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Condensed Phases and Intermolecular Forces

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Presentation on theme: "Condensed Phases and Intermolecular Forces"— 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 Dispersion Dipole-Dipole Covalent Ionic Metallic
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 Particles want to clump together
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 [this substance = a gas at room temperature]
It’s a balancing act! Kinetic Energy Intermolecular Forces [this substance = a gas at room temperature]

12 Intermolecular Forces vs. 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 temperature
Intermolecular Forces 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):
Dispersion forces Dipole-Dipole forces Hydrogen bonds

17 occur between nonpolar molecules
1. Dispersion Forces: weakest IMF occur between nonpolar molecules

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

19 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 if charge cloud not symmetrical will induce asymmetry in neighbor’s charge cloud!

21 Nonpolar means no poles Can’t tell one end of molecule from other end
Nonpolar molecules 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: H2, N2, O2, Cl2, F2, I2, Br2 symmetrical molecules: CH4, C2H6, C3H8

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 I2 > Br2 > Cl2 > F2 C8H18 > C5H12 > C3H8 > CH4

24 Boiling point of N2 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 Molecules Polar means molecule has poles: (+) & (-) geometry and electron distribution are not symmetrical

27 What do you know about charge?
Opposites Attract! this time, situation is permanent! Examples: HI, CH3Cl

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


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:
NH3 Ar N2 HCl HF Ne O2 HBr CH3NH2 Hydrogen bonding Dispersion forces Dipole-dipole forces Dispersion Dipole-dipole

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