)]

15. since T is measure of average KE, changing T can change phase changing T changes average KE of particles T change can allow change in phase

16. 3 types of IMF: 1. Dispersion forces 2. Dipole-Dipole forces 3. Hydrogen bonds

17. 1. Dispersion Forces: weakest IMF occur between non-polar molecules

18. dispersion forces & non-polar molecules • instantaneous and momentary change • electron cloud will fluctuate • results from motion of electronsdue to attractive • forces

19. Non-polar atoms/molecules non-polar means no poles can’t tell one end of atom/molecule from other end electrons are evenly distributed charge is evenly distributed atom/molecule: symmetrical

20. Non-polar Atoms/Molecules: monatomic atoms: He, Ne, Ar, Kr, Xe, Rn 0 diatomic elements: H2, N2, O2, Cl2, F2, I2, Br2 0 small symmetrical molecules: CO2 , CX4 0 carbon-hydrogen molecules: CH4, C2H6, C3H8

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

22. 2. Dipole-Dipole forces: • intermediate IMF • occur between polar molecules

23. What do you know about charge? Opposites Attract! this time, uneven distribution of electrons (charge) is permanent! examples:

24. Polar Molecules: • geometry NOTsymmetrical(asymmetrical) • uneven electron distribution • permanentseparation of charge • has poles: • one end partly (-) and • one end partly (+)

25. neighboring molecules orient themselves according to their opposing attractive charges

26. 3. Hydrogen Bonding forces: • strongest IMF • subtype of dipole-dipole attractive forces • attractive force occurs between H in one molecule • and • F, O or N in neighboring molecule • H-F H-O or H-N

27. Hydrogen Bonding Force H-O N-H 0 this attractive force occurs between molecules with FON!!!

28. Hydrogen Bonding Force hydrogen bonding force is special subtype of dipole-dipole attractive forces F, O, and N are all small and electronegative strong electron attraction H has only 1 electron, so if being pulled away H proton is almost “naked” H end of molecule is always positive & F, O, or N end is always negative

29. Strength of Hydrogen Bonding Force fluorine most electronegative element, so H-F bonds are mostpolar and exhibit strongest hydrogen bonding attractive forces H-F > H-O > H-N

30. IMF vs. Physical Properties • If strength of IMF then: • boiling point  • melting point  • heat of fusion  • heat of vaporization  while: • evaporation rate  • Ifstrength of IMF then: • boiling point  • melting point  • heat of fusion  • heat of vaporization  while: • evaporation rate 

31. boiling point of N2 is 77 K (-196˚C) IMF are very, very weak forces (dispersion)

32. Hydrogenbonding: • strongest IMF • influences physical props a great deal

33. IMFvs.Temp IMFmoreimportant astempislowered lowtemperature=low evaporation rate high temperature = high evaporation rate

34. 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