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 2

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 4

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

Na+

H +

O H +

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dipole-dipole intermolecular forces: polar ↔ polar

H H

H C

kJ/mol

O

H



+

+



Cl ─ I I ─ Cl

+



I ─ Cl

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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 nonpolar

dipole

induced dipole 11

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 !!)

small electronegative atom with lone pair (N, O, F)

-

+

-

─B: ······ H─A─

hydrogen bonded to electronegative atom N,O,F

H-bond small electronegative atoms:

.. H ─ F.. : H2O

.. H─O .. ─

.. O= ..

.. H─N─ or :N≡

kJ/mol

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HW8: 75. Zumdahl #16.24 greater bond polarity; greater H-bond stability F(g) +

HF(g)



(exothermic) ΔH=  155kJ/mol

FHF

(CH3)2C=O(g) +HF(g)  (CH3)2C=OHF

ΔH=  46kJ/mol

 H2OHOH (ice)

ΔH=  21kJ/mol

H2O(g) + H2O(g)

weaker weakest where would

N HO

and

N  HN

fall in the above series? 16

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)

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)

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

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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) 22

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 melting point, strength of intermolecular forces

]~

 LE

increased polarizability increased freezing point

boiling point in Kº

problem 73. #16.18 a,c a. highest boiling point HBr, Kr, or Cl2 HBr > [Cl2>?Kr] c. lowest vapor pressure at 25ºC Cl2, Br2, or I2 I2 < Br2 < Cl2 23

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

greater molecular surface  greater dispersion forces  higher boiling points

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van 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 higher energy than those in ‘bulk’; liquids for spherical droplets to minimize surface area 29

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 31

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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solubility and intermolecular forces (two polar liquids)

C2H5OH + H2O → C2H5OH (aq) ethyl alcohol

H H H H .. H ─C─C─O─H .. + H2O → H ─C─C─O H H

H H

H

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solubility and intermolecular forces (nonpolar + polar)

C6H14 + H2O → C6H14 (aq) hexane

H H H H H H H─C─C─C─C─C─C─H

H H H H H H only weak dispersion and dipole-induced dipole forces among hexane and water molecules

immiscible 38

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 42

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 hydrophilic polar

hydrophobic nonpolar

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)

(phospholipids detergent-like molecules) http://www.uic.edu/classes/bios/bios100/lecturesf04am/phospholipid.jpg

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

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

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micelles and membranes

micelles http://www.chemistry.nus.edu.sg/2500/grease.jpg http://fig.cox.miami.edu/~cmallery/255/255chem/gk2x20.gif

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

delocalized bonds conjugated -system

interesting properties of graphene sp2 carbons

unhybridized p-orbitals

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Chemistry 1B, Fall 2013 Week of December 2nd-9th • 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 49

final exam: 8:00-11:00AM on Monday, 9th December

see you there !!

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Finis !!! BUT one MORE THING

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