1

Intermolecular forces holding molecules together.

Intermolecular Forces

- Surface Tension -

Cohesive forces:

interactions

between like

particles.

Adhesive forces:

interactions

between unlike

particles.

Intermolecular Forces

- Cohesive vs Adhesive Forces -

2

Two liquids that mix completely together are said to

be MISCIBLE - otherwise they are IMMISCIBLE.

Nonpolar CCl4

CuSO4(aq) Nonpolar octane (C8H18)

Polarity and Solubility

Mixing of Polar Liquids

CH3CH2 - O - H

CH3CH2 - O - H

CH3CH2 - O - H

• •

• •

• •

• •

• •

• •

- +

O H

H -

+

+

O H

H -

+

+

O H

H -

+

+

- +

- +

Polar molecules

can break up

groups of other

polar molecules

through dipole-

dipole

interactions,

resulting in a

thorough mixing

3

Mixing of Polar and Nonpolar Liquids

H H

O

+ +

-

H H

O

+ +

-

H H

O

+ +

-

H - C - C - C - H

H H H

H H H

H - C - C - C - H

H H H

H H H

The weak London forces present in the CH3CH2CH3 molecules are

not strong enough to break the hydrogen bonds, so the two liquids

are immiscible.

Mixing Nonpolar Liquids

H - C - C - C - C - C - C -H

H H H

H H H

H H H

H H H

C Cl

Cl

Cl

Cl

London forces are the only interactions, but the strength

of these forces between CCl4 molecules is comparable to

those between C6H14 molecules, so the two liquids are

miscible with each other.

General Observation:

4

More Solubility Examples

More than one intermolecular force may need to

be considered when examining solubility.

Solubility decreases

as relative energy of

H-bonding decreases

and dispersion

increases.

Combination of Forces

5

Extensive dispersion forces limit solubility of octanol in water.

Octanol

Combination of Forces – cont’d

Hydrophobic (“water-fearing”)

• Interaction that repels water, diminishes water solubility.

Hydrophilic (“water-loving”)

• Interaction that attracts water, promotes water solubility.

Hydrophobic Hydrophilic

Solubility Behavior

6

Sample Exercise 10.5

Which of these compounds should be very soluble in water and which should have limited solubility in water: carbon tetrachloride (CCl4), ammonia (NH3), hydrogen fluoride (HF), oxygen (O2)?

Chemistry In Action: The Killer Lake

Lake Nyos, West Africa

8/21/86

CO2 Cloud Released

1700 Casualties

Trigger?

• earthquake

• landslide

• strong Winds

7

Henry’s law - the solubility of a gas in a liquid is proportional to

the pressure of the gas over the solution ().

c = kP

c is the concentration (M) of the dissolved gas

P is the pressure of the gas over the solution

k is a constant (mol/L•atm) that depends only

on temperature

low P

low c

high P

high c

Solubility of Gases in Water

8

Sample Exercise 10.6

Calculate the solubility of oxygen in water in moles per liter at 1.00 atm pressure and 20°C. The mole fraction of O2 in air is 0.209. (Remember that the sum of the mole fractions of all the gases in a mixture equals 1.) The Henry’s law constant for oxygen at 20°C is 1.3 × 10-3 mol/L . atm.

States of matter as a function of temperature and pressure

Pre

ssu

re

Temperature

solid

liquid

gas

Phase Diagrams

9

Triple Point

Critical Point

Critical Temperature

Critical Pressure

Supercritical Fluid

Equilibrium Lines

Phase Diagram of Water

On the left is water; water is unusual because the solid is

less dense than the liquid. For most substances, the solid is

more dense than the liquid like for benzene (on the right).

Some Remarkable Properties of Water

10

Unique Property of Water

- density decreases when it freezes -

H-bonding results in cage-like

structure in solid state; less

dense than liquid state.

11

Water and Aquatic Life

Importance of Density:

• Lakes/rivers freeze from top down, allowing fish and

aquatic life to survive below.

• As surface waters warm or cool, nutrient-rich bottom

waters cycle to the surface; oxygen-rich surface

waters cycle to the bottom.

Phase Diagram for CO2

12

1. Positive slope for the solid-liquid interface (normal)

2. Sublimation occurs at room temperature under 1

atm of pressure.

3. There is a “critical point” at 73 atm and 31 oC

(varies from substance to substance).

4. Above the critical point the substance is known as

a “supercritical fluid” (not a liquid; not a gas).

Increasing the pressure normally would convert a

gas to a liquid but doesn’t happen.

5. Supercritical fluids have enhanced solvation

abilities, i.e. the fluid will dissolve greater amounts

of solute than normal.