Intermolecular Forces:

1
Intermolecular Forces
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• What holds everything together
• (Chapter 14)

2
Intramolecular forces (bonds)
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• Hold atoms together in molecules
• Have high energy associated with them
• its difficult to break molecules into their
  individual atoms
• Different types based upon what is going on with
  the electrons (electron clouds)

3
Types of bonds
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• Ionic
• attraction between fully charged molecules/ atoms
  
• NaCl, made from Na and Cl- or
• Ca(OH)2, made from Ca2 and 2OH-
• Covalent
• electrons are shared between atoms,
• water (H2O) and
• sugar (C6H12O6)
• Can be polar or nonpolar
• Based on
• electronegativity
• VSEPR geometry (shape)

4
Intermolecular forces (IMFs)
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• Hold molecules together
• Much weaker than intramolecular forces
• Intramolecular bonds are usually 100x or even
  1000x stronger
• (kJ are units of energy like Calories 1Cal
  4.184kJ)
• 1000cal 1Cal
• 1cal 4.184J

5
Figure 14.2 Intermolecular forces exist between
molecules. Bonds exist within molecules.
0
6
Why do we care?
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• The strength of the IMFs determine the state of
  matter
• Solid, liquid, or gas
• Not plasma- intramolecular bonds are broken to
  get plasmas

7
Solids, Liquids, and Gases
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shape volume density energy motion with Energy level of organization strength of IMFs
Gas indefinite variable with P and T variable with volume change high high molecules freely moving with great distance compared to molecular size between them very low low
Liquid indefinite constant constant moderate high molecules freely moving past each other but in close proximity to each other low moderate
Solid definite constant constant low low vibration only as molecules are basically fixed in place high high
all at room temperature, 25C small variations
occur due to temperature changes, very little
variable with pressure changes
8
0

• Things with strong IMFs tend to be solids at room
  temperature
• Things with weak IMFs tend to be gases at room
  temperature
• Medium IMFs tend to be in between-
• liquids, yes, but with varying characteristics
• Amorphous solids long transition between solid
  and liquid states- gets soft, then melts (like
  wax)
• Crystalline solids definite, clear melting point
  (no soft transition- ie ice)

9
Types of IMFs
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• In order of increasing strength
• London dispersion forces
• Dipole- dipole
• Hydrogen bonds

10
London dispersion forces
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• LDFs occur in all molecules, but are the only
  forces that are present in nonpolar molecules
  such as diatomic molecules and atomic substances
• CO2, N2, He
• They occur because the electron clouds around
  molecules are not always evenly distributed.
• When the electron clouds are unevenly
  distributed, temporary partial charges result

11
Figure 14.6 Atoms with spherical electron
probability.
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14.6 The atom on the left develops an
instantaneous dipole.
12
LDFs, cont
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• These temporary partial charges are called
  induced or temporary dipoles
• This temporary dipole forming in a nonpolar
  substance is strong enough to cause a dipole to
  occur in a neighboring molecule

13
Figure 14.3 (a) Interaction of two polar
molecules. (b) Interaction of many dipoles in a
liquid.
0
14
LDfs, cont
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• Basically, everything lines up temporarily, but
  long enough to keep everything together
• Common in gases

15
See LDFs at work here
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• http//antoine.frostburg.edu/chem/senese/101/liqui
  ds/faq/h-bonding-vs-london-forces.shtml
• These dipoles fluctuate they do not last very
  long, but they do occur frequently enough to have
  a significant effect overall

16
Dipole- dipole forces
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• Are stronger than LDfs because they occur in
  polar molecules that already have permanent
  dipole moments (in other words, partial charges
  already exist)
• Are AKA as van der Waals interactions at times,
  but in actuality both induced dipole attractions
  and dipole-dipole attractions are van der Waals
  forces

17
Examples
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• HCl and other acids
• HCN
• NH3
• except HF, which does something else

18
0

• What would happen between polar and nonpolar
  molecules? (Do forces of attraction exist? Do
  the molecules repel?) Explain!

19
Hydrogen bonding
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• Are stronger than dipole-dipole forces or LDFs
• Occurs in only the most polar bonds
• between molecules containing H-F, H-O and H-N
  bonds only
• Are the reason that water is so different from
  any material from similar atoms, like H2S

20
Figure 14.4 Hydrogen bonding among water
molecules.Norton Interactive IMFs
tutorialSelect Hydrogen bonding in water from
bottom of list
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21
0

• http//www.northland.cc.mn.us/biology/Biology1111/
  animations/hydrogenbonds.html
• (note I am not responsible for the music on the
  above web site)
• Polarity and hydrogen bond formation
• Ice at different temperatures

22
Which is ice? Which is liquid water? Explain.
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• Ice at different temperatures

23
Water is special because
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• It has a high specific heat, meaning that it
  takes a lot of energy to raise the temperature of
  a sample of water by even 1 degree
• Specific heat of water (c) 1 cal/ gC or 4.184J
  /gC
• The solid phase is LESS dense than the liquid
  phase, so ice floats on water
• Its a good solvent for many substances due its
  polarity
• H2O is liquid at RT, where H2S is a gas

24
Figure 14.5 The boiling points of covalent
hydrides.
0
25
Water is special
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• And water would not be special without hydrogen
  bonding
• H bonding plays vital roles in
• DNA (holding together the chains of DNA)
• Protein shape (and therefore the proteins
  function think hair!)

26
H bonding in dna1
27
H bonding in DNA
28
Amino Acids- they make proteins
29
(No Transcript)
30
Protein Structures
31
Protein Structure and H Bonding
32
For the next slides
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• Determine polarity of group
• Determine type of IMFs are possible in group
• Determine if the group will be highly soluble in
  water

33
IMFs in proteins
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34
Sickle Cell Anemia
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• Glu (glutamic acid) replaced by Val (valine)

35
0

• What would happen if a molecule capable of
  H-bonding comes into contact with
• A nonpolar substance
• A polar substance that does not H-bond

36
0

• Strength increases from left to right when ions
  are involved, attractive forces are greater than
  when they are not involved.
• http//cwx.prenhall.com/bookbind/pubbooks/blb/chap
  ter11/medialib/blb1102.html

37
Dealing with this pic
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• Ion- dipole forces
• Ionic Bonding
• Basically electrostatic attractive forces
  between positive and negative charges
• Strong

38
IMFs influence
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• Boiling point/ Melting Point
• Viscocity
• Surface Tension
• Capillary Action
• Vapor pressure/ rate of evaporation
• State of Matter (at room temp)
• Density falls here, but can vary even within
  state

39
IMFs and mass
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• The mass of a material makes a difference, so
  yes, mass (size) matters
• Larger molecules have stronger forces than
  similar molecules that are smaller (in terms of
  mass)

40
Figure 14.5 The boiling points of covalent
hydrides.
0
41
Boiling points and masses of noble gases
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• Helium -269C 4.00 g/mol
• Neon -246C 20.18 g/mol
• Argon -186C 39.95 g/mol
• Krypton -152C 83.80 g/mol
• Xenon -108C 131.3 g/mol
• radon -62C 222 g/mol
• Larger atoms have larger e- clouds, which
  lead to greater polarizability

42
Name Molecular Melting Boiling State at
Name Formula Point Point 25oC
Name   (oC) (oC)  
methane CH4 -183 -164 gas
ethane C2H6 -183 -89 gas
propane C3H8 -190 -42 gas
butane C4H10 -138 -0.5 gas
pentane C5H12 -130 36 gas
hexane C6H14 -95 69 gas
heptane C7H16 -91 98 gas
octane C8H18 -57 125 gas
nonane C9H20 -51 151 liquid
decane C10H22 -30 174 liquid
undecane C11H24 -25 196 liquid
dodecane C12H26 -10 216 liquid
eicosane C20H42 37 343 liquid
triacontane C30H62 66 450 solid
0
Saturated Hydrocarbons, or Alkanes As melting
point increases, boiling point increases (saturat
ed hydrocarbons are hydrocarbons with as many Hs
as possible)

43
Shape also matters
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• Butane, bp -0.5 degrees C
• 2-methylpropane -11.7 degrees C
• Butane has a higher boiling point because the
  dispersion forces are greater. The molecules are
  longer (and so set up bigger temporary dipoles)
  and can lie closer together than the shorter,
  fatter 2-methylpropane molecules.
• Also, the molecules can stack with each other
  better

H
44
0
Butane and 2-methylpropane                      
                                                  
                                                  
                                                  
                                                
                                                  
                                                  
                                                  
                 Compare the properties of these
two compounds n-butane . . . . . . . . . . . .
. . . . .. . . . . . . . . . . . .
2-methylpropane 0.601 . . . . . . . . . . . . . .
. . relative density (liquid) . . . . . . . . . .
. . . . . . 0.551 1.348 . . . . . . . . . . . . .
. . . refractive index (liquid) . . . . . . . . .
. . . . . . .1.351 - 0.5 . . . . . . . . . . .. .
. . . . . boiling point (oC) . . . . . . . . . .
. . . . . . . . .. . - 11.7 - 138.3 . . . . . . .
. . . . . . . . . melting point (oC) . . . . . .
. . . . . . . . .. . . . - 159.6 It is clear that
the different carbon skeletons make a difference
to the properties, especially the melting and
boiling points.
45
Fats v OilsSaturated v. Unsaturated
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• Molecular size, bond order, and bond orientation
• How different IMFs result in differences in food
  molecules

46
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• A carbon exists where two lines intersect
• Atoms other than C and H are written in
• Hs are not usually written out-
• They fill in to complete octets on other atoms

47
Random cis trans fats
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• (Omega 3 and Omega 6 fats have the double bonds
  on the 3 or 6th carbon)

48
fatty acids and triglycerides
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• 3 Fatty acid chains (above) join with a glycerol
  molecule (top right) to form a triglyceride
  (right, saturated)

49
Triglyceride formation
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50
Triglycerides
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• Oils
• More unsaturated FAs
• Liquid at RT
• Fats
• More saturated FAs
• Solid at RT

51
Fatty acid pamlmitic stearic oleic
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16C sat MP 62.9C 18C sat MP 69.6C
18C unsat MP 13C
52
Oleic v linoleic acid
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• Melting Points C
• Oleic acid 13
• Linoleic Acid -5

53
0
54
Why?
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• Why do the melting points differ between
• Palmitic Acid (16 C, sat)
• Stearic Acid (18 C, sat)
• Oleic Acid (18C, mono unsat)
• Linoleic Acid (18C, polyunsat. 2)
• (Explain the impact of number of carbons and the
  number of double bonds)

55
WHY DOES THIS HAPPEN?
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• Proximity of atoms regular shape allows the IMFs
  to hold everything in place (to stack)
  molecules rather than have the irregular shapes
  slide past each other

56
Triglycerides
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• In unsaturated triglycerides, the molecules can
  not stack
• In the saturated molecules, the fatty acids are
  tightly packed and stacked

57
0

• More carbons, higher MP
• The more double bonds, the lower the MP

58
0

• Of the following, which would have the highest
  MP? The lowest?
• Lauric
• 12C, unsat, MP 44C
• Stearic
• 18C, sat, MP 70C
• Arachodonic
• 20, unsat, MP -50C

Elmhurst
59
0

• Which fats are saturated? Unsaturated?
• What type of IMF would predominate?
• Rank the molecules in order from lowest to
  highest MP.

60
Percent Fatty Acids in
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• Percent Fatty acids in selected triglycerides

61
Cis and trans fats
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62
Cis- v Trans- fats
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• Cis- fats are naturally occurring fats from
  animal products
• Trans- fats occur from modifying oils chemically
• Partially hydrogenating oils
• Adding Hs causes double bonds to convert to
  single bonds
• Unsaturated to saturated conversion
• Due to steric hindrance, when the H is added,
  they convert some cis bonds to trans bonds
• Why do manufacturers make trans fats for use in
  foods?
• Trans fats cost less (vegetable sources v. animal
  sources)
• Fats in foods are usually more desirable that
  oils-
• Less greasy
• Can control how solid the fats are by controlling
  the number of double bonds
• Better/ easier to cook with (especially in baked
  goods)

63
0

• Saturated Fats These are considered to be the
  bad fats. They are called saturated because
  their carbon structures are completely filled
  (saturated) with hydrogen atoms. Their chemical
  structure is very linear which allows for a
  stacking effect to occur. This is what promotes
  the solidifying effect of most saturated fats
  (butter, lard, most animal fats). This
  solidification may also occur in the body which
  partly explains the artery-clogging effects
  linked to saturated fats. Examples of saturated
  fats include myristic acid, palmitic acid,
  stearic acid, arachidic acid, and lignoceric
  acid. These fats may raise cholesterol levels in
  the body and should be used in moderation

64
Why are trans- fats bad?
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• The trans- double bonds
• Are more reactive in the body
• Promote free radical formation
• Leads to destruction of biomolecules
• Are more likely to clog arteries
• Due to shape, get caught in body
• Promote cholesterol levels to increase, since
  they can be used to make cholesterol in the body
• We dont have the enzymes to process the trans-
  fats
• (we can process cis- fats)

65
0

• Good fat/ Bad fat?

• http//www.nhlbi.nih.gov/chd/Tipsheets/images/satf
  atgraph.gif

66
Spider silk monomer
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• (amino acid)
• Amino acid R groups

Kevlar monomer
67
Silk
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• Silk and proteins

68
Viscosity
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• Viscosity is the resistance to flow
• The greater the viscosity, the greater the
  resistance to flow
• Determined
• How quickly a fluid flows through a tube under
  gravitational force (slower more viscous)
• Or by
• Determining rate at which steel sphere fall
  through the liquid (more viscous more slowly)
• Changes as temperature changes

69
What is surface tension?
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• Resistance of a liquid to an increase in its
  surface area (Zumdahl)
• Free energy per unit surface area (Tinoco, Sauer,
  Wang and Puglisi)
• Force per unit length (mNm-1, or dyne/cm)
• Laymans terms How much something spreads out on
  a surface
• Beading up high surface tension
• Spreading out low surface tension

70
Surface Tension
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(High surface tension) (Low surface tension)

• The molecules of water have more adhesion to the
  (polar) glass than to each other (cohesion)
• The Hg has more cohesive forces than attraction
  to the glass

• Cohesion Molecules sticking (due to IMFs) to the
  same molecule in a pure compound
• Adhesion Molecules sticking (due to IMFs) to
  other molecules adjacent to the pure compound
• (not a mixture- at a surface interface)

http//home.earthlink.net/dmocarski/chapters/chap
ter7/main.htm
71
Wetting and Dewetting
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http//www.mpikg-golm.mpg.de/gf/1
http//www.mpikg-golm.mpg.de/gf/1
72
0

• Wetting is how water (in this case) adheres to a
  surface when the surface tension is lowered, the
  material becomes wetter.
• Surface tension of water is 73 dyne/ cm
• http//home.att.net/larvalbugrex/striders.html

73
What causes surface tension?
0

• Surface tension is a result of the imbalance of
  forces at the surface (or interface)

http//www.kibron.com/Science/
http//home.earthlink.net/dmocarski/chapters/chap
ter7/main.htm
74
Surface tension of
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  mNm-1 Temperature (C)
Platinum 1819 200
Mercury 487 15
Water 71.97 25
Water 58.85 100 (liquid)
Benzene 28.9 20
Acetone 23.7 20
n- Hexane 18.4 20
Molten Iron 17 1600
Silicon Oil 16.9 25
Neon 5.2 -247

(Tinoco, Sauer, Wang, and Puglisi)
http//www.boldinventions.com/tsun_sim_2.html
75
Does temperature matter?
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surface tension (mNm-1)
-8 77
-5 76.4
0 75.6
5 74.9
10 74.22
15 73.49
18 73.05
20 72.75
25 71.97
30 71.18
40 69.56
50 67.91
60 66.18
70 64.4
80 62.6
100 58.9
temperature ( C)

• Yes- part of the reason that we wash in warm
  water (at times), not cold the fabric gets
  wetter
• As temperature increases, surface tension
  decreases
• (Surface tension given for water against air)

http//scienceworld.wolfram.com/physics/SurfaceTen
sion.html
76
Temperature and IMFs
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• IMFs in a substance change in strength in a
  substance as temperature changes
• This influences certain properties of the
  substances
• Surface tension
• Viscosity
• Capillary action
• Vapor pressure
• (but not BP, MP)

77
Capillary Action
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• Capillary action a phenomenon associated with
  surface tension and resulting in the elevation or
  depression of liquids in capillaries (from
  www.dictionary.com)
• The molecules of water have more adhesion to the
  (polar) glass than to each other (cohesion)
• The Hg has more cohesive forces than attraction
  to the glass
• (glass is polar)

http//home.earthlink.net/dmocarski/chapters/chap
ter7/main.htm
78
Vaporization and Vapor Pressure
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• The molecules in a sample of a liquid move at
  various speeds
• (average speed is the temperature some have more
  energy, some have less, but the overall KE is
  temperature)
• Sometimes molecules at the surface have
  sufficient speed to overcome the attractive
  forces and leave the liquid surface (thus
  vaporizing)

79
Figure 14.9 Microscopic view of a liquid near
its surface.
0
80
Dynamic equilibrium
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• Dynamic equilibrium is the state where there is
  simultaneous and equal vaporization and
  condensation of the substance
• In a closed container, at some pressure, the
  amount that vaporizes will equal the amount
  condensing on the surface of the liquid
• This is the equilibrium vapor pressure

81
VP and IMFs

• Stronger IMFs equal LOWER vapor pressures
• Weaker IMFs equal high vapor pressures
• Substance with very low IMFs and therefore high
  vapor pressures evaporate very quickly and easily
• Called volatile substance
• Mass and shape important, just like with boiling
  point
• Heavier lower VP ex oil
• Lighter higher VP ex alcohol
• More volatile
• Think propane (C3H8) v. gasoline (C8H18)

82
VP and Boiling
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• This vaporization occurs at any temperature, but
  occurs more rapidly as temperature increases
• Molecules at the surface would have to have more
  speed to overcome the IMFs
• Boiling is the point at which the vapor pressure
  equals the external pressure on the surface of
  the liquid

83
Boiling and VP, cont
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• Liquids have some air dissolved in them in tiny
  invisible bubbles
• As water vaporizes in the liquid, it is added to
  the bubbles
• Also, the gas bubbles are expanding because they
  are being heated this causes an increase in
  volume, but not mass
• At this point, 2 things are going on
• This decreases density, causing the bubbles to
  float to the surface
• Also, as gas expands, the pressure increases
• When the pressure of the bubble increases to
  greater than the vapor pressure at the surface,
  the liquid is boiling
• All molecules must be vaporized before a further
  increase in temperature can occur

84
Boiling Point and Elevation
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• As elevation on the Earths surface increases,
  the atmospheric pressure decreases
• (smaller column of air pushing down on the area
  therefore less pressure)
• Boiling point changes as the atmospheric pressure
  changes
• If you could decrease the pressure without
  changing temperature, the substance would boil at
  a lower temperature
• A decrease in pressure results in a decrease in BP

85
Figure 14.14 The formation of the bubble is
opposed by atmospheric pressure.
0
86
Energy Changes Accompanying Changes of State
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• Each change of state is accompanied by a change
  in the energy of the system
• Whenever the change involves the disruption of
  intermolecular forces, energy must be supplied
• The disruption of intermolecular forces
  accompanies the state going towards a less
  ordered state
• As the strengths of the intermolecular forces
  increase, greater amounts of energy are required
  to overcome them during a change in state
• Takes more energy to go from
• a liquid to a gas
• than
• from a solid to a liquid
• Removing energy allows the molecules to self-
  organize, and results in an more ordered state

87
Heat of Fusion
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• The melting process for a solid is also referred
  to as fusion
• The enthalpy change associated with melting a
  solid is often called the heat of fusion (? Hfus)
  
• Ice ?Hfus 6.01 kJ/mol
• ? H is a change (?) in enthalpy (H), a measure of
  energy that is much like heat, but takes into
  account a few other factors

88
Heat of Vaporization
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• The heat needed for the vaporization of a liquid
  is called the heat of vaporization (? Hvap)
• Water ? Hvap 40.67 kJ/mol
• Vaporization requires the input of heat energy

89
0

• Less energy is needed to allow molecules to move
  past each other than to separate them totally,
• so ?Hfus lt ? Hvap

90
The heating/cooling curve for water heated or
cooled at a constant rate.
0
91
Energy/ disorder diagram
0
92
0

• Think of IMFs like magnets stronger magnets hold
  things more firmly together
• The more firm the connections, the less molecular
  motion can occur with the same amount of Energy
  added
• Adding (or removing) energy from the system can
  overcome (or increase) the IMFs, and cause a
  change in state
• Add Energy, move from S -gt L -gt G
• Remove Energy, move from G -gt L -gt S

93
0

• Air conditioners take advantage of Energy changes
  to remove heat energy from a warm indoor
  environment by vaporizing condensed gas
• On the outdoor portion of the AC unit, the gas is
  condensed to a liquid, sending the heat energy to
  the environment

94
0

• Vacuum chamber demo

95
Phase Diagram
0

• Due to changes in pressure and temperature, a
  substance can exist in all three states under
  specific conditions
• The Triple Point
• Think foggy icy days

96
Energy and IMFs

• Remember
• Kinetic energy is the Energy associated with
  moving particles
• Heat is the RANDOM KE of an object
• (as opposed to directional motion)
• Temperature is the measure of the AVERAGE KE in a
  substance
• When IMFs are disturbed due to E changes, the
  properties of the substance change, even to the
  point of changing state

97

• Explain how the lava lamp works (not you plug it
  in! On a molecular level, explain what is
  happening to the materials and their IMFs)

98
Calculations(I know you are excited about this)
99
Heat of fusion
100
Heat of fusion
101
Heat of fusion
102
calorimetry
103
calorimetry
104
calorimetry
105
calorimetry
106
VP and IMFs

• Stronger IMFs equal higher vapor pressures
• Weaker IMFs equal high vapor pressures
• Substance with very low IMFs and therefore high
  vapor pressures evaporate very quickly and easily
• Called volatile substance
• Mass and shape important, just like with boiling
  point
• Heavier lower VP ex oil
• Lighter higher VP ex alcohol
• More volatile
• Think propane (C3H8) v. gasoline (C8H18)

107
VP and IMFs
0

• Stronger IMFs equal lower vapor pressures
• Weaker IMFs equal high vapor pressures
• Substance with very low IMFs and therefore high
  vapor pressures evaporate very quickly and easily
• Called volatile substance
• Mass and shape important, just like with boiling
  point
• Heavier lower VP ex oil
• Lighter higher VP ex alcohol
• More volatile
• Think propane (C3H8) v. gasoline (C8H18)

108
IMFs in proteins
0
109
Sickle Cell Anemia
0

• Glu (glutamic acid) replaced by Val (valine)