Ch 11: Intermolecular Forces and Types of Solids

1
Ch 11 Intermolecular Forces and Types of Solids

• Brown, LeMay
• AP Chemistry
• Monta Vista High School
• Credits Adapted from Kots, Weaver and Trichels
  PPT

2
Inter-molecular Forces

• Have studied INTRAmolecular forcesthe forces
  holding atoms together to form molecules.
• Now turn to forces between molecules
  INTERmolecular forces.
• Forces between molecules, between ions, or
  between molecules and ions.

3
11.1 Intermolecular Forces (IMF)

• IMF lt intramolecular forces (covalent, metallic,
  ionic bonds)
• IMF strength solids gt liquids gt gases
• Boiling points and melting points are good
  indicators of relative IMF strength.

4
Summary of Intermolecular Forces

• Ion-Ion forces
• Ion-dipole forces
• Dipole-dipole forces
• Special dipole-dipole force hydrogen bonds
  (sometimes treated as a separate IMF)
• Forces involving non polar molecules induced
  forces (LDFs)

5
Intermolecular Forces Summary
6
11.2 Types of IMF

• Electrostatic forces act over larger distances
  in accordance with Coulombs law
• Ion-ion forces strongest found in ionic
  crystals (i.e. lattice energy)

http//chemmovies.unl.edu/ChemAnime/IONSIZED/IONSI
ZED.html Ion size and LE
7

1. Ion-dipole between an ion and a dipole (a
   neutral, polar molecule/has separated partial
   charges)

• Increase with increasing polarity of molecule and
  increasing ion charge.

Ex Compare IMF in Cl- (aq) and S2- (aq).
lt
http//chemmovies.unl.edu/ChemAnime/NACL1D/NACL1D.
html NaCl dissolving in Water
8
Attraction Between Ions and Permanent Dipoles

• Water is highly polar and can interact with
  positive ions to give hydrated ions in water.

9
Attraction Between Ions and Permanent Dipoles

• Attraction between ions and dipole depends on ion
  charge and ion-dipole distance.
• Measured by ?H for Mn H2O f M(H2O)xn

10

1. Dipole-dipole weakest electrostatic force (Not
   all IMFs, LDFs weaker than dipole-dipole) exist
   between neutral polar molecules

• Increase with increasing polarity (dipole moment)
  of molecule

Ex What IMF exist in NaCl (aq)?
11
Dipole-Dipole Forces

• Influence of dipole-dipole forces is seen in the
  boiling points of simple molecules.
• Compd Mol. Wt. Boil Point
• N2 28 -196 oC
• CO 28 -192 oC
• Br2 160 59 oC
• ICl 162 97 oC

12
Partner Activity

• Discuss with your partner the difference between
  ion-dipole and dipole-dipole interactions, in
  terms of the following
• How they are formed
• Strength
• Examples

13

• Hydrogen bonds (or H-bonds)
• H is unique among the elements because it has a
  single e- that is also a valence e-.
• When this e- is hogged by a highly EN atom (a
  very polar covalent bond), the H nucleus is
  partially exposed and becomes attracted to an
  e--rich atom nearby.
• http//www.youtube.com/watch?vLGwyBeuVjhU

14

• H-bonds form with H-XX', where X and X' have
  high EN and X' possesses a lone pair of e-
• X F, O, N (since most EN elements) on two
  molecules

F-H O-H N-H
F O N
15
Hydrogen Bonding

• A special form of dipole-dipole attraction, which
  enhances dipole-dipole attractions.

H-bonding is strongest when X and Y are N, O, or F
16

• H-bonds explain why ice is less dense than water.

http//en.wikipedia.org/wiki/Water_28molecule29
Density_of_water_and_ice
17
Ex Boiling points of nonmetal hydrides

• Conclusions
• Polar molecules have higher BP than nonpolar
  molecules
• ? Polar molecules have stronger IMF
• BP increases with increasing MW
• ? Heavier molecules have stronger IMF

Boiling Points (ºC)

• NH3, H2O, and HF have unusually high BP.
• ? H-bonds are stronger than dipole-dipole IMF

18
H-Bonding Between Methanol and Water
-?
?
-?
19
H-Bonding Between Two Methanol Molecules
-?
?
-?
H-bond
20
Hydrogen Bonding in H2O

• H-bonding is especially strong in water because
• the OH bond is very polar
• there are 2 lone pairs on the O atom
• Accounts for many of waters unique properties.

http//www.visionlearning.com/library/flash_viewer
.php?oid1435mid120 Animation of Ice
21
Hydrogen Bonding in H2O

• Ice has open lattice-like structure.
• Ice density is lt liquid.
• And so solid floats on water.

Snow flake www.snowcrystals.com
22
Hydrogen Bonding in H2O

• Ice has open lattice-like structure.
• Ice density is lt liquid and so solid floats on
  water.

http//www.visionlearning.com/library/flash_viewer
.php?oid1380mid57 H bonding in Water
One of the VERY few substances where solid is
LESS DENSE than the liquid.
23
Hydrogen Bonding

• H bonds leads to abnormally high boiling point of
  water.

See Screen 13.7
24
Boiling Points of Simple Hydrogen-Containing
Compounds
See Active Figure 12.8
25
Methane Hydrate
26
Hydrogen Bonding in Biology

• H-bonding is especially strong in biological
  systems such as DNA.
• DNA helical chains of phosphate groups and
  sugar molecules. Chains are helical because of
  tetrahedral geometry of P, C, and O.
• Chains bind to one another by specific hydrogen
  bonding between pairs of Lewis bases.
• adenine with thymine
• guanine with cytosine

27
Double helix of DNA
Portion of a DNA chain
28
Base-Pairing through H-Bonds
29
Hydrogen Bonding in Biology
Hydrogen bonding and base pairing in DNA.
30
H Bonding Activity

• With your elbow partner, draw the following on
  the same sheet of paper taking turns
• Water Molecule
• Dipole of this water molecule
• Another water molecule
• Hydrogen Bonding Between these molecules
• Structure of Ice
• Reflect on your beautiful drawings and give each
  other high fives.

31

• There is no strict cutoff for the ability to
  form H-bonds (S forms a biologically important
  hydrogen bond in proteins).
• Hold DNA strands together in double-helix

Nucleotide pairs form H-bonds
DNA double helix
32
Inductive forces

• Arise from distortion of the e- cloud induced by
  the electrical field produced by another particle
  or molecule nearby.
• London dispersion between polar or nonpolar
  molecules or atoms
• Proposed by Fritz London in 1930
• Must exist because nonpolar molecules form liquids

Fritz London(1900-1954)
33

• How they form
• Motion of e- creates an instantaneous dipole
  moment, making it temporarily polar.

• Instantaneous dipole moment induces a dipole in
  an adjacent atom
• Persist for about 10-14 or 10-15 second
• Ex two He atoms

34
FORCES INVOLVING INDUCED DIPOLES

• How can non-polar molecules such as O2 and I2
  dissolve in water?

The water dipole INDUCES a dipole in the O2
electric cloud.
Dipole-induced dipole
http//antoine.frostburg.edu/chem/senese/101/liqui
ds/faq/h-bonding-vs-london-forces.shtml Dipole-Dip
ole and LDFs
35
FORCES INVOLVING INDUCED DIPOLES

• Solubility increases with mass the gas

36
FORCES INVOLVING INDUCED DIPOLES

• Process of inducing a dipole is polarization
• Degree to which electron cloud of an atom or
  molecule can be distorted in its polarizability.

37
IM FORCES INDUCED DIPOLES

• Consider I2 dissolving in ethanol, CH3CH2OH.

38
FORCES INVOLVING INDUCED DIPOLES
Formation of a dipole in two nonpolar I2
molecules.
Induced dipole-induced dipole
http//chemmovies.unl.edu/ChemAnime/LONDOND/LONDON
D.html LDFs
39
FORCES INVOLVING INDUCED DIPOLES

• The induced forces between I2 molecules are very
  weak, so solid I2 sublimes (goes from a solid to
  gaseous molecules).

40
Intermolecular Forces
See Figure 12.12
41
LiquidsSection 12.4

• In a liquid
• molecules are in constant motion
• there are appreciable intermolec. forces
• molecules close together
• Liquids are almost incompressible
• Liquids do not fill the container

42
Liquids

• The two key properties we need to describe are
  EVAPORATION and its oppositeCONDENSATION

Evaporation f
Add energy
break IM bonds
make IM bonds
Remove energy
r condensation
43
LiquidsEvaporation

• To evaporate, molecules must have sufficient
  energy to break IM forces.

Breaking IM forces requires energy. The process
of evaporation is endothermic.
44
LiquidsDistribution of Energies

• Distribution of molecular energies in a liquid.
• KE is propor-tional to T.

See Figure 12.13
45
Vapor Pressure
46
Equilibrium Vapor Pressure
http//www.mhhe.com/physsci/chemistry/essentialche
mistry/flash/vaporv3.swf Vapor Pressure
47
Liquids

• HEAT OF VAPORIZATION is the heat reqd (at
  constant P) to vaporize the liquid.
• LIQ heat f VAP
• Compd. ?vapH (kJ/mol) IM Force
• H2O 40.7 (100 oC) H-bonds
• SO2 26.8 (-47 oC) dipole
• Xe 12.6 (-107 oC) induced dipole

48
Equilibrium Vapor Pressure the
Clausius-Clapeyron Equation

• Clausius-Clapeyron equation used to find
  ?vapH.
• The logarithm of the vapor pressure P is
  proportional to ?vapH and to 1/T.
• ln P (?vapH/RT) C

49
Surface Tension

• SURFACE TENSION also leads to spherical liquid
  droplets.

50
11.3 Properties resulting from IMF

1. Viscosity resistance of a liquid to flow
2. Surface tension energy required to increase the
   surface area of a liquid

51
Liquids

• Intermolec. forces also lead to CAPILLARY action
  and to the existence of a concave meniscus for a
  water column.

52
Capillary Action

• Movement of water up a piece of paper depends on
  H-bonds between H2O and the OH groups of the
  cellulose in the paper.

53

• 3. Cohesion attraction of molecules for other
  molecules of the same compound
• 4. Adhesion attraction of molecules for a
  surface

54

• Meniscus curved upper surface of a liquid in a
  container a relative measure of adhesive and
  cohesive forces
• Ex

Hg
H2O
(cohesion rules)
(adhesion rules)
55
Geckos!

• Geckos feet make use of London dispersion forces
  to climb almost anything.
• A gecko can hang on a glass surface using only
  one toe.
• Researchers at Stanford University recently
  developed a gecko-like robot which uses synthetic
  setae to climb walls
• http//www.visionlearning.com/library/module_viewe
  r.php?mid57
• Jesus Lizard

http//en.wikipedia.org/wiki/Van_der_Waals27_forc
e
56

• London dispersion forces (induced dipole-induced
  dipole) increase with
• Increasing MW, of e-, and of atoms
  (increasing of e- orbitals to be distorted)
• Boiling points
• Effect of MW Effect of atoms
• pentane 36ºC Ne 246C
• hexane 69ºC CH4   162C
• heptane 98ºC
• ??? effect
• H2O 100C
• D2O 101.4C
• Longer shapes (more likely to interact with
  other molecules)
• C5H12 isomers 2,2-dimethylpropane 10C
• pentane 36C

57
Summary of IMF
Van der Waals forces
58
Ex Identify all IMF present in a pure sample of
each substance, then explain the boiling points.
BP(C) IMF Explanation
HF 20
HCl -85
HBr -67
HI -35
Lowest MW/weakest London, but most polar/strongest dipole-dipole and has H-bonds
Low MW/weak London, moderate polarity/dipole-dipole and no H-bonds
Medium MW/medium London, moderate polarity/dipole-dipole and no H-bonds
Highest MW/strongest London, but least polar bond/weakest dipole-dipole and no H-bonds
London, dipole-dipole, H-bonds
London, dipole-dipole
London, dipole-dipole
London, dipole-dipole
59
11.4 Phase Changes

• Processes
• Endothermic melting, vaporization, sublimation
• Exothermic condensation, freezing, deposition

I2 (s) and (g)
60
Water Enthalpy diagram or heating curve
61
11.5 Vapor pressure
Pressure cooker 2 atm
Normal BP 1 atm
10,000 elev 0.7 atm
29,029 elev (Mt. Everest) 0.3 atm

• A liquid will boil when the vapor pressure equals
  the atmospheric pressure, at any T above the
  triple point.

62
11.6 Phase diagrams CO2

• Lines 2 phases exist in equilibrium
• Triple point all 3 phases exist together in
  equilibrium (X on graph)
• Critical point, or critical temperature
  pressure highest T and P at which a liquid can
  exist (Z on graph)

Temp (ºC)

• For most substances, inc P will cause a gas to
  condense (or deposit), a liquid to freeze, and a
  solid to become more dense (to a limit.)

63
Phase diagrams H2O

• For H2O, inc P will cause ice to melt.

64

65

66
Group Activity
Get in groups of four. Two people will need to
draw and two will need to explain Choose
roles. First drawer draws a phase diagram of
water. Second drawer draws a phase diagram of
CO2 Third member explains the waters phase
diagram Fourth member explains CO2s phase
diagram.
67
11.7-8 Structures of solids

• Amorphous without orderly structure
• Ex rubber, glass
• Crystalline repeating structure have many
  different stacking patterns based on chemical
  formula, atomic or ionic sizes, and bonding

68
Types of crystalline solids (Table 11.6)
Type Particles Forces Notable properties Examples
Atomic Atoms London dispersion Poor conductors Very low MP Ar (s),Kr (s)
69
Molecular Molecules (polar or non-polar) London dispersion, dipole-dipole, H-bonds Poor conductors Low to moderate MP CO2 (s), C12H22O11, H2O (s)
Carbon dioxide (dry ice)
Sucrose
Ice
70
Ionic Anions and cations Electrostatic attractions Hard brittle High MP Poor conductors Some solubility in H2O NaCl, Ca(NO3)2
71
Covalent (a.k.a. covalent network) Atoms bonded in a covalent network Covalent bonds Very hard Very high MP Generally insoluble Variable conductivity C (diamond graphite) SiO2 (quartz) Ge, Si, SiC, BN
Diamond
Graphite
SiO2
72
Metallic Metal cations in a diffuse, delocalized e- cloud Metallic bonds Excellent conductors Malleable Ductile High but wide range of MP Cu, Al, Fe