TOPIC: Intermolecular ForcesPart 1: Dispersion Forces

Do Now: How do particle diagrams of liquids & solids compare to those of gases?

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

***Separation of charge is responsible for the forces between

the Molecules***…

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

1. Dispersion Forces (van der waals):

● weakest IMF ● occur between nonpolar (symmetrical) molecules

●Click here for animation (slide 4 of 13)

• Nonpolar = no poles (no + or -)• 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 induce asymmetry in neighbor’s charge cloud!

4 categories of Nonpolar Molecules- all these have DISPERSION FORCES(you need to memorize)

Noble Gas –group 18: He, Ne, Ar, Kr, Xe, Rn

7 diatomic elements H2, N2, O2, Cl2, F2, I2, Br2

Pure Hydrocarbons – molecules with only C and H

General formula CxHy : examples = CH4, C2H6, C3H8

these 3 small symmetrical molecule CO2, CF4, CCl4

Dispersion Forces and SizeThe larger the molecule, the great

the Dispersion forces = stronger IMFB/c, the larger the electron cloud, the greater the fluctuations in charge Noble Gases: Rn has greater

dispersion forces = strongest IMF Diatomic Elements: I2 is larger then F2,

so I2 is larger (way more electrons) so greater dispersion forces, I2 is a solid at room temp. F2 is much smaller (less electrons) weaker dispersion forces, F2 is a gas at room temp.

You try…Which has the greatest dispersion forces

between it’s molecules?

C3H8 C8H18 CH4

C5H12

Which is most likely a liquid/solid (not a gas) at room temp?

C3H8 C8H18 CH4

C5H12

Which is most likely a gas at room temp?C3H8 C8H18 CH4

C5H12

The weaker the IMF, the lower the boiling point (BP)

Br2 = boils at 58.8°C, 137.8°F

Compared to

Water = boils at 100°C, 212°F

So water must have stronger IMF

TOPIC: Intermolecular ForcesPart 2: Dipole- Dipole and Hydrogen Bonding

Do Now: List the 4 categories of Nonpolar Molecules – all of these have DISPERSION FORCES Noble Gas –group 18:

He, Ne, Ar, Kr, Xe, Rn

7 diatomic elements

H2, N2, O2, Cl2, F2, I2, Br2

Pure Hydrocarbons – molecules with only C and H

General formula CxHy : examples = CH4, C2H6, C3H8

these 3 small symmetrical molecule

CO2, CF4, CCl4

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.

2. Dipole-dipole forces: • Stronger then dispersion forces• occur between polar (asymmetrical)

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

Polar Molecule shows permanent separation of charge; has poles: one end partially (-) & one end partially (+);Asymmetrical

3. Hydrogen bonds:

• strongest IMF • occur between molecules that have an :

H-F H-O or H-N bonds ONLY

Strongest Intermolecular Force

Hydrogen Bonding

Dipole-Dipole

Dispersion

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, so strongest IMF

H-F is stronger than H-O which is stronger than H-N

(H-bonding…sound like FON to me!!!)

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

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

H-F > H-O > H-N

IMF vs Physical PropertiesIf 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

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

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)

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

apart (overcome IMF)

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