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Condensed Phases and Intermolecular Forces. Fundamentals  How do particle diagrams of liquids & solids compare to those of gases?

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Presentation on theme: "Condensed Phases and Intermolecular Forces. Fundamentals  How do particle diagrams of liquids & solids compare to those of gases?"— Presentation transcript:

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 of forces need to be aware of

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

7 #1: Intramolecular Forces  Intramolecular forces = attractive forces that hold particles together in bonds (ionic, covalent, or metallic)  Intra means “within”  Intramolecular forces = 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  “Competition” between strength of IMF & KE determines phase

10  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

11 It’s a balancing act! Intermolecular Forces Kinetic Energy [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 will change average KE

14 Changing the temperatureIntermolecularForces Kinetic Energy

15 Intermolecular Forces:  ≈ 5% to 15% of strength of intramolecular or bonding forces  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 1. Dispersion Forces: ● weakest IMF ● occur between nonpolar molecules

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

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

20 Dispersion Forces & Nonpolar molecules instantaneous and momentary fluctuate results from motion of electrons induce if charge cloud not symmetrical will induce asymmetry in neighbor’s charge cloud!

21 Nonpolar molecules Nonno Nonpolar means no poles Can’t tell one end of molecule from other end electrons are evenly distributed Symmetrical

22 Examples of Nonpolar Molecules  monatomic gas molecules:  He, Ne, Ar, Kr, Xe, Rn  diatomics if both atoms are same:  H 2, N 2, O 2, Cl 2, F 2, I 2, Br 2  symmetrical molecules:  CH 4, C 2 H 6, C 3 H 8

23 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  I 2 > Br 2 > Cl 2 > F 2  C 8 H 18 > C 5 H 12 > C 3 H 8 > CH 4

24 Boiling point of N 2 is 77 K (-196˚C) IMF are very weak dispersion forces

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

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

27 What do you know about charge?  Opposites Attract!  this time, situation is permanent!  Examples: HI, CH 3 Cl

28 Hydrogen Bonding H-O N-H Occurs between molecules with H-F, H-O, or H-N bonds (FON!!!)

29 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

30 Hydrogen bonding: strongest IMF influences physical props a great deal

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32 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

33 IMF vs. Physical Properties  If IMF  then:  Boiling point   Melting point   Heat of Fusion   Heat of Vaporization  while:  Evaporation Rate 

34 Intermolecular Force vs. Temperature  IMF more important as temperature is lowered  Low temperature – low evaporation rate  High temperature – high evaporation rate

35 Indicate type of IMF for each molecule:  NH 3  Ar N2N2  HCl  HF  Ne O2O2  HBr  CH 3 NH 2 Hydrogen bonding Dispersion forces Dipole-dipole forces Hydrogen bonding Dispersion Dipole-dipole Hydrogen bonding


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