Ch 10: Intermolecular Forces and Types of Solids

AP ChemistrySequoyah High School

210.1: 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.

310.1: Types of IMF1. Electrostatic forces: act over larger

distances in accordance with Coulomb’s law

http://dwb4.unl.edu/ChemAnime/attractive_forces.htm

a.Ion-ion forces: strongest; found in ionic crystals (i.e. lattice energy)

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

2d

QQF

4b. 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.

2d

QQF

Cl-

S2-<

Ex: Compare IMF in Cl- (aq) and S2- (aq).http://wps.prenhall.com/wps/media/objects/439/449969/Media_Portfolio/Chapter_14/Dissolution_NaCl_Water.MO

5c. Dipole-dipole: weakest electrostatic force; exist between neutral polar molecules Increase with increasing

polarity (dipole moment) of molecule

Ex: What IMF exist in NaCl (aq)?http://antoine.frostburg.edu/chem/senese/101/liquids/faq/h-bonding-vs-london-forces.shtml

http://usm.maine.edu/~newton/Chy251_253/Lectures/CarbonylReduction/AldehydesKetones.html

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d. 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.visionlearning.com/library/module_viewer.php?mid=57

( ice and water simulation)http://www.youtube.com/watch?v=LGwyBeuVjhU ( H bond

movie)http://www.youtube.com/watch?

v=lkl5cbfqFRM&feature=related ( H bonding in water)

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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

8 * 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

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

http://www.youtube.com/watch?v=PcoiLAsUvqc&feature=related http://www.youtube.com/watch?v=gmjLXrMaFTg&feature=relatedhttp://en.wikipedia.org/wiki/Water_%28molecule%29#Density_of_water_and_ice

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Ex: Boiling points of nonmetal hydrides

Boili

ng P

oin

ts (

ºC)

Conclusions:

Polar molecules have higher BP than nonpolar molecules

∴ Polar molecules have stronger IMF

BP increases with increasing MW

∴ Heavier molecules have stronger IMF

NH3, H2O, and HF have unusually high BP.

∴ H-bonds are stronger than dipole-dipole IMF

112. 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)

12How they form:1. Motion of e- creates an instantaneous

dipole moment, making it “temporarily polar”.

2. Instantaneous dipole moment induces a dipole in an adjacent atom• * Persist for about 10-14 or 10-15 second

http://dwb4.unl.edu/ChemAnime/LONDOND/LONDOND.html

Ex: two He atoms

13* 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://en.wikipedia.org/wiki/Van_der_Waals%27_force

14London dispersion forces 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 –246°C hexane 69ºC CH4   –162°Cheptane 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

Van der Waals forces

16Ex: Identify all IMF present in a pure sample of each substance, then explain the boiling points.

BP(⁰C)

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

1710.2: 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

183. Cohesion: attraction of molecules for other molecules of the same compound

4. Adhesion: attraction of molecules for a surface

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5. Meniscus: curved upper surface of a liquid in a container; a relative measure of adhesive and cohesive forcesEx:

Hg H2O(cohesion rules) (adhesion rules)

2010.8: Phase ChangesProcesses: Endothermic: melting,

vaporization, sublimation Exothermic:

condensation, freezing, deposition

I2 (s) and (g)

Microchip

21Water: Enthalpy diagram or heating curve

J/g) 334(mQ

TmQ )CJ/g 4.18(

TmQ )CJ/g 87.1(

TmQ )CJ/g 06.2(

J/g) 2602(mQ

TmcQ mHQ

2210.8: Vapor pressure

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

http://glencoe.com/sites/common_assets/advanced_placement/chemistry_chang9e/animations/chang_2e/vapor_pressure.swfhttp://dwb4.unl.edu/ChemAnime/VPTEMPD/VPTEMPD.htmlhttp://dwb4.unl.edu/ChemAnime/VP3LIQD/VP3LIQD.html

Pressure cooker ≈ 2 atm

Normal BP = 1 atm

10,000’ elev ≈ 0.7 atm

29,029’ elev (Mt. Everest) ≈ 0.3 atm

2310.9: 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)

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.)

Temp (ºC)

24Phase diagrams: H2O

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

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*

26

*

2710.3-7: Structures of solids

Amorphous: without orderly structureEx: rubber, glass

Crystalline: repeating structure; have many different stacking patterns based on chemical formula, atomic or ionic sizes, and bonding

Types of crystalline solids (Table 11.6)

Type Particles ForcesNotable

propertiesExample

s

Atomic AtomsLondon

dispersion

Poor conductors

Very low MP

Ar (s),Kr (s)

Molecular

Molecules

(polar or non-

polar)

London dispersion, dipole-

dipole, H-bonds

Poor conductors

Low to moderate MP

CO2 (s),

C12H22O11,

H2O (s)

SucroseCarbon dioxide (dry ice)

Ice

Ionic

Anions and

cations

Electrostatic attractions

Hard & brittle

High MPPoor conductors

Some solubility in H2O

NaCl,Ca(NO3)2

Covalent (a.k.a.

covalent network)

Atoms bonded

in a covalent network

Covalent bonds

Very hardVery high MP

Generally insoluble

Variable conductivity

C (diamond

& graphite)

SiO2

(quartz)

Ge, Si, SiC, BN

DiamondGraphite SiO2

Metallic

Metal cations in a diffuse, delocalized e- cloud

Metallic bonds

Excellent conductors

MalleableDuctileHigh but wide range of MP

Cu, Al, Fe