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Ch 11: Intermolecular Forces and Types of Solids. Brown, LeMay AP Chemistry Monta Vista High School. Inter- molecular Forces. Have studied INTRA molecular forces—the forces holding atoms together to form molecules. Now turn to forces between molecules — INTER molecular forces.

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ch 11 intermolecular forces and types of solids

Ch 11: Intermolecular Forces and Types of Solids

Brown, LeMay

AP Chemistry

Monta Vista High School

inter molecular forces
Inter-molecular Forces

Have studied INTRAmolecular forces—the 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.

11 1 intermolecular forces imf
11.1: Intermolecular Forces (IMF)
  • IMF < intramolecular forces (covalent, metallic, ionic bonds)
  • IMF strength: solids > liquids > gases
    • Boiling points and melting points are good indicators of relative IMF strength.
summary of intermolecular forces
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)
11 2 types of imf
11.2: Types of IMF
  • Electrostatic forces: act over larger distances in accordance with Coulomb’s law
    • Ion-ion forces: strongest; found in ionic crystals (i.e. lattice energy)

http://chemmovies.unl.edu/ChemAnime/IONSIZED/IONSIZED.html

Ion size and LE

slide7

d+

d+

d+

d+

d-

d-

d+

d+

d-

d-

Cl-

S2-

d+

d+

d+

d+

d-

d-

d+

d+

  • 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).

<

http://chemmovies.unl.edu/ChemAnime/NACL1D/NACL1D.html

NaCl dissolving in Water

attraction between ions and permanent dipoles
Attraction Between Ions and Permanent Dipoles

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

attraction between ions and permanent dipoles9
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)x]n+

slide10
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)?

dipole dipole forces
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

partner activity
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
slide13
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?v=LGwyBeuVjhU

slide14
H-bonds form with H-X•••X', 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

hydrogen bonding
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

slide16
H-bonds explain why ice is less dense than water.

http://en.wikipedia.org/wiki/Water_%28molecule%29#Density_of_water_and_ice

ex boiling points of nonmetal hydrides
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
hydrogen bonding in h 2 o
Hydrogen Bonding in H2O

H-bonding is especially strong in water because

  • the O—H bond is very polar
  • there are 2 lone pairs on the O atom

Accounts for many of water’s unique properties.

http://www.visionlearning.com/library/flash_viewer.php?oid=1435&mid=120

Animation of Ice

hydrogen bonding in h 2 o21
Hydrogen Bonding in H2O

Ice has open lattice-like structure.

Ice density is < liquid.

And so solid floats on water.

Snow flake: www.snowcrystals.com

hydrogen bonding in h 2 o22
Hydrogen Bonding in H2O

Ice has open lattice-like structure.

Ice density is < liquid and so solid floats on water.

http://www.visionlearning.com/library/flash_viewer.php?oid=1380&mid=57

H bonding in Water

One of the VERY few substances where solid is LESS DENSE than the liquid.

hydrogen bonding23
Hydrogen Bonding

H bonds leads to abnormally high boiling point of water.

PLAY MOVIE

See Screen 13.7

hydrogen bonding in biology
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

slide27

Double helix of DNA

Portion of a DNA chain

hydrogen bonding in biology29
Hydrogen Bonding in Biology

Hydrogen bonding and base pairing in DNA.

h bonding activity
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.
slide31
* 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

inductive forces
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)

slide33
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
forces involving induced dipoles
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/liquids/faq/h-bonding-vs-london-forces.shtml

Dipole-Dipole and LDFs

forces involving induced dipoles35
FORCES INVOLVING INDUCED DIPOLES

Solubility increases with mass the gas

forces involving induced dipoles36
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.
im forces induced dipoles

d

-

I-I

I-I

d

The alcohol temporarily creates or INDUCES a dipole in I2.

+

d

-

d

O

-

O

R

H

R

H

d

+

d

+

IM FORCES — INDUCED DIPOLES

Consider I2 dissolving in ethanol, CH3CH2OH.

forces involving induced dipoles38
FORCES INVOLVING INDUCED DIPOLES

Formation of a dipole in two nonpolar I2 molecules.

Induced dipole-induced dipole

http://chemmovies.unl.edu/ChemAnime/LONDOND/LONDOND.html

LDFs

forces involving induced dipoles39
FORCES INVOLVING INDUCED DIPOLES

The induced forces between I2 molecules are very weak, so solid I2sublimes (goes from a solid to gaseous molecules).

PLAY MOVIE

intermolecular forces
Intermolecular Forces

See Figure 12.12

liquids section 12 4
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

PLAY MOVIE

liquids

LIQUID

VAPOR

Liquids

The two key properties we need to describe are EVAPORATION and its opposite—CONDENSATION

Evaporation f

Add energy

break IM bonds

make IM bonds

Remove energy

r condensation

liquids evaporation
Liquids—Evaporation

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

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

liquids distribution of energies

higher T

lower T

Number of molecules

0

Molecular energy

Minimum energy req’d to break IM forces and evaporate

Liquids—Distribution of Energies

Distribution of molecular energies in a liquid.

KE is propor-tional to T.

See Figure 12.13

equilibrium vapor pressure
Equilibrium Vapor Pressure

http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/vaporv3.swf

Vapor Pressure

in class activity
In Class Activity
  • Your teacher will explain 
liquids48
Liquids

HEAT OF VAPORIZATION is the heat req’d (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

equilibrium vapor pressure the clausius clapeyron equation
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
surface tension
Surface Tension

SURFACE TENSION also leads to spherical liquid droplets.

11 3 properties resulting from imf
11.3: Properties resulting from IMF
  • Viscosity: resistance of a liquid to flow
  • Surface tension: energy required to increase the surface area of a liquid
liquids52

concave

ADHESIVE FORCES

meniscus

between water

and glass

H

O in

2

COHESIVE FORCES

glass

between water

tube

molecules

Liquids

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

capillary action
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.

PLAY MOVIE

slide54
3. Cohesion:attraction of molecules for other molecules of the same compound

4. Adhesion:attraction of molecules for a surface

slide55
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)

geckos
* 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_viewer.php?mid=57

Jesus Lizard

http://en.wikipedia.org/wiki/Van_der_Waals%27_force

slide57
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 36ºCNe –246°C

hexane 69ºCCH4   –162°C

heptane 98ºC

??? effect:

H2O 100°C

D2O 101.4°C

  • “Longer” shapes (more likely to interact with other molecules)

C5H12 isomers: 2,2-dimethylpropane 10°C

pentane 36°C

summary of imf
Summary of IMF

Van der Waals forces

slide59

Ex: Identify all IMF present in a pure sample of each substance, then explain the boiling points.

11 4 phase changes
11.4: Phase Changes

Processes:

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

I2 (s) and (g)

Microchip

11 5 vapor pressure
11.5: Vapor pressure
  • A liquid will boil when the vapor pressure equals the atmospheric pressure, at any T above the triple point.

Pressure cooker ≈ 2 atm

Normal BP = 1 atm

10,000’ elev ≈ 0.7 atm

29,029’ elev (Mt. Everest)≈ 0.3 atm

11 6 phase diagrams co 2
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.)
phase diagrams h 2 o
Phase diagrams: H2O
  • For H2O, inc P will cause ice to melt.
group activity
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 water’s phase diagram

Fourth member explains CO2’s phase diagram.

11 7 8 structures of solids
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
slide72

Graphite

Diamond

SiO2