Chem 102

General Chemistry II

Lecture 2
Instructor: Assoc. Prof. Umut Aydemir
 GENERAL CHEMISTRY
P R I N C I P L E S A N D M O D E R N A P P L I C AT I O N S
ELEVENTH EDITION

PETRUCCI HERRING MADURA BISSONNETTE

Intermolecular Forces:
Liquids and Solids 12
PHILIP DUTTON
UNIVERSITY OF WINDSOR
DEPARTMENT OF CHEMISTRY AND BIOCHEMISTRY

Slide 12 - 2 General Chemistry: Chapter 12 Copyright © 2017 Pearson Canada Inc.

 Intermolecular Forces: Liquids and Solids
CONTENTS

12-1 Intermolecular Forces

12-2 Some Properties of Liquids

12-3 Some Properties of Solids

12-4 Phase Diagrams

Network Covalent Solids and Ionic
12-5
Solids

12-6 Crystal Structures

Energy Changes in the Formation

12-7
of Ionic Crystals

Slide 12 - 3 General Chemistry: Chapter 12 Copyright © 2017 Pearson Canada Inc.

 The van der Waals Equation of State

One of the earliest and most important improvements on the ideal

gas equation of state was proposed in 1873 by the Dutch physicist
Johannes van der Waals. The van der Waals equation of state is:

To obtain this equation, the ideal gas law—which ignores

interactions between molecules—requires two modifications to
describe the effects of the forces between molecules, which are
repulsive at short distances and attractive at large distances.

Slide 12 - 4
 The van der Waals Equation of State

The constants a and b are obtained by fitting experimental

P-V-T data for real gases. The units for these constants are
a: atm L2 mol-2
b: L mol-1
when R has the units L atm mol-1 K-1.

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 The van der Waals Equation of State

Slide 12 - 6
 Intermolecular Forces
For many purposes, the detailed shape of the
potential is less important than two
characteristic parameters: the depth and
location of the potential minimum. A simple
expression frequently used to model these
interactions between atoms is the Lennard–
Jones potential:

where e is the depth and s is the distance at

which V(R) passes through zero.

This potential has an attractive part, proportional to R-6, and a repulsive

part, proportional to R-12. The minimum is located at 21/6 s or 1.22 s,
where s is the value of R at which V(R) = 0.
Slide 12 - 7 General Chemistry: Chapter 12 Copyright © 2017 Pearson Canada Inc.
 Intermolecular Forces: Origins in Molecular Structure

❑ Intermolecular forces are distinguished from intramolecular

forces, which lead to the covalent chemical bonds.

❑ Intramolecular forces between atoms in the covalent bond

establish and maintain the structure of discrete molecules:
They are strong, directional, and comparatively short
ranged.

Slide 12 - 8 General Chemistry: Chapter 12 Copyright © 2017 Pearson Canada Inc.

 Intermolecular Forces: Origins in Molecular Structure

Intermolecular forces differ from intramolecular forces in several

important ways:
1. Intermolecular forces are generally weaker than covalent chemical
bonds. For example, it takes 239 kJ to break 1 mol of Cl—Cl covalent
bonds, but only 1.2 kJ to overcome 1 mol of Ar—Ar attractions.
2. Intermolecular forces are much less directional than covalent chemical
bonds.
3. Intermolecular forces operate at longer range than covalent chemical
bonds.

All intermolecular and intramolecular forces arise because matter is

composed of electrically charged particles whose interactions with one
another are all described by Coulomb’s Law. It is useful to distinguish
different classes of forces based on their strength, directionality, and range.

Slide 12 - 9 General Chemistry: Chapter 12 Copyright © 2017 Pearson Canada Inc.

 Ion–Ion Forces
❑ Ions of like charge repel one another, and ions of unlike charge attract
one another. These ion–ion forces can be as strong as those in the
covalent bond, and they are long ranged.
❑ Ion–ion forces are not directional; each ion interacts equally strongly
with neighboring ions on all sides. Ion–ion forces lead to the formation of
ionic bonds through the Coulomb stabilization energy.

Slide 12 - 10 General Chemistry: Chapter 12 Copyright © 2017 Pearson Canada Inc.

 Dipole–Dipole Forces
❑ The dominant force between polar molecules is the
dipole–dipole force.

FIGURE 12-2
Electrostatic potential maps and properties of CF4 and CHF3

Slide 12 - 11 General Chemistry: Chapter 12 Copyright © 2017 Pearson Canada Inc.

 Dipole–Dipole Forces
❑ The dominant force between polar molecules is the
dipole–dipole force.

A molecule of HCl can be represented as having a small net negative charge on the Cl
end, balanced by a small net positive charge on the H end. The forces between two HCl
molecules depend on their orientations. (a) The oppositely charged ends (blue arrows)
are closer than the ends with the same charge (red arrows). This gives a net attractive
force. (b) Here, the opposite is true, and the net force is repulsive.
Slide 12 - 12 General Chemistry: Chapter 12 Copyright © 2017 Pearson Canada Inc.
 Dipole–Dipole Interactions

FIGURE 12-1
Dipole-Dipole Interactions

Slide 12 - 13 General Chemistry: Chapter 12 Copyright © 2017 Pearson Canada Inc.

 Dipole–Dipole Interactions

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 Hydrogen Bonding

FIGURE 12-4
Comparison of boiling points of some hydrides of the elements of groups 14, 15, 16, and 17

Slide 12 - 15 General Chemistry: Chapter 12 Copyright © 2017 Pearson Canada Inc.

 Hydrogen Bonds
The dramatic deviations from these systematic trends shown by HF,
NH3, and especially H2O indicate the strength and importance of the
special type of bond that is common to these cases, a hydrogen bond.
Such a bond forms when an H atom bonded to an F, O, or N atom
(highly electronegative atoms) also interacts with the lone electron pair
of another such atom nearby.

A single hydrogen bond between water molecules forms a dimer. This bond is far weaker
than a covalent bond but still strong enough to resist dissociation at room temperature.
The shared hydrogen (H) atom at the center approaches the neighboring oxygen (O)
atom quite closely.
Slide 12 -6
 Hydrogen Bonds

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 Hydrogen Bonds

The hydrogen bond that forms is weaker than an ordinary O-H covalent bond,
but the interaction is significantly stronger than most other intermolecular
interactions. Like most hydrogen bonds, that in water is nearly linear but
asymmetric, with the H atom closer to and more strongly bound to one of the O
atoms.
Slide 12 - 18
 Hydrogen Bonding

FIGURE 12-6
The hydrogen bond illustrated

Slide 12 - 19 General Chemistry: Chapter 12 Copyright © 2017 Pearson Canada Inc.

 Hydrogen Bonding in Water

FIGURE 12-7
Hydrogen bonding in water

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 Hydrogen Bonding in Water

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 Hydrogen Bonding in Water

Ice

FIGURE 12-8
Solid and liquid densities compared Solid paraffin
Slide 12 - 22 General Chemistry: Chapter 12 Copyright © 2017 Pearson Canada Inc.
 Intermolecular and Intramolecular Hydrogen Bonding

Electrostatic potential There is no intramolecular

map of salicylic acid hydrogen bonding in
showing intramolecular para-hydroxybenzoic acid
hydrogen bonding.
(isomer of salicylic acid)
Slide 12 - 23 General Chemistry: Chapter 12 Copyright © 2017 Pearson Canada Inc.
 Hydrogen Bonding in Living Matter

Hydrogen bonding between

guanine (left) and cytosine (right)
in DNA
Slide 12 - 24 General Chemistry: Chapter 12 Copyright © 2017 Pearson Canada Inc.
 Ion–Dipole Forces
These electrostatic forces occur when a polar molecule
is near an ion. The interaction between a polar solvent
molecule, such as water, and a dissolved ion is the most
common case of ion–dipole interaction.

Solvation of ions in liquid water. The water molecules have dipole moments; thus, the
oxygen (O) atoms bear small, negative charges, whereas the hydrogen (H) atoms bear
small, positive charges. (a) Positive ions are attracted to neighboring water molecules in
aqueous solution by ion–dipole forces. (b) Negative ions form hydrogen bonds with water,
with a nearly linear bond from O to H to the anion.
Slide 12 - 25 General Chemistry: Chapter 12 Copyright © 2017 Pearson Canada Inc.
 Charge-Induced Dipole Forces: Polarizability
❑ The electrons in a nonpolar molecule or atom are distributed symmetrically,
but the distribution can be distorted by an approaching electrical charge. An
argon (Ar) atom has no dipole moment, but an approaching Na+, with its
positive charge, attracts the electrons on the side near it more strongly than
those on the far side. By tugging on the nearby electrons harder, Na+
induces a temporary dipole moment in the Ar atom.

As an ion approaches an atom or molecule, its electrostatic field distorts the distribution
of the outer electrons. The effect of this distortion is to create a dipole moment that exerts
an attractive force back on the ion.
Slide 12 - 26 General Chemistry: Chapter 12 Copyright © 2017 Pearson Canada Inc.
Induced Dipole–Induced Dipole Forces: London Dispersion Forces
❑ Helium (He) atoms, like the atoms of the other noble gases, are electrically neutral
and nonpolar, so none of the forces discussed so far explains the observed fact that
there are attractions between He atoms. We know such attractions must exist,
because helium becomes a liquid at 4.2 K and 1 atm. Attractions between neutral,
nonpolar atoms or molecules arise from the London dispersion forces (often called
van der Waals forces) that exist between all atoms and molecules.

A fluctuation of the electron distribution on one atom induces a corresponding temporary dipole
moment on a neighboring atom. The two dipole moments interact to give a net attractive force,
called a “dispersion force.”
Slide 12 - 27 General Chemistry: Chapter 12 Copyright © 2017 Pearson Canada Inc.
 Comparison of Potential Energy Curves

The potentials illustrated here include

Coulomb (R-1), dipole–dipole (R-3),
dispersion (R-6), and repulsive (R-12)
potentials.

For comparison, the covalent bond

(intramolecular force) for Cl2 is also
shown. The ion–ion interaction of K+
with Cl- is the strongest (stronger
even than the covalent interaction in
Cl2), followed by the interaction
between two HCl molecules (dipole–
dipole and dispersion) and the Ar–Ar
interaction (dispersion only).

Slide 12 - 28 General Chemistry: Chapter 12 Copyright © 2017 Pearson Canada Inc.

 Summary of van der Waals Forces

Dispersion (London) forces exist between all molecules. These

forces increase with molecular size and depend on molecular
shapes
Forces associated with permanent dipoles involve
displacements of electron pairs in bonds, rather than molecules
as a whole.
When comparing substances of:
comparable molecular sizes,
dipole forces can produce significant differences.
widely different molecular sizes,
dispersion forces are usually more significant than dipole forces.
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 Intermolecular Forces and Properties

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 Intermolecular Forces and Properties

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 Summary of Noncovalent Interactions

Slide 12 - 32 General Chemistry: Chapter 12 Copyright © 2017 Pearson Canada Inc.

 Summary of Noncovalent Interactions

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 Other Reference Books

PRINCIPLES OF MODERN CHEMISTRY, Oxtoby, Gillis and

Nachtrieb, Saunders, 8th edition

Slide 12 - 34 General Chemistry: Chapter 12 Copyright © 2017 Pearson Canada Inc.