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 Hbonding, dipole-induced dipole, induced
dipole-induced dipole
Solids
Liquids
Phase Diagrams
Vapor pressure and
temperature, Critical
T & P, Surface
tension and
viscosity
Show relation of solid,
liquid, and gas phases
with change in T and
P
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.
E α
Q1Q2
d
Attractive force will increase as
charge on ion increases, and
decrease as ionic radius
increases.
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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+ O2Na+ FCa2+ F-
Q1Q2 = (+2)(-2) = -4
Q1Q2 = (+1)(-1) = -1
Q1Q2 = (+2)(-1) = -2
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Sample Exercise 10.1 cont’d
The values of d are
dCaO =
dNaF =
dCaF2 =
Ca2+ 100 pm
Na+ 102 pm
Ca2+ 100 pm
+ O2+ F+ F-
140 pm = 240 pm
133 pm = 235 pm
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.
Dispersion forces
Van der Waal’s forces
Types of Intermolecular Forces
4
Ion-Dipole Interactions
Hydration Spheres
5
Dipole-Dipole Interactions
The positive and negative ends of polar molecules interact
with each other, resulting in a net force of attraction.
+
-
+
-
Dipole-Dipole: Hydrogen Bonding
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.
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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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The Double Helix of DNA is held
together by hydrogen bonding
adenine
thymine
white = hydrogen
blue = nitrogen
black = carbon
red = oxygen
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The polymer Nylon is also held
together by hydrogen bonding
DISPERSION FORCES
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 dipoleinduced dipole interactions.
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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 H H
H H H H H H
H-C-C-C-H
H - C - C - C - C - C - C -H
H H H
H H H H H H
H H H
H H H H H H
H-C-C-C-H
H - C - C - C - C - C - C -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
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