Ch 11: Intermolecular Forces and Types of Solids

1. Ch 11: Intermolecular Forces and Types of Solids Brown, LeMay AP Chemistry Monta Vista High School

2. Inter-molecular Forces Have studied INTRAmolecular forces—the forces holding atoms together to form molecules. Now turn to forces between molecules —INTERmolecular forces. Forces between molecules, between ions, or between molecules and ions.

3. 11.1: 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.

4. Summary of Intermolecular Forces • Ion-Ion forces • Ion-dipole forces • Dipole-dipole forces • Special dipole-dipole force: hydrogen bonds (sometimes treated as a separate IMF) • Forces involving non polar molecules: induced forces (LDFs)

5. Intermolecular Forces Summary

6. 11.2: Types of IMF • Electrostatic forces: act over larger distances in accordance with Coulomb’s law • Ion-ion forces: strongest; found in ionic crystals (i.e. lattice energy) http://chemmovies.unl.edu/ChemAnime/IONSIZED/IONSIZED.html Ion size and LE

7. d+ d+ d+ d+ d- d- d+ d+ d- d- Cl- S2- d+ d+ d+ d+ d- d- d+ d+ • 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. Ex: Compare IMF in Cl- (aq) and S2- (aq). < http://chemmovies.unl.edu/ChemAnime/NACL1D/NACL1D.html NaCl dissolving in Water

8. Attraction Between Ions and Permanent Dipoles Water is highly polar and can interact with positive ions to give hydratedions in water.

9. Attraction Between Ions and Permanent Dipoles Attraction between ions and dipole depends on ion charge and ion-dipole distance. Measured by ∆H for Mn+ + H2O f [M(H2O)x]n+

10. Dipole-dipole: weakest electrostatic force (Not all IMFs, LDFs weaker than dipole-dipole); exist between neutral polar molecules • Increase with increasing polarity (dipole moment) of molecule Ex: What IMF exist in NaCl (aq)?

11. Dipole-Dipole Forces Influence of dipole-dipole forces is seen in the boiling points of simple molecules. Compd Mol. Wt. Boil Point N2 28 -196 oC CO 28 -192 oC Br2 160 59 oC ICl 162 97 oC

12. Partner Activity • Discuss with your partner the difference between ion-dipole and dipole-dipole interactions, in terms of the following: • How they are formed • Strength • Examples

13. 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.youtube.com/watch?v=LGwyBeuVjhU

14. 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

15. Hydrogen Bonding A special form of dipole-dipole attraction, which enhances dipole-dipole attractions. H-bonding is strongest when X and Y are N, O, or F

16. H-bonds explain why ice is less dense than water. http://en.wikipedia.org/wiki/Water_%28molecule%29#Density_of_water_and_ice

17. Ex: Boiling points of nonmetal hydrides Conclusions: • Polar molecules have higher BP than nonpolar molecules • ∴ Polar molecules have stronger IMF • BP increases with increasing MW • ∴ Heavier molecules have stronger IMF Boiling Points (ºC) • NH3, H2O, and HF have unusually high BP. • ∴ H-bonds are stronger than dipole-dipole IMF

18. H-bond H-Bonding Between Methanol and Water - + -

19. H-Bonding Between Two Methanol Molecules - + - H-bond

20. Hydrogen Bonding in H2O H-bonding is especially strong in water because • the O—H bond is very polar • there are 2 lone pairs on the O atom Accounts for many of water’s unique properties. http://www.visionlearning.com/library/flash_viewer.php?oid=1435&mid=120 Animation of Ice

21. Hydrogen Bonding in H2O Ice has open lattice-like structure. Ice density is < liquid. And so solid floats on water. Snow flake: www.snowcrystals.com

22. Hydrogen Bonding in H2O Ice has open lattice-like structure. Ice density is < liquid and so solid floats on water. http://www.visionlearning.com/library/flash_viewer.php?oid=1380&mid=57 H bonding in Water One of the VERY few substances where solid is LESS DENSE than the liquid.

23. Hydrogen Bonding H bonds leads to abnormally high boiling point of water. PLAY MOVIE See Screen 13.7

24. Boiling Points of Simple Hydrogen-Containing Compounds See Active Figure 12.8

25. Methane Hydrate

26. Hydrogen Bonding in Biology H-bonding is especially strong in biological systems — such as DNA. DNA — helical chains of phosphate groups and sugar molecules. Chains are helical because of tetrahedral geometry of P, C, and O. Chains bind to one another by specific hydrogen bonding between pairs of Lewis bases. —adenine with thymine —guanine with cytosine

27. Double helix of DNA Portion of a DNA chain

28. Base-Pairing through H-Bonds

29. Hydrogen Bonding in Biology Hydrogen bonding and base pairing in DNA.

30. H Bonding Activity With your elbow partner, draw the following on the same sheet of paper taking turns: • Water Molecule • Dipole of this water molecule • Another water molecule • Hydrogen Bonding Between these molecules • Structure of Ice • Reflect on your beautiful drawings and give each other high fives.

31. * 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

32. 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)

33. How they form: • Motion of e- creates an instantaneous dipole moment, making it “temporarily polar”. • Instantaneous dipole moment induces a dipole in an adjacent atom • * Persist for about 10-14 or 10-15 second • Ex: two He atoms

34. FORCES INVOLVING INDUCED DIPOLES How can non-polar molecules such as O2 and I2 dissolve in water? The water dipole INDUCES a dipole in the O2 electric cloud. Dipole-induced dipole http://antoine.frostburg.edu/chem/senese/101/liquids/faq/h-bonding-vs-london-forces.shtml Dipole-Dipole and LDFs

35. FORCES INVOLVING INDUCED DIPOLES Solubility increases with mass the gas

36. FORCES INVOLVING INDUCED DIPOLES • Process of inducing a dipole is polarization • Degree to which electron cloud of an atom or molecule can be distorted in its polarizability.

37. d - I-I I-I d The alcohol temporarily creates or INDUCES a dipole in I2. + d - d O - O R H R H d + d + IM FORCES — INDUCED DIPOLES Consider I2 dissolving in ethanol, CH3CH2OH.

38. FORCES INVOLVING INDUCED DIPOLES Formation of a dipole in two nonpolar I2 molecules. Induced dipole-induced dipole http://chemmovies.unl.edu/ChemAnime/LONDOND/LONDOND.html LDFs

39. FORCES INVOLVING INDUCED DIPOLES The induced forces between I2 molecules are very weak, so solid I2sublimes (goes from a solid to gaseous molecules). PLAY MOVIE

40. Intermolecular Forces See Figure 12.12

41. LiquidsSection 12.4 In a liquid • molecules are in constant motion • there are appreciable intermolec. forces • molecules close together • Liquids are almost incompressible • Liquids do not fill the container PLAY MOVIE

42. LIQUID VAPOR Liquids The two key properties we need to describe are EVAPORATION and its opposite—CONDENSATION Evaporation f Add energy break IM bonds make IM bonds Remove energy r condensation

43. Liquids—Evaporation To evaporate, molecules must have sufficient energy to break IM forces. Breaking IM forces requires energy. The process of evaporation is endothermic.

44. higher T lower T Number of molecules 0 Molecular energy Minimum energy req’d to break IM forces and evaporate Liquids—Distribution of Energies Distribution of molecular energies in a liquid. KE is propor-tional to T. See Figure 12.13

45. Vapor Pressure

46. Equilibrium Vapor Pressure http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/vaporv3.swf Vapor Pressure

47. In Class Activity • Your teacher will explain 

48. Liquids HEAT OF VAPORIZATION is the heat req’d (at constant P) to vaporize the liquid. LIQ + heat f VAP Compd. ∆vapH (kJ/mol) IM Force H2O 40.7 (100 oC) H-bonds SO2 26.8 (-47 oC) dipole Xe 12.6 (-107 oC) induced dipole

49. Equilibrium Vapor Pressure & the Clausius-Clapeyron Equation • Clausius-Clapeyron equation — used to find ∆vapH˚. • The logarithm of the vapor pressure P is proportional to ∆vapH and to 1/T. • ln P = –(∆vapH˚/RT) + C

50. Surface Tension SURFACE TENSION also leads to spherical liquid droplets.