10.4_Attractive_Forces

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Chapter 10 Structures of Solids
and Liquids
10.4
Attractive Forces between
Particles
Basic Chemistry
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1
Ionic Bonds
In ionic compounds, ionic bonds
• are strong attractive forces
• hold positive and negative ions together
Basic Chemistry
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2
Dipole-Dipole Attractions
In covalent compounds, polar
molecules
• exert attractive forces called
dipole-dipole attractions
• form strong dipole attractions
called hydrogen bonds between
hydrogen atoms bonded to F, O,
or N, and other atoms that are
strongly electronegative
Basic Chemistry
Copyright © 2011 Pearson Education, Inc.
3
Dipole-Dipole Attractions
Basic Chemistry
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4
Dispersion Forces
Dispersion forces are
• weak attractions between nonpolar molecules
• caused by temporary dipoles that develop when
electrons are not distributed equally
Basic Chemistry
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5
Comparison of Bonding and Attractive Forces
Basic Chemistry
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6
Mass and Boiling Points
Nonpolar covalent compounds such as alkanes family have
higher boiling points as
• the mass of the compounds increases
• Because more dispersion forces form
Basic Chemistry
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7
Melting Points and Attractive Forces
• Ionic bonds require large amounts of energy to break apart.
Ionic compounds have very high melting points.
• Hydrogen bonds are the strongest type of dipole-dipole
attractions. They require more energy to break than other
dipole attractions. Compounds with hydrogen bonds have
moderate melting points.
Basic Chemistry
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8
Melting Points and Attractive Forces
(continued)
• Dipole-dipole attractions are weaker than hydrogen
bonds, but stronger than dispersion forces. They have
low to moderate melting points.
• Dispersion forces are weak and little energy is needed
to break them. Compounds with dispersion forces
have the lowest melting points.
Basic Chemistry
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9
Melting Points
Basic Chemistry
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10
Learning Check
Identify the major type of attractive force in each of the
following substances:
1) ionic bonds
2) dipole-dipole attractions
3) hydrogen bonds
4) dispersion forces
A.
B.
C.
D.
E.
NCl3
H2O
Br2
KCl
NH3
Basic Chemistry
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11
Solution
Identify the major type of attractive force in each
of the following substances:
1) ionic bonds
2) dipole-dipole attractions
3) hydrogen bonds 4) dispersion forces
2
3
4
1
3
A. NCl3
B. H2O
C. Br2
D. KCl
E. NH3
Basic Chemistry
Copyright © 2011 Pearson Education, Inc.
12
Learning Check
Identify the compound in each pair that has the higher
melting point. Explain.
A. NCl3 or NH3
B. HBr or Br2
C. KCl or HCl
Basic Chemistry
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13
Solution
A. NH3 would have a higher melting point than NCl3 because the
hydrogen bonds in NH3 are stronger than the dipole-dipole
attractions in NCl3.
B. HBr would have a higher melting point than Br2 because the
dipole-dipole attractions in HBr are stronger than the dispersion
forces in Br2
C. KCl would have a higher melting point than HCl because the ionic
bonds in KCl are stronger than the dipole-dipole attractions in
HCl.
Basic Chemistry
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14
Molecular Polarity
Polarity affects the intermolecular forces of
attraction.
– Melting points and Boiling points
– Solubilities: Like dissolves like
Basic Chemistry
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polarity affect the melting point of a
molecule
• A more polar molecule will have a higher
melting point. A polar molecule will have a
strong dipole-dipole bond and will require
more energy to break that bond.
Basic Chemistry
Copyright © 2011 Pearson Education, Inc.
16
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