Condensed Phases and Intermolecular Forces

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

Part of answer has to do with forces between separate molecules
2 broad categories of forces need to be aware of

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

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

# 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

Intermolecular forces determine phase
“Competition” between strength of IMF & KE determines phase

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

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

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

Why Temperature Changes Affect Phase
Since T is measure of average KE, changing T can change phase Changing T will change average KE

Changing the temperature
Intermolecular Forces Kinetic Energy

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

3 types of intermolecular forces (IMF):
Dispersion forces Dipole-Dipole forces Hydrogen bonds

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

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

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

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!

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

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

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

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

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

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

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

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

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

Hydrogen bonding: strongest IMF influences physical props a great deal

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

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

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

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