The boiling point of a substance is a measure of the amount of energy needed to:

increase the speed of the molecules in the liquid phase to that of their gas phase

(The heavier it is, the higher the boiling point)

overcome any intermolecular forces (“molecular stickiness”)

Consider the following polar molecules:

Substance Boiling Point (C)Molar Mass Dipole moment

HF 2020 1.83

H2O 10018 1.80

HCl -8536 1.08

H2S -6034 0.97

SO2 -1064 1.61

The polarity of a molecule is more important in determining boiling point than its mass.

Water’s unusual properties are due to its:

Surface tension: uneven distribution of the intermolecular dipole-dipole interactions increases the strength of the intermolecular hydrogen bonding at the surface

Adhesive properties: the small size and high polarity of water causes it to adhere (wet) other polar substances

Cohesive properties: a large molecular dipole moment causes water to stick to itself

Non-polar polyesterPolar spider silk (a polypeptide)

The three common states of matter are:

Definite volumeDefinite shape

Definite volumeIndefinite shape

Indefinite volumeIndefinite shape

Three basic assumptions:

1. All matter is composed of small particles (atoms, molecules, ions). The amount of space (volume) that the particles take up depends upon the distance between the particles and not on the size of the particles themselves.

2. These particles are in constant random motion.

3. The particles undergo elastic collisions (no loss of energy) with each other and the walls of their container.

Absolute zero: The temperature at which ALL molecular motion stops. (0 K = -273°C = -460°F)

Solids: The particles in solids can be arranged in three different ways.

1. In a regular repeating pattern that extends over long distances.

A glassy material:Short-range order only

A crystalline material:Has long-range order

2. With a pattern that only repeats over short distances.

3. Amorphous solids have no long or short range order.

Charcoal:Amorphous carbon

Graphitic carbon:‘glassy’ carbon

Diamond:Crystalline carbon

Allotropes: different structural forms of the same element

Polar bonds

• In covalent bonds, electrons aren’t shared equally between the two nuclei when there is a difference in electronegativity.*

Electronegativity: Ability of an atom to attract electrons when in a molecule. Trend: Increases left right, bottom top

H F+ -

The permanent positive and negative charges on the molecule cause HF to be POLAR.

When electrons in a covalent bond are concentrated near one of the nuclei, the bond is POLAR.

H F Polar covalent bond

H H Nonpolar covalent bond

If the electrons are shared equally between the nuclei, the bond is NONPOLAR.

+ -

Now, you try some...

Do the following have polar or nonpolar bonds?

F2

HCl

BCl3

S8

H2O

C60

SO2

BN

nonpolar

nonpolar

nonpolarpolar

polar

polar

polar

polar

Because of the ASYMMETRIC bent structure of water, water is a polar molecule.

O

HH• Imagine that you’re holding up a mirror through one of the

atoms in the molecule. If you can find a place where the molecule is different on either side of the mirror, the molecule is polar.

Both bonds in water are polar.

How does this affect the polarity of the molecule?

What about CO2? C OO

This molecule is perfectly SYMMETRIC, which makes it NONPOLAR

VE = 16, 4 bonds to C

Are these planar molecules polar or nonpolar?

B

F

F

F

B

F

F

H

B

H

H

H

nonpolar polar nonpolar

Are these linear molecules polar or nonpolar?

O C O C O S C O

polar nonpolar polar

Three main types

Intermolecular Forces: Whether a substance is a liquid, solid or gas depends upon the strength of the intermolecular forces between adjacent molecules.

OOHH

OOHHLet’s get

to stickin’!!

Let’s get to

stickin’!!

1. H-bonding

2. Dipole-dipole

3. Induced dipole-induced dipole (also called London Dispersion Forces, or LDF)

STRONGEST

weakest

1. H-bonding: Is a special case of dipole-dipole interaction because it is significantly stronger. It occurs whenever H is

bonded to either N, O or F.

C F

H

H

H

CH

H

H

OH

C

H

H

H

OH

+

-

No intermolecularH-bonding possible

H-bonding between H on OH and O on next OH

2. Dipole-dipole: interaction between polar molecules that do not have H-bonding (ex: H2S, PH3, HCl)

3. Induced dipole-induced dipole: A temporary redistribution of the electron cloud around a NONPOLAR molecule induces a temporary dipole in an adjacent NONPOLAR molecule.

C6H14C6H14

It is because of the increasing size of the electron cloud that:

H H O ON N F F Cl Clare all gases at

room temp.

Br Br I Iwhile is a liquid and is a solid.

Phase Diagrams

SOLID

LIQUID

GAS

CriticalPoint

TriplePoint

vaporizationcondensation

sublimationdeposition

meltingfreezing

• Critical point: Temperature and pressure beyond which it is impossible to condense the vapor into a liquid

• Triple point: Temperature and pressure at which all three phases coexist

• Normal boiling point: Temperature at which a liquid will boil under 1 atm of external pressure

• Normal freezing point: Temperature at which a liquid will freeze under 1 atm of external pressure

What are the normal boiling and freezing points of water in C?

Normal BP: 100 CNormal FP: 0 C

Temperature (C)

Pre

ssur

e (a

tm)5.11

1.00

-78

Phase Diagram for Carbon Dioxide

Why is it not possible to have liquid CO2 at atmospheric pressure?

Because CO2 won’t condense at pressures below 5.11 atm.

1.00atm

0C 100C

Q: In terms of IMF’s, why is water’s phase diagram so different from the phase diagram for CO2?

CO2 H2O

A: There is intermolecular hydrogen bonding present in water, but only LDFs in CO2. Therefore, it takes more energy to separate water molecules from each other.