Jeffrey Mack

California State University,

Sacramento

Chapter 12 Intermolecular Forces and Liquids

• Why is water usually a liquid and not a gas?

• Why does liquid water boil at such a high

temperature for such a small molecule?

• Why does ice float on water?

• Why do snowflakes have 6 sides?

• Why is I2 a solid whereas Cl2 is a gas?

• Why are NaCl crystals little cubes?

All of these questions may be answered by

“Intermolecular Forces”

Why?

• The forces holding solids and liquids together are

called intermolecular forces.

• Intermolecular Forces are the attractions and

repulsions between molecules.

• They are NOT chemical bonds.

• The intermolecular forces of a substance may

exhibit are a function of:

1. charge (ions vs. neutrals)

2. polarity (molecular shape, dipoles)

3. molar mass

Intermolecular Forces

Intermolecular forces influence chemistry in many

ways:

• They are directly related to properties such as

melting point, boiling point, and the energy

needed to convert a solid to a liquid or a liquid to

a vapor.

• They are important in determining the solubility

of gases, liquids, and solids in various solvents.

• They are crucial in determining the structures of

biologically important molecules such as DNA

and proteins.

Intermolecular Forces

C–H (413 kJ/mol)

C=C (610 kJ/mol)

C–C (346 kJ/mol)

CN (887 kJ/mol)

Covalent Bonding Forces for Comparison of Magnitude

Intermolecular forces are much weaker than the bonds that

make up compounds.

20 to 30 kJ/mol

D(H-Cl) = 432 kJ/mol

Covalent Bonding Forces for Comparison of Magnitude

The forces that govern charged particles are defined by

Coulomb’s law.

These are the strong forces that lead to salts with high melting

temperatures.

2

Q QF k

r

NaCl, mp = 800

°C

Q = the charges on the cation and

anion

r = the distance between

k = a constant

MgO, mp = 2800 °C H2O, mp = 0 °C

Greater charge = stronger attraction

Greater distance = weaker attraction

Ion-Ion Forces: Formal Charges

The polar nature of water provides for attractive forces between ions and water.

Attractions Between Ions & Permanent Dipoles

When a cation exists in solution, it is surrounded by the

negative dipole ends of water molecules.

When as anion exists in solution, it is surrounded by

the positive dipole ends of water molecules.

+

Solvation of Ions

As the size of the ion increases, the

exothermicity of the process decreases.

This is due to the weaker ion-dipole forces.

Enthalpies of Hydration: A Measure of Ion-Dipole Forces

As the size of the ion increases, the

exothermicity of the process decreases.

This is due to the weaker ion-dipole forces.

Enthalpies of Hydration: A Measure of Ion-Dipole Forces

Water is highly polar and can interact with positive ions to give hydrated ions in water.

Attraction Between Ions & Permanent Dipoles

Molecular Geometry Non-Polar Molecule

Linear Both atoms the same (outer the same for linear tri-atomic)

Trigonal Planar All bonding groups the same

Tetrahedral All bonding groups the same

Trigonal bipyramidal All bonding groups the same or both axial groups the same and all three equatorial groups the same,

Octahedral All bonding groups the same or all groups trans to one another the same.

Any deviations of symmetry yield a polar molecule.

Molecular Polarity

Dipole-dipole forces bind molecules having permanent dipoles to one another.

Dipole-Dipole Forces

Compound Molar Mass

(amu)

Dipole

Moment

(D)

BP

(K)

CH3CH2CH3 44.1 0.1 231

CH3OCH3 46.07 1.3 248

CH3Cl 40.49 1.9 249

CH3CN 41.05 3.9 355

molar Mass

Dipole

BP

0

50

100

150

200

250

300

350

400

1

2

3

4

Dipole Moment

Boiling point

Molar Mass

As the polarity for a

given set of molecules

with similar molar

masses increases, the

boiling point increases.

Dipole-Dipole Forces

A special form of dipole-dipole attraction, which

enhances dipole-dipole attractions.

H-bonding is strongest when X and Y are N, O, or F

Hydrogen Bonding

capillary action surface tension why ice floats

The water molecules

network with one

another.

H–bonding in water

brings about a

network of

interactions which

explain phenomena

such as:

Hydrogen Bonding

Molecules at surface behave differently than those in the

interior.

• Molecules at surface experience net INWARD force of attraction.

• This leads to SURFACE TENSION — the energy reqired to break the surface.

Surface Tension

SURFACE TENSION also leads to spherical

liquid droplets.

Surface Tension

IMF’s also lead to CAPILLARY action and to the

existence of a concave meniscus for a water column.

concave

meniscus

ATTRACTIVE FORCES

between water

and glass

COHESIVE FORCES

between water

molecules

H2O in

glass

tube

Capillary Action

Movement of water up a piece of paper is a

result of H-bonds between H2O and the OH

groups of the cellulose in the paper.

Capillary Action

• Ice, H2O(s) floats because it is less dense than

water, H2O(l).

• The H–bonds allow the molecules in the liquid phase

to to approach closer than normal for non H–bonding

liquids.

• This is why water has its maximum density at 4 °C.

Ice has open lattice-like structure.

Ice density is < liquid and so solid floats on water.

One of the VERY few

substances where

solid is LESS DENSE

than the liquid.

Hydrogen Bonding in H2O

The Consequences of Hydrogen Bonding

Boiling Points of Simple Hydrogen-Containing Compounds

18 g/mol

20 g/mol

17 g/mol

16 g/mol

Notice that water has an

unusually high bp for its Mwt...

This is a result of

hydrogen bonding!

H-bonding leads to

abnormally high

boiling point of water.

Hydrogen Bonding

Dipole-induced

dipole

How can non-polar molecules such as O2 and

I2 dissolve in water?

The water dipole INDUCES a dipole in the O2

electron cloud.

Forces Involving Induced Dipoles

How can non-polar molecules such as O2 and I2

dissolve in water?

The water dipole INDUCES a dipole in the O2

electron cloud.

Once polarized, the O2 is attracted to additional

water molecules.

Induced Dipole Forces

The degree to which electron cloud of an atom or molecule can

be distorted is measured by its polarizability.

The larger the molecule, the more easily it is polarized.

As the electrons in a molecule become more loosely held and

more spread out, the greater the degree of polarizibility in

the molecule.

The explains the trend we see in solubility.

Induced Dipole Forces

Formation of a dipole in two nonpolar I2 molecules.

Induced dipole-

induced dipole

Forces Involving Induced Dipoles

London dispersion forces exist between all

molecules.

London dispersion forces are a function of molecular

polarizability.

The Polarizability of a molecule is measured by the

ease with which an electron cloud can be distorted.

The larger the molecule (the greater the number of

electrons) the greater polarizability. The greater the

surface area available for contact, the greater the

dispersion forces.

London dispersion forces therefore increase as

molecular weight increases.

London Dispersion Forces

For molecules with the same relative polarizability, the forces

scale with molar mass:

CH4

C2H6

C3H8

C4H10

Note the linear relation between bp and molar mass.

Higher Mwt. = larger induced dipoles.

Molecule BP (oC)

CH4 (methane) - 161.5

C2H6 (ethane) - 88.6

C3H8 (propane) - 42.1

C4H10 (butane) - 0.5

London Dispersion Forces

The induced forces between I2 molecules are very weak, so solid I2 sublimes (goes from a solid to gaseous molecules).

Forces Involving Induced Dipoles

Intermolecular Forces Summary

Intermolecular Forces

• Of the three states of matter, liquids are the

most difficult to describe precisely.

• Under ideal conditions the molecules in a gas

are far apart and are considered to be

independent of one another.

• The structures of solids can be described easily

because the particles that make up solids are

usually in an orderly arrangement.

• The particles of a liquid interact with their

neighbors, like the particles in a solid, but, unlike

in solids, there is little long-range order.

Properties of Liquids

Liquids

• Particles are in constant

motion.

• Particles are in close

contact.

• Liquids are almost

incompressible

• Liquids do not fill the

container.

• Intermolecular forces are

relevant.

Properties of Liquids

Breaking IM forces requires energy.

The process of vaporization is

therefore endothermic.

In order for a

liquid to vaporize,

sufficient energy

must be available

to overcome the

intermolecular

forces.

Liquids: Vaporization

The HEAT OF VAPORIZATION is the heat required to

vaporize the liquid at constant P.

vapH

Liquid + energy = Vapor

Notice how the types of forces greatly affects the Hvap and boiling point.

Compound IMF ∆vapH (kJ/mol) BP

H2O

SO2

Xe

H-bonds

Dipole

London

40.7

26.8

12.6

100 °C

47 °C

107 °C

Liquids: Enthalpy of Vaporization

When molecules of liquid are in the vapor state, they

exert a VAPOR PRESSURE.

The EQUILIBRIUM VAPOR

PRESSURE is the pressure

exerted by a vapor over a liquid

in a closed container.

rate of evaporation = the rate of condensation.

At equilibrium,

Liquids: Enthalpy of Vaporization

When molecules of liquid

are in the vapor state, they

exert a VAPOR PRESSURE

EQUILIBRIUM VAPOR

PRESSURE is the pressure

exerted by a vapor over a

liquid in a closed container

when the rate of evaporation

= the rate of condensation.

Vapor Pressure

Recall from kinetic molecular theory…

As Temp increases, so does the average KE of the

particles.

This means that there are more particles that can

escape into the gas phase!

Vapor Pressure

Liquid boil when Pvap = Patm

(Vapor pressure equals

atmospheric pressure.

Boiling Point

As the external pressure is lowered, the vapor pressure equals the external pressure at a lower temperature. Boiling therefore occurs at a reduced temperature.

Boiling Point at Reduced Pressure

When can cools, vapor pressure of water drops. Pressure inside of the can is less than that of atmosphere, which collapses the can.

Consequences of Vapor Pressure Changes

The vapor pressure of a liquid is seen to increase

exponentially with temperature.

Equilibrium Vapor Pressure

Liquid in flask evaporates and exerts pressure on

manometer.

Measuring Equilibrium Vapor Pressure

A plot of lnPvap vs. yields a

slope of:

∆vapH° is related to T and P by

the Clausius-Clapeyron

equation

vap

vap

HlnP C

RT

1

T

vap

slope :

H

R

D-

y-intercept

= C

The Temperature Dependence of Vapor Pressure Goes As:

Molecules in the Liquid

State vapH Volatility

Equilibrium Vapor

Pressure

Boiling Point

Strong IMF’s

More Endothermic

Low Low High

Weak IMF’s

Less Endothermic

High High Low

Liquids: IMF’s Summary