Lecture Notes Chem 150 - K. Marr

1. Lecture Notes Chem 150 - K. Marr Chapter 12 Intermolecular Attractions & the Properties of Liquids & Solids Silberberg 3 ed

2. Intermolecular Forces: Liquids, Solids, and Phase Changes 12.1 An Overview of Physical States and Phase Changes 12.2 Quantitative Aspects of Phase Changes 12.3 Types of Intermolecular Forces 12.4 Properties of the Liquid State 12.5 The Uniqueness of Water 12.6 The Solid State: Structure, Properties, and Bonding 12.7 Advanced Materials

3. Table 12.1 A Macroscopic Comparison of Gases, Liquids, and Solids State Shape and Volume Compressibility Ability to Flow Gas Conforms to shape and volume of container high high Liquid Conforms to shape of container; volume limited by surface very low moderate Solid Maintains its own shape and volume almost none almost none

4. Chapter 12: Intermolecular Attractions & the Properties of Liquids & Solids • Why do the gas laws work with almost any gas? • Gases are alike • Mainly empty space  Weak intermolecular attractions Liquid: Conforms to shape of container; volume limited by surface Solid: Maintains its own shape and volume Gas: Conforms to shape and volume of container

5. Why aren’t there liquid laws and solid laws? • Little empty space between molecules: Particles close together in S’s and L’s • Stronger and quite varied intermolecular attractionsin S’s & L’s than in gases...Why? • Attractions decrease as distance between molecules increase:I.M.F.a 1/ d2 • Attractions dependent on chemical composition • Polar vs Nonpolar molecules e.g. Water vs Carbon Dioxide(sublimes. @ -58.5 oC)

6. Intermolecular Attractions: Bonds between Molecules • Much weaker than Chemical Bonding within molecules • Chemical Bonds (ionic and covalent) determine chemical properties • Intermolecular bonds determine physical properties e.g. density, mp, bp, solubility, vapor pressure, etc.

7. Kinds of Intermolecular Attractions • Dipole-Dipole Attractions • Hydrogen Bonds (H-FON Bonds) • London Forces (dispersion forces) • Ion-Dipole (e.g. spheres of hydration) Chapter 13 • Induced Dipole forces Chapter 13 • Ion induced • Dipole induced

8. How do IMF’s affect Heats of Vaporization and Fusion?

10. Relative magnitudes of forces in Molecular compounds Covalent bonds>>>>>Hydrogen bonding>> Dipole-dipole interactions>>>>> London forces

11. What kind of IMF??

13. Dipole-Dipole Attractions • Dipoles are polar molecules • Molecules w/ polar bonds and asymmetric distribution of charge • What determines if a bond is nonpolar, polar or ionic? • What determines if a molecule with polar bonds is polar or nonpolar • Much weaker than covalent bonds • Important in maintaining the shape of many biological molecules: e.g. Proteins

17. Hydrogen Bonds (H-FON Bonds) • Special kind of dipole-dipole interaction • Found in HF and molecules containing O-H or N-H bonds • 5x’s the strength of a typical dipole-dipole bond • ~ 5% the strength of a covalent bond • Important in biological molecules e.g. DNA, proteins

19. Myoglobin

20. Hemoglobin

21. Hydrogen bonding is responsible for the expansion of water when it freezes.

22. Exercise: Which of the following molecules display hydrogen bonding? • Methane, CH4 • methyl ether, CH3OCH3 • Hydrogen peroxide, H2O2 • methyl alcohol, CH3OH

23. London Forces: Attractionsbetween temporary dipoles Electrostatic Attraction

24. Random movement of electronsmay cause temporary charge imbalances London Force

25. London (dispersion) forces between nonpolar molecules

26. Why are London forces the greatest in large molecules?

27. London Forces (dispersion forces) exist in all molecules • Ave. strength <<< Dipole-Dipole interactions • Result from temporary charge imbalances • Due to the random movement of electrons • Nucleus of one atom attracts electrons from a neighboring atom. • At the same time, the electrons in one particle repel the electrons in the neighbor and create a short lived charge imbalance.

28. Relationship between atomic size and the strength of London forces • Greatest in large atoms • Electron clouds more easily distorted • Halogens and Noble Gases BP increase w/ molar mass • Ion Induced Dipoles • dipoles can be induced by ions • attractions exist between ions and dipoles

29. London ForcesEffect of molecular surface area Cyclopentane, BP = 49.3 oC

30. IMF’s inNonpolar Organic Molecules • What kind of attractive forces are present? • What role do molecular size and surface area play? • Linear molecules have more surface area than if they are folded into a sphere. • Linear molecules have higher melting and boiling points because of the increased attractions.

31. Predicting the Relative Boiling Points of Substances • The substance with the strongest intermolecular attractions will have the higher BP. Why? • More energy needed to separate molecules  higher boiling temperature e.g. Halogens and noble gases

32. Cooling Curve: H2O (g)  H2O (s) What is happening in to KE and PE at each part of the curve? • DHfus = + 6.01 kJ/mol DHvap = + 40.7 kJ/mol

34. Molar Heat of Vaporization, DHvap Heat absorbed when one mole liquid is changed to one mole of vapor at constant T and P • Depends on strength of IMF’s • Endothermic (results in PE elevation) • For water: DHvap = + 40.7 kJ/mol @ 100oC

35. Molar Heat of fusion, DHvap Heat absorbed when 1 mole solid is changed to 1 mole of liquid at constant T and P. • Depends on strength of IMF’s • Endothermic (results in PE elevation) • For water: DHfus = + 6.01 kJ/mol

36. Quantitative Aspects of Phase Changes Within a phase A change in heat is associated with a change in average KE and, therefore, a change in temperature. q = (mass)(Specific Heat)(Dt) During a phase change A change in heat occurs at constant temperature, which is associated with a change in PE, as the average distance between molecules changes--Bond IMF formation is exothermic, IMF breaking is endothermic q = (moles of substance)(enthalpy of phase change)

37. Calculation of the Heat of Fusion of Ice • Use the data below to calculate the heat of fusion of water. • A piece of ice at zero Celsius melts in 100.0 g water until the water’s temperature also becomes zero. • Initial water temp. = 44.0 oC • Mass of ice that melted = 56.0 g. • Specific heat of water, CH2O = 4.184 J/g oC • Calculate the % error and explain the source of the error. DHfus H2O = + 6.01 kJ/mol

38. Application Questions • The molar heat of vaporization of water at 25 oC is 43.99 kJ/mol. How many kilojoules of heat would be required to vaporize 125 mL (125 g) of water at 25 oC? • Answer: 305kJ • How much heat would be needed to convert 125 mL water at 25.oC to steam at 100.0 oC? The heat of vaporization of water at 100.0 oC is 40.657 kJ/mol and the specific heat of water is 4.184 J/goC. • Answer: 321 kJ

39. General Properties of Liquids and Solids • Macroscopic properties depend on Microscopic properties • Microscopic properties of L’s and S’s • Molecules tightly packed • Strong intermolecular attractions

41. Macroscopic properties of S’s and L’s • Compressibility • Little to none. Why? • Diffusion (ability to flow) (T6) • Slow in liquids • Like moving in a crowded room • Nonexistent in Solids • Particles not free to move

42. Macroproperties Liquids Why are raindrops spherical? • Increases stability by maximizing the number of IMF’s and decreasing surface tension • Surface Tension = • E needed to increase the surface area of a liquid by a given amount (J/m2) • Depends on nature of intermolecular forces

43. The Molecular Basis of Surface Tension • Surface molecules experience fewer intermolecular forces than interior molecules Liquids minimize surface area by forming spherical surfaces  Lowers PE, thus increases stability ( e.g. Raindrops, overfilled glass) • Surface molecules are at higher P.E. than interior molecules • Recall: Bond formation results in P.E. lowering

48. Macroproperties Liquids • Wetting of a Surface by a Liquid • Spreading of a Liquid across a surface • Caused by attraction of liq. molecules to surface molecules • Why are Liquids with a Low Surface Tension Good Wetters?........ • Low Surface Tension means weak IM Forces • Which wets solids surfaces better, hydrocarbons (gasoline/oil ) or water?.......

49. Evaporation and Sublimation Where molecules leave surface and enter vapor space around them • Evaporation: L  Vapor • Sublimation: S  Vapor

50. Evaporation: L Vapor (T9) • Factors that affect the rate of evaporation • Surface area, Temp., Strength of IMF’s • Why does evaporation occur at temp.’s below the BP? • Why does an increase in temp. increase the rate of evaporation? • Why does sweating cool you? • Why does a fan cool you?