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Condensed Phases and Intermolecular Forces

Condensed Phases and Intermolecular Forces. Fundamentals. How do particle diagrams of liquids & solids compare to those of gases?. Describe relative positions and motions of particles in each of 3 phases. The Question.

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Condensed Phases and Intermolecular Forces

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  1. Condensed Phases and Intermolecular Forces

  2. Fundamentals How do particle diagrams of liquids & solids compare to those of gases?

  3. Describe 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 forces between separate molecules 2 broad categories offorces need to be aware of

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

  7. #1: Intramolecular Forces Intramolecular forces: attractive forces that hold particles together in chemical bonds (ionic, covalent, or metallic) Intra means “within” Intramolecular forces = chem bonding forces

  8. # 2: Intermolecular Forces (IMF) (aka: van der Waals forces) Inter means “between” or “among” Intermolecular forces: forces between neighboring molecules Intermolecular forces are weaker than Intramolecular forces

  9. Intermolecular forces determine phase phase is determined by: “competition” between strength of IMF and KE

  10. If IMF are strong, substance will be solid or liquid at roomtemp • particles stay close due to attractive forces • If IMF are weak, substance will be gas at roomtemp • little attractive forces since particles far apart & free to spread apart

  11. It’s a balancing act! Kinetic Energy Intermolecular Forces [this substance = a gas at room temperature]

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

  13. Why Temperature Changes Affect Phase since T is measure of average KE, changing T can change phase changing T changes average KE of particles

  14. Changing the temperature Intermolecular Forces Kinetic Energy

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

  16. 3 types of intermolecular forces (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 • fluctuate • results from motion of electrons if charge cloud not symmetrical willinduce asymmetry in neighbor’s charge cloud!

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

  20. Examples of Nonpolar 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 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

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

  23. What do you know about charge? Opposites Attract! this time, situation is permanent! examples: HI, CH3Cl

  24. Dipole-dipole Forces & Polar Molecules molecule: - asymmetrical - shows permanent separation of charge - has poles: one end partly (-) & one end partly (+)

  25. Polar Molecules polar means molecule haspoles:(+)&(-) geometry and electron distribution areNOTsymmetrical

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

  27. Hydrogen Bonding Force H-O N-H 0 occurs between molecules with H-F, H-O, or H-N bonds & neighbor’s H-F, H-O, or H-N (FON!!!)

  28. Hydrogen Bonding Force Hydrogen bonding force is extreme case of dipole-dipole force 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 H-F > H-O > H-N

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

  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

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