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

Forces

INTERMOLECULAR INTRAMOLECULAR

Dispersion

Dipole-Dipole

Hydrogen Bonding

Covalent

Ionic

Metallic

#1: Intramolecular ForcesIntramolecular 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 Intermolecular forces determine phasephase

“Competition” between strength of IMF & KE determines phase

If IMF are strong, substance will be solid or liquid at room tempParticles want to clump together

If IMF are weak, substance will be gas at room temp

Particles free to spread apart

It’s a balancing act!

Intermolecular Forces

Kinetic Energy

[this substance = a gas at room temperature]

Intermolecular Forces vs. Kinetic Energy

Intermolecular Forces

Kinetic Energy

[this substance = a condensed phase (solid/liquid)]

Why Temperature Changes Affect PhaseSince T is measure of average KE, changing T can change phase

Changing T will change average KE

Changing the temperature

Intermolecular Intermolecular ForcesForces

KineticEnergy

Intermolecular Forces:≈ 5% to 15% of strength of

intramolecular or bonding forces

Account for phase at room tempAccount for phase at room temp

Strong IMF condensed phase

Weak IMF gas phase

3 types of intermolecular forces (IMF):

1.Dispersion forces2.Dipole-Dipole forces3.Hydrogen bonds

1. 1. Dispersion ForcesDispersion Forces:

● weakest IMF ● occur between nonpolar molecules

2. 2. Dipole-dipole Dipole-dipole forcesforces:

• intermediate IMF • occur between polar molecules

3. 3. Hydrogen bondsHydrogen 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 induceinduce asymmetry in neighbor’s charge cloud!

Nonpolar molecules

NonNonpolar means nono 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 beRn > Xe > Kr > Ar > Ne > HeI2 > 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 means molecule has poles: (+) & (-) geometry and electron distribution are not symmetrical

Polar Molecules

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 bondingF, 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 BondingFluorine 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. TemperatureIMF more important as

temperature is loweredLow temperature – low evaporation rate

High temperature – high evaporation rate

Indicate type of IMF for each molecule:NH3

ArN2

HClHFNeO2

HBrCH3NH2

• Hydrogen bonding• Dispersion forces• Dispersion forces• Dipole-dipole

forces• Hydrogen bonding• Dispersion• Dispersion• Dipole-dipole• Hydrogen bonding