TOPIC: Intermolecular Forces

How do particle diagrams of liquids & solids compare to those

of gases?

Describe relative positions and motions of particles in each of 3 phases

SOLID LIQUID GAS

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 (called intermolecular forces)

Intermolecular forces between molecules. They are weaker. Intramolecular forces are between individual atoms (we will learn this later)

Intramolecular forces

Intermolecular forces

Intermolecular Forces-IMF

Inter means “between” or “among”Intermolecular forces = forces

between neighbouring compounds

All molecules have Dispersion forces (the regents calls these Van der Waals)

2 other types of forces (IMF): 1. Dipole-Dipole forces2. Hydrogen bonds

-if one of these are present, they are more important.

Most atoms don’t have a charge, unless they are ions, so we often refer to them as having partial charges and write them like this…

This separation of Charge is responsible

For the forcesBetween themolecules

1. Dispersion Forces (van der waals)1. Dispersion Forces (van der waals):

● weakest IMF ● occur between nonpolar molecules

●Click here for animation (slide 4 of 13)

• NonNonpolar means nono poles (+/-)

• Can’t tell one end of molecule from other end

• electrons are evenly distributed

•instantaneous and momentary•fluctuate•results from motion of electrons

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

4 categories of Nonpolar Molecules(you need to memorize)

Noble Gas molecules: He, Ne, Ar, Kr, Xe, Rn

diatomics if both atoms are same: (7) H2, N2, O2, Cl2, F2, I2, Br2

Pure Hydrocarbons (CxHy): CH4, C2H6, C3H8

small symmetrical molecule CO2, CF4, CCl4

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

2. 2. Dipole-dipole forcesDipole-dipole forces: • intermediate IMF • occur between polar molecules (they have a

partial charge at each pole – one is typically much larger than the other)

• Click here for animation (slide 3 of 13)

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

3. 3. Hydrogen bondsHydrogen bonds:

• strongest IMF • occur between molecules that have: H-F H-O or H-N bonds ONLY

Hydrogen Bonding

H-O N-H

Occurs between molecules with H-F, H-O, or H-N bonds

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

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

(H-bonding…sound like FON to me!!!)

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

H-F > H-O > H-N

Strongest Intermolecular

Force

Hydrogen Bonding

Dipole-Dipole

Dispersion

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

OH

H

OH

H

H-Bonding = strongest IMFmuch harder to “pull” molecules apart

C

Dispersion Forces= weakest IMFmuch easier to “pull” molecules apart

C H

H

HH

H

H

HH

IMF vs.

Physical Properties

If IMF then: Boiling point Melting point Heat of Fusion Heat of Vaporization

while: Evaporation Rate

Change from solid to liquid w/o changing temp

Change from liquid to gas w/o changing temp

Rate at which conc. will go from liquid to gas

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

#1 reason = IMF

#2 reason = temperature (avg. KE)

IMF vs. Temp

IMF more important as temp is lowered

Low temperature – low evaporation rate High temperature – high evaporation rate

Intermolecular forces determine phaseIntermolecular forces determine phase“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 tempParticles free to spread apart

It’s a balancing act!

Intermolecular Forces

weak

Kinetic Energy

High (fast)

[this substance = a gas at room temperature]

Intermolecular Forces

strong Kinetic Energy

Low (slow)

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

REMEMBER…

Temp = average KEIf we change T we change KEIncrease KE will help “pull” molecules

apart (overcome IMF)