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CHAPTER 10 Forces Between Ions and Molecules

Scientists are interested in how matter behaves under unusual

circumstances. For example, before the space station could

be built, fundamental research into materials properties had to

be undertaken.

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Solids and Liquids

Intermolecular Forces

Ion-ion, Ion-dipole, dipole-dipole and H-

bonding, dipole-induced dipole, induced

dipole-induced dipole

Liquids

Vapor pressure and

temperature, Critical

T & P, Surface

tension and

viscosity

Phase Diagrams

Show relation of solid,

liquid, and gas phases

with change in T and

P

Solids

Unit cells, metal

structures, formulas

and structures of ionic

compounds,

Molecular, network,

and amorphous solids

Properties of Solids

Lattice energy, heat of

fusion, melting point

Interactions Between Ions in Salts

Coulomb’s Law:

Energy (E) of interaction is

directly proportional to the

charges of the ions (Q1, Q2),

and inversely proportional to the

distance (d) between them.

Attractive force will increase as

charge on ion increases, and

decrease as ionic radius

increases.

d

QQ α E 21

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Ionic Radii

Sample Exercise 10.1

List the ionic compounds CaO, NaF, and CaF2 in order of decreasing strength of the attraction between their ions.

Ca2+ O2- Q1Q2 = (+2)(-2) = -4

Na+ F- Q1Q2 = (+1)(-1) = -1

Ca2+ F- Q1Q2 = (+2)(-1) = -2

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The values of d are

dCaO = Ca2+ 100 pm + O2- 140 pm = 240 pm

dNaF = Na+ 102 pm + F- 133 pm = 235 pm

dCaF2 = Ca2+ 100 pm + F- 133 pm = 233 pm

Substituting the values of d and Q1Q2 into Coulomb’s Law for each compound:

ECaO ∞ (-4)/240 = -0.017

ENaF ∞ (-1)/235 = -0.0043

ECaF2 ∞ (-2)/233 = -0.0086

The predicted order of decreasing strength of ionic interactions is CaO > CaF2 > NaF.

Sample Exercise 10.1 cont’d

Types of Intermolecular Forces

Van

der

Waal’s f

orc

es

Dis

pers

ion

fo

rces

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Ion-Dipole Interactions

Hydration Spheres

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Dipole-Dipole Interactions

+ - + -

The positive and negative ends of polar molecules interact

with each other, resulting in a net force of attraction.

The strongest type of dipole-dipole interaction

involving a hydrogen and either F, O, or N

About 10 % as strong as an ordinary covalent

bond so approximately 15-40 kJ/mol.

Dipole-Dipole: Hydrogen Bonding

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H H

O

+ +

-

H H

O

+ +

-

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Sample Exercise 10.2

Dimethyl ether (C2H6O) has a molar mass of 46.07

g/mol and a boiling point of -24.9°C. Ethanol

(C2H6O) has the same formula and therefore the

same molar mass but a boiling point of 78.5°C.

Explain this difference in boiling points. The

structures are shown in Figure 10.9.

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adenine thymine

white = hydrogen

blue = nitrogen

black = carbon

red = oxygen

The Double Helix of DNA is held

together by hydrogen bonding

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The polymer Nylon is also held

together by hydrogen bonding

Polarizability - a measure of the extent to which the

electron cloud of an atom or molecule can be

distorted by an external electric charge.

In general, larger atoms or

molecules are more easily

polarizable than smaller

ones (more shells, etc), and

so experience larger dipole-

induced dipole interactions.

DISPERSION FORCES

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Dipole-Induced Dipole Interactions

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Induced dipole - Induced dipole Interactions

(London Forces)

Even nonpolar molecules

and uncombined atoms

have attractive forces

between them, otherwise

they would never

condense or solidify.

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Straight-chain Alkanes (CnH2n+2)

Which alkane has greater attractive

intermolecular forces ?

H - C - C - C - H

H H H

H H H

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

H H H

H H H

H H H

H H H

H - C - C - C - H

H H H

H H H

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

H H H

H H H

H H H

H H H

The molecule with the longer chain because there are

more points of “attachment” via London Forces.

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Summary of Intermolecular Forces

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Summary of Intermolecular Forces