Lectures 24-25 Intermolecular forces · 2012. 12. 1. · Chemistry 1B, Fall 2012 Lectures 24-25....

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Chemistry 1B, Fall 2012Lectures 24-25

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Lectures 24-25

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 Forcesand 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

HekJ/mol

kJ/mol

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more dipole – induced dipole

dipole induceddipole

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)

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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,FH-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) ö FHF- ΔH= - 155kJ/mol

(CH3)2C=O(g) +HF(g) ö (CH3)2C=OºHF ΔH= - 46kJ/mol

H2O(g) + H2O(g) ö H2OºHOH (ice) ΔH= - 21kJ/mol

~N ºH~O~ ~N º H~N~

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)

strongmoderate

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 pointHBr, Kr, or Cl2

c. lowest vapor pressure at 25ºCCl2, 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

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25van der Waals forces

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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 higherenergy than those in ‘bulk’; liquids forspherical droplets to minimize surfacearea

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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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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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‘ 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

hydrophobicnonpolar

hydrophilicpolar

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 membraneshttp://www.cem.msu.edu/~reusch/VirtualText/Images3/bilyrstr.gif

micelleshttp://www.chemistry.nus.edu.sg/2500/grease.jpg

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

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Finis !!!

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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

p-system

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Lectures 24-25 Intermolecular forces · 2012. 12. 1. · Chemistry 1B, Fall 2012 Lectures 24-25....

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