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Lecture 24

Intermolecular forces

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where we’ve been and where the (almost) last 1B lectures take us

• have studied intramolecular forces among atoms or ions within a ‘molecule’

• covalent forces

• ionic forces

• metallic bonding

• extended covalent bonding (graphite, diamond, graphene)

• coordinate covalent (transition metal complexes; Lewis acid-base)

• now what about intermolecular forces among differing molecules

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examples of phenomena that depend on intermolecular forces

• physical states (phases) and phase changes

( solid liquid gas )

• secondary and tertiary structure of biologically important

molecules

(how differing parts of a large molecule interact to form its

full 3-D structure)

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physical states and intermolecular forces (Fig. 16.1; Silber table 12.1)

Intermolecular

forces (vs T)

weak

moderate

strong

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types of intramolecular (bonding) and intermolecular force

• intramolecular

ionic

covalent

metallic

coordinate covalent (transition metal complexes; Lewis acid-base)

• intermolecular ion-dipole

hydrogen bonding

dipole-dipole

ion-induced dipole

dipole-induced dipole

dispersion (London, van der Waals)

see handout: Intermolecular Forces

and slide #19 (Silberberg Table 12.2)

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energies of intramolecular (bonding) ‘forces’

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ion-dipole intermolecular forces: ion (polar) ↔ polar

H

O

H

+ -

+

Na+

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+ I ─ Cl

+ I ─ Cl

+ Cl ─ I

H

H C O

H

dipole-dipole intermolecular forces: polar ↔ polar

H

kJ/mol

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ordering by dipole-dipole forces (figure 16.2)

Lower T Higher T

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ion-induced_dipole and dipole-induced_dipole (polar ↔ nonpolar)

isolated

He kJ/mol

kJ/mol

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more dipole – induced dipole

dipole induced dipole

nonpolar

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dispersion forces (instantaneous dipoles): (non-polar ↔ non-polar)

kJ/mol

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dispersion forces (instantaneous dipoles; figure 16.5)

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dispersion forces (animation)

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

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hydrogen bonds (very important !!)

- + -

─B: ······ H─A─

H2O

small electronegative atom

with lone pair (N, O, F) hydrogen bonded to

electronegative atom N,O,F

H-bond

.. .. .. .. H ─ F : H ─ O ─ O= H─N─ or :N≡ .. .. ..

small electronegative atoms:

kJ/mol

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where would and

fall in the above series?

HW8: 75. Zumdahl #16.24

F(g) + HF(g) FHF ΔH= 155kJ/mol

(CH3)2C=O(g) +HF(g) (CH3)2C=OHF ΔH= 46kJ/mol

H2O(g) + H2O(g) H2OHOH (ice) ΔH= 21kJ/mol

N HO N HN

weaker

weakest

greater bond polarity; greater H-bond stability

(exothermic)

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hydrogen bonds in biological molecules (RNA and DNA)

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hydrogen bonds in biological molecules (protein secondary structure)

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summary (Silberberg: table 12.2)

strong

moderate

weaker

depends stay tuned

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FINAL EXAM WILL BE FROM EXAMPLES IN LECTURE

Now some factoids and examples.

Problems on final will be based on understanding

of these specific examples !!

‘Take Home’ message on each slide !!

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molecular structure and intermolecular forces (problem 71 and 72)

72. Zumdahl #16.15 Identify the most important types of

interparticle forces present in the solids of each of

the following substances

examples: a. Ar; e. CH4 ; k. CHCl3 ; l. NH3

71. What are the most important intermolecular forces

between the following molecules and atoms:

a. NaCl (aq)

b. Fe2+ and O2

c. CH3Cl and CCl4

d. examples from table in handout (slide #19)

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polarizability: strength of induced and spontaneous dipoles

• polarizability: how “free” the electrons in an atom or

molecule are to ‘slosh around’

• induced and spontaneous dipoles are larger if atom or

molecule is more polarizable

• periodic trends in polarizability:

increases down a group (outer electrons further away)

decreases across a period (higher Zeff, more tightly held)

anions are more polarizable than parent neutral atom (lower Zeff)

cations are less polarizable than parent atom (higher Zeff)

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boiling points, melting points, vapor pressure and intermolecular forces

(nonpolar compounds, Table 16.2; table 16.8, Silberberg fig. 12.7)

greater polarizability

greater intermolecular forces

higher melting (freezing) and boiling points, lower vapor pressure

boiling point in Kº

problem 73. #16.18 a,c

a. highest boiling point

HBr, Kr, or Cl2

c. lowest vapor pressure at 25ºC

Cl2, Br2, or I2

HBr > [Cl2>?Kr]

I2 < Br2 < Cl2

LE

] ~

melting point, strength of

intermolecular forces

increased polarizability

increased freezing point

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boiling points and intermolecular forces (nonpolar compounds; 16.19a)

greater molecular surface greater dispersion forces

higher boiling points

25 van der Waals forces

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Mercury News

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more gecko (‘Getting a Grip’, p 769)

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boiling points and intermolecular forces (Prob 16.19, Silb. fig. 12.8)

molecules with equivalent “molecular weight”

(ie ‘size’ and polarizability and intermolecular dispersion forces)

polarity (dipole moment)

and boiling point

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surface tension (Zumdahl fig. 16.6, 16.7; Silb fig. 12.19)

intermolecular forces differ for molecules at surface and in bulk

extra: molecules at surface have higher

energy than those in ‘bulk’; liquids for

spherical droplets to minimize surface

area

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surface tension (Silberberg table 12.3; sample problem 16.29)

greater intermolecular forces greater surface tension

IMF

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concave vs convex meniscus (Zumdahl fig 16.7; Silberberg fig. 12.20)

H2O greater forces with glass than H2O concave

and high capillarity

Hg greater forces with Hg than glass convex

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why does ice float (see figure 16.12)

• H2O is polar and can form hydrogen bonds

(macho intermolecular forces)

• High surface tension and capillarity

• Hydrogen bonds form very open structure in solid H2O (ice)

giving ice a lower density than H2O liquid. ICE FLOATS!!

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ice bomb !!!!

http://www.jce.divched.org/JCESoft/CCA/pirelli/pages/cca2icebomb.html

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solubility and intermolecular forces

NaCl(s) → Na+(aq) + Cl- (aq)

C2H5OH + H2O → C2H5OH (aq)

C6H14 + H2O → C6H14 + H2O → C6H14 (aq)

C6H14 + CCl4 → solution

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solubility and intermolecular forces

whether a substance dissolves in ‘solvent’ (solubility),

or two liquids mix (miscibility) is determined by two

factors:

• things like to get ‘mixed up’, S[olutions] Happen

unless too endothermic (entropy, chem 1C)

• things like to give off heat (stability of

‘products’, interparticle forces in products vs

those in reactants; chem 1B)

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solubility and intermolecular forces (ionic solids + polar solvent)

NaCl(s) → Na+(aq) + Cl- (aq)

[ion-ion] [ion-dipole]

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C2H5OH + H2O → C2H5OH (aq)

ethyl alcohol

solubility and intermolecular forces (two polar liquids)

H H

H ─C─C─O─H + H2O

H H .. .. H H

→ H ─C─C─O

H H H

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solubility and intermolecular forces (nonpolar + polar)

C6H14 + H2O → C6H14 (aq) hexane

only weak dispersion and dipole-induced dipole forces

among hexane and water molecules

immiscible

H H H H H H

H─C─C─C─C─C─C─H

H H H H H H

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nonploar molecules: hydrophobic

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solubility and intermolecular forces (nonpolar + nonpolar)

C6H14 + CCl4 → solution

does dissolve

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solubility and intermolecular forces

‘ in general’ (likes dissolve in likes)

polar molecules will form solutions with polar

molecules

nonpolar molecules will form solutions with nonpolar

molecules

polar and nonpolar substances will not form solutions

http://www.jce.divched.org/JCESoft/CCA/pirelli/pages/cca2like.html

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practical applications of immiscibility

lava lamps

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hydrophilic vs hydrophobic

hydrophilic: ‘likes’ water; polar molecules or

polar parts of molecules

hydrophobic: ‘dislikes’ water; ‘likes’

nonpolar environments; nonpolar

molecules or parts of molecules

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soap and detergents: hydrophilic + hydrophobic

hydrophobic nonpolar

hydrophilic polar

soap detergent

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soap- ‘takes the grime right down the drain”

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micelles in biology (cell and other membranes)

http://fig.cox.miami.edu/~cmallery/255/255chem/mcb2.20.micelle.jpg

(phospholipids

detergent-like molecules)

http://www.uic.edu/classes/bios/bios100/lecturesf04am/phospholipid.jpg

http://sps.k12.ar.us/massengale/images/cellmembranes15.jpg

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micelles and membranes

bilayer membranes http://www.cem.msu.edu/~reusch/VirtualText/Images3/bilyrstr.gif

micelles http://www.chemistry.nus.edu.sg/2500/grease.jpg

http://fig.cox.miami.edu/~cmallery/255/255chem/gk2x20.gif

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Graphene (2010 Nobel Prize) lecture 9

Graphene is a one-atom-thick planar sheet of sp2-bonded carbon

atoms that are densely packed in a honeycomb crystal lattice.

sp2 carbons unhybridized p-orbitals

delocalized interesting

bonds properties

conjugated of graphene

-system

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• Week of December 2-6- all regular sections and office hours

• Monday December 2

Sample Exam on eCommons

‘Cumulative Review Outline’ on Chem1B WWW Handouts

Lecture: Review Kinetics and Spectroscopy

• Tuesday December 3

Extra Office Hours: Switkes 2:15-3:15PM

• Wednesday December 4

Lecture: Intermolecular Forces

HW #9 (kinetics) WA due; solutions on eCommons

• Thursday December 5

Sample Exam Key on eCommons

•Friday December 6

Last lecture (get yourselves there !!)

Extra Office Hours: Switkes 9:00-10:00AM

• Saturday December 7

Class Review Session- 11-12:30AM, Thimann 3 (Gene S.)

• Monday December 9

FINAL EXAM 8:00-11:00 AM M110

Chemistry 1B, Fall 2013

Week of December 2nd-9th

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final exam: 8:00-11:00AM on

Monday, 9th December

see you there !!

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Finis !!! BUT one MORE THING