Lecture 24 Intermolecular Forces
Lecture 24 Intermolecular forces
1 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)
3 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) 5 energies of intramolecular (bonding) ‘forces’
6 ion-dipole intermolecular forces: ion (polar) ↔ polar
- H + Na+ O H +
7 dipole-dipole intermolecular forces: polar ↔ polar
H H
H C O kJ/mol
H
+ Cl ─ I
+ + I ─ Cl I ─ Cl
8 ordering by dipole-dipole forces (figure 16.2)
Lower T Higher T 9 ion-induced_dipole and dipole-induced_dipole (polar ↔ nonpolar)
isolated He kJ/mol
kJ/mol
10 more dipole – induced dipole
nonpolar
dipole induced dipole
11 dispersion forces (instantaneous dipoles): (non-polar ↔ non-polar)
kJ/mol
12 dispersion forces (instantaneous dipoles; figure 16.5)
13 dispersion forces (animation)
http://chemmovies.unl.edu/ChemAnime/LONDOND/LONDOND.html 14 hydrogen bonds (very important !!)
- + - small electronegative atom ─B: ······ H─A─ hydrogen bonded to with lone pair (N, O, F) electronegative atom N,O,F H-bond small .. electronegative ..atoms: .. .. H ─ F : H ─ O ─ O= H─N─ or :N≡ ......
H2O kJ/mol
15 HW8: 75. Zumdahl #16.24
greater bond polarity; greater H-bond stability (exothermic) F(g) + HF(g) FHF ΔH= 155kJ/mol
(CH3)2C=O(g) +HF(g) (CH3)2C=O HF ΔH= 46kJ/mol
H2O(g) + H2O(g) H2O HOH (ice) ΔH= 21kJ/mol
weaker
weakest
where would N H O and N HN
fall in the above series? 16 hydrogen bonds in biological molecules (RNA and DNA)
17 hydrogen bonds in biological molecules (protein secondary structure)
18 summary (Silberberg: table 12.2)
strong moderate
weaker
depends stay tuned 19 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 !!
20 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) 2+ b. Fe 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
21 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
problem 73. #16.18 a,c a. highest boiling point increased polarizability HBr, Kr, or Cl2 HBr > [Cl >?Kr] increased freezing point 2 c. lowest vapor pressure at 25ºC
Cl2, Br2, or I2 I2 < Br2 < Cl2 boiling point in Kº 23 boiling points and intermolecular forces (nonpolar compounds; 16.19a)
greater molecular surface greater dispersion forces higher boiling points
24 van der Waals forces 25 nanofur from NANOSYS (Palo Alto)
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26 more gecko (‘Getting a Grip’, p 769)
27 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 28 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
30 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!!
32 ice bomb !!!!
http://www.jce.divched.org/JCESoft/CCA/pirelli/pages/cca2icebomb.html
33 solubility and intermolecular forces
NaCl(s) → Na+(aq) + Cl- (aq)
C2H5OH + H2O → C2H5OH (aq)
C6H14 + H2O → C6H14 + H2O → C6H14 (aq)
C6H14 + CCl4 → solution
34 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)
35 solubility and intermolecular forces (ionic solids + polar solvent)
NaCl(s) → Na+(aq) + Cl- (aq) [ion-ion] [ion-dipole]
36 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
37 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
39 solubility and intermolecular forces (nonpolar + nonpolar)
C6H14 + CCl4 → solution
does dissolve
40 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 41 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
43 soap and detergents: hydrophilic + hydrophobic
hydrophobic hydrophilic nonpolar polar
soap detergent
44 soap- ‘takes the grime right down the drain”
45 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
46 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
47 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 interesting bonds properties conjugated of graphene -system sp2 carbons unhybridized p-orbitals 48 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 !!
50 Finis !!!
BUT one MORE THING
51