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Chapter 10: Intermolecular Forces (IMFs)
“Everything sticks to Everything”
Ultimate source of all Intermolecular Forces (IMF’s )
Interactive force between opposite charges
• IMFs = “stickyness”
– Differences in strength of stickyness
∝
• IMFs affect
– chemical reactivity
· • Interactive force will increase
– boiling point (Tb), melting/ freezing point (Tf)
– as charge increases
– Vapor pressure, viscosity, capillary action
– as distance decreases
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Coulomb’s law application
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Relative strengths (strongest to weakest)
Predict which salt in the following pairs exhibits the greater ion–ion
interactions.
1. Bonding Forces
strongest
• Ionic bonding
• Covalent bonding
a) MgO and NaF
2. Non Bonding
• Ion- dipole
• Hydrogen dipole- dipole
b) MgO and MgS
• Ordinary dipole-dipole
• Ion-induced diploe
• London Dispersion
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Ion – Dipole interaction
weakest
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Sphere of hydration
Attractive force between an ion and a polar molecule (has a permanent dipole)
•
Example: Hydrated ions
– sphere of hydration
hydrated Na+ ion
NaCl (s)
hydrated Cl- ion
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Hydrogen Dipole- Dipole Interactions
Hydrogen Dipole- Dipole Interactions
Attractive force between polar molecules
• Strongest is hydrogen dipole–dipole interaction
• Occurs when H atom directly bonded to highly eneg element (F, O, N)
– interaction between H atom and an O, N, or F atom in another
molecule
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Hydrogen Dipole- Dipole Interactions
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Affect of hydrogen dipole-dipole forces
Stronger IMF’s ⇒ higher Tb and Tf
• Unusually high Tb’s
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Affect of hydrogen dipole-dipole forces
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Interactions between non-polar molecules
Dispersion (London) Forces
• caused by temporary dipoles in the molecules
– Temporary (induced) dipole ⇒ momentary uneven distribution of
electrons
H-bonding between
complementary sites on
double stranded DNA
• Polarizability
– Relative ease with which the electron cloud in a molecule, ion, or
atom can be distorted, inducing a temporary dipole
– “squishiness” of electron cloud
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Induced dipoles
Induced dipoles
Momentary shift in e– density = dispersion
Proximity of polar molecule causes dipole-induced dipole
dipole - induced dipole
temporary (induced) dipoles
nonpolar
nonpolar
nonpolar
polar
London dispersion forces
London dispersion forces
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Strength of Dispersion Forces
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Kinetic Energy vs. Interactive Forces (IMFs)
Factors affecting strength of dispersion:
KE =
lowest
intermediate
highest
• Size of atoms/molecules
– Larger ⇒ more polarizable ⇒ stronger dispersion forces
• Molecular shape
– Larger surface area ⇒ more polarizable ⇒ stronger dispersion
forces
– Linear molecules have higher dispersion forces than branched
molecules of similar molecular weight
IMFs dominate
KE dominates
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Predict the trends in boiling point
FW (g/mole)
F2
Tb (K)
FW (g/mole)
He
4
38
Ne
20
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Trends in boiling point
FW (g/mole)
Tb (K)
FW (g/mole)
Tb (K)
F2
38
85
He
4
74
Ne
20
Cl2
71
239
Ar
27
40
87
Tb (K)
Cl2
71
Ar
40
Br2
160
332
Kr
84
120
Br2
160
Kr
84
I2
254
457
Xe
131
165
I2
254
Rn
222
211
Xe
131
Rn
222
For similar species:
size/ weight ↑ polarizability ↑ dispersion forces ↑ Tb (Tf, viscosity & etc)
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Trends in boiling point
Predict trends in boiling point (IMFs)
CH3
CH3-CH2-CH2-CH2-CH3
-
-
CH3-CH2-CH-CH3
CH3-C-CH3
-
CH3
CH3
2,2-dimethyl propane
For similar species:
size/ weight ↑ polarizability ↑ dispersion forces ↑ Tb (Tf, viscosity & etc)
n-Pentane
2-methylbutane
All have molecular formula C5H12 (and thus same FW)
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Predict trends in boiling point (IMFs)
-
CH3-CH2-CH-CH3
Molecular shape and surface area
There are more
points at which
dispersion forces
act.
CH3-CH2-CH2-CH2-CH3
-
-
CH3
CH3-C-CH3
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CH3
CH3
Tb = 9oC
Tb = 28oC
There are fewer
points at which
dispersion forces
act.
Tb = 36oC
For similar species:
surface area ↑ polarizability ↑ dispersion forces ↑ Tb (& Tf, viscosity, etc)
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Predict boiling point (IMF) trends
Predict boiling point (IMF) trends
Rank the following compounds in order of increasing boiling point:
CH2CH2CH3OH
CH3CH2CH2CH3
propanol, 60.11 g/mole
butane, 58.14 g/mole
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Chemistry
Rank the following compounds in order of increasing boiling point:
CH3CH2OCH3
ethylmethyl ether, 60.11 g/mole
CH3CH2CH2CH3
butane, 58.14 g/mole
Tb = -0.5oC
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CH3CH2OCH3
ethylmethyl ether, 60.11 g/mole
Tb = 7.4oC
CH2CH2CH3OH
propanol, 60.11 g/mole
Tb = 97oC
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Polarity and boiling point
Other Properties affected by IMFs
more polar⇒ stronger IMFs ⇒ higher Tb
Vapor pressure
•
the pressure due to gas phase particles above its liquid phase
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Factors affecting vapor pressure
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Vapor pressure
Temperature
• Increased temperature increases vapor pressure
– increased ave KE of particles ⇒ more particle can “escape” liquid
Vapor pressure increases as
temperature increases.
Intermolecular forces
• Stronger IMFs ⇒ lower vapor pressure
– increased stickiness means fewer particle can “escape” liquid
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Vapor pressure
Biodiesel
Stronger IMFs than regular diesel or octane
Vapor pressure decreases as the
strength of IMFs increases.
•
<
ethanol
<
water
>
water
O
H3C O
>
C
C15H31
biodiesel
Vapor Pressure
diethyl ether
lower Tf
– cars and buses running on biodiesel encounter problems in
colder weather
IMFs
diethyl ether
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ethanol
polar ⇒ stronger IMFs
lower vapor pressure
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CH3-(CH2)6-CH3
octane
non-polar ⇒ weaker IMFs
higher vapor pressure
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Saponification/ making soap
Saponification/ making biodiesel
H2C O
C C15H31
OH
H
OH
O
H
OH
O
O
HC O
C C15H31
H2C O
C C15H31
+
O
H
C C15H31
H2C OH
H2C O
C C15H31
O
O
base (aq)
C C15H31
O
O
O
O
+
HC OH
+
HC O
C C15H31
H2C O
C C15H31
OH
H3C
OH
H3C
OH
H3C O
O
C C15H31
H2C OH
H3C O
“soap”
glycerol
methanol
high viscosity
low vapor pressure
H3C O
+
C C15H31
biodiesel
HC OH
H2C OH
glycerol
lower viscosity
higher vapor pressure
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Surface Tension
C C15H31
O
oil
water
H2C OH
C C15H31
O
base (aq)
O
O
oil
H3C
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Viscosity
Surface tension is the energy required to increase the surface area of a liquid.
viscosity is resistance to flow
A surface molecule experiences a net attraction downward. This
causes a liquid surface to have the smallest area possible.
stronger IMFs ⇒ higher viscosity
An interior molecule is attracted by
others on all sides.
Stronger IMFs ⇒ higher surface tension.
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“slow as molasses in January”
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Header
January 15th, 1919;
A storage tank burst on Boston's waterfront releasing two million gallons of
molasses in a 15 ft-high, 160 ft-wide wave that raced through the city's north
end at 35 mph.
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1. Item
•
bullet
– sub-bullet
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