Chapter Eight. Molecules and Materials. Chapter Eight Slide 1 of 99

Transcription

1 Chapter Eight Molecules and Materials Chapter Eight Slide 1 of 99

2 Chapter Preview Intramolecular forces determine such molecular properties as molecular geometries and dipole moments. Intermolecular forces determine the macroscopic physical properties of liquids and solids. Three states of matter: solids, liquids, and gases. In gases and liquids, motion is mainly translational. In solids, motion is mainly vibrational. Chapter Eight Slide 2 of 99

3 Chapter Eight Slide 3 of 99

4 States of Matter Compared Chapter Eight Slide 4 of 99

5 Dipole-Dipole Forces Dipole-dipole forces arise when permanent dipoles align themselves with the positive end of one dipole directed toward the negative ends of neighboring dipoles. When molecules come close to one another, repulsions occur between like-charged regions of dipoles. A permanent dipole in one molecule can induce a dipole in a neighboring molecule, giving rise to a dipole-induced dipole force. Chapter Eight Slide 5 of 99

6 Dipole-Dipole Interactions d µ 1 Dipole moment r µ = δ d µ 2 E = - 2(µ 1 µ 2 )/4πεr 3

7 Dispersion Forces A dispersion force is the force of attraction between an instantaneous dipole and an induced dipole. Also called a London force after Fritz London who offered a theoretical explanation of these forces in The polarizability of an atom or molecule is a measure of the ease with which electron charge density is distorted by an external electrical field. The greater the polarizability of molecules, the stronger the intermolecular forces between them. Chapter Eight Slide 7 of 99

8 Dispersion Forces Illustrated r µ E = - 2(µ 2 α)/r 6 Mean instantaneous dipole polarizability

9 Predicting Physical Properties of Molecular Substances Dispersion forces become stronger with increasing molar mass and elongation of molecules. In comparing nonpolar substances, molar mass and molecular shape are the essential factors. Dipole-dipole and dipole-induced dipole forces are found in polar substances. The more polar the substance, the greater the intermolecular force is expected to be. Because they occur in all molecular substances, dispersion forces must always be considered. Often they predominate. Chapter Eight Slide 9 of 99

10 Molecular Shape and Polarizability Chapter Eight Slide 10 of 99

11 Chapter Eight Slide 11 of 99

12 Effect of Molecular Weight & Dipole Moment Compound M.W. Dipole (D) moment b.p. ( 0 C) CH 3 Cl CH 2 Cl CHClF CF CCl Chapter Eight Slide 12 of 99

13 Hydrogen Bonds A hydrogen bond is an intermolecular force in which a hydrogen atom covalently bonded to a non-metal atom in one molecule is simultaneously attracted to a non-metal atom of a neighboring molecule. The strongest hydrogen bonds are formed if the non-metal atoms are small and highly electronegative. Usually occurs with nitrogen, oxygen, and fluorine atoms. X H Y X, Y = F, O, N, Cl, S (highly electronegative elements) Chapter Eight Slide 13 of 99

14 Hydrogen Bonds in Water

15 Hydrogen Bonding in Ice Chapter Eight Slide 15 of 99

16 Solid water is less dense than liquid water due to hydrogen bonding. Chapter Eight Slide 16 of 99

17 Chapter Eight Slide 17 of 99

18 Hydrogen bonding is also the reason for the unusually high boiling point of H 2 O, HF and NH 3. Chapter Eight Slide 18 of 99

19 Intramolecular Hydrogen Bonds Hydrogen Bonding In Salicylic Acid Chapter Eight Slide 19 of 99

20 The structures of proteins, substances essential to life, are determined partly by hydrogen bonding. Hydrogen Bonding in Polypeptide Chains α-helix Secondary Structure Chapter Eight Slide 20 of 99

21 Intermolecular Hydrogen Bonds Hydrogen Bonding between Acetic Acid molecules Chapter Eight Slide 21 of 99

22 Hydrogen Bonding between the base pairs of two strands of a DNA molecules: A-T and C-G Chapter Eight Slide 22 of 99

23 Vaporization and Condensation Liquid Vaporization Condensation Vapor Vaporization is the conversion of a liquid to a gas. The enthalpy of vaporization ( H vapn ) is the quantity of heat that must be absorbed to vaporize a given amount of liquid at a constant temperature. Condensation is the change of a gas to a liquid. H condn = - H vapn Chapter Eight Slide 23 of 99

24 Chapter Eight Slide 24 of 99

25 Crystal Structure of Diamond Chapter Eight Slide 25 of 99

26 Crystal Structure of Graphite van der Waals force Covalent bond Chapter Eight Slide 26 of 99

27 Structure of a Buckyball Covalent bond Chapter Eight Slide 27 of 99

28 Structure of Carbon Nanotube SWCNT- single-wall carbon nanotube MWCNT- multiplewall carbon nanotube Chapter Eight Slide 28 of 99

29 Some Enthalpies of Vaporization Chapter Eight Slide 29 of 99

30 Vapor Pressure The vapor pressure of a liquid is the partial pressure exerted by the vapor when it is in dynamic equilibrium with a liquid at a constant temperature. The vapor pressures of liquids increases with temperature. A vapor pressure curve is a graph of vapor pressure as a function of temperature. Chapter Eight Slide 30 of 99

31 Liquid-Vapor Equilibrium Chapter Eight Slide 31 of 99

32 Vapor Pressure Curves a) Carbon disulfide: CS 2 b) Methanol: CH 3 OH c) Ethanol: CH 3 CH 2 OH d) Water: H 2 O e) Aniline: C 6 H 5 NH 2 The temperature of the line at P= 760 mmhg with a vapor pressure curve is the normal boiling point. Chapter Eight Slide 32 of 99

33 Chapter Eight Slide 33 of 99

34 Vapor Pressure Of Water Chapter Eight Slide 34 of 99

35 Vapor Pressure as a Function of Temperature Clausius-Clapeyron equation: ln(p 2 /P 1 ) = H vap /R(1/T 1-1/T 2 ) where, H vap = enthalpy of vaporization Chapter Eight Slide 35 of 99

36 Generalized Phase Diagram Fusion curve Sublimation curve Vapor Pressure curve Triple point Critical point Supercritical fluid Chapter Eight Slide 36 of 99

37 Phase Diagram For H 2 O normal melting point normal boiling point Chapter Eight Slide 37 of 99

38 Boiling Point The boiling point of a liquid is the temperature at which its vapor pressure becomes equal to the external pressure. The normal boiling point is the boiling point at 1 atm. Chapter Eight Slide 38 of 99

39 Critical Point The critical temperature, Tc, is the highest temperature at which a liquid and vapor can coexist in equilibrium as physically distinct states of matter. The critical pressure, Pc, is the vapor pressure at the critical temperature. The condition corresponding to a temperature of Tc and a pressure of Pc is called the critical point. Chapter Eight Slide 39 of 99

40 Critical Temperature and Pressure of Various Substances Chapter Eight Slide 40 of 99

41 The Critical Point Chapter Eight Slide 41 of 99

42 Phase Changes Involving Solids The conversion of a solid to a liquid is called melting, or fusion, and the temperature at which a solid melts is its melting point. The enthalpy of fusion, H fusion, is the quantity of heat required to melt a given amount of solid. Sublimation is the process of a molecules passing directly from the solid to the vapor state. Enthalpy of sublimation, H subln, is the sum of the enthalpies of fusion and vaporization. The triple point is the point at which the vapor pressure curve and the sublimation curve meet. Chapter Eight Slide 42 of 99

43 Phase Diagram For CO 2 Sublimation of dry ice Chapter Eight Slide 43 of 99

44 Phase Diagram For HgI C Chapter Eight Slide 44 of 99

45 Phase Diagram For Liquid Crystal Chapter Eight Slide 45 of 99

46 Liquid Liquid crystal Chapter Eight Slide 46 of 99

47 Chapter Eight Slide 47 of 99

48 Chapter Eight Slide 48 of 99

49 Chapter Eight Slide 49 of 99

50 Some Enthalpies of Fusion Chapter Eight Slide 50 of 99

51 Crystal Structure of Solid Elements Chapter Eight Slide 51 of 99

52 Unit Cells In Cubic Crystal Structures simple cubic (primitive cubic) bcc fcc Chapter Eight Slide 52 of 99

53 Primitive cubic Body-centered cubic Face-centered cubic Chapter Eight Slide 53 of 99

54 Occupancies per Unit Cells Primitive cubic: a = 2r 1 atom/unit cell occupancy = [4/3(πr 3 )]/a 3 = [4/3(πr 3 )]/(2r) 3 = 0.52 = 52% Body-centered cubic: a = 4r/(3) 1/2 2 atom/unit cell occupancy = 2 x [4/3(πr 3 )]/a 3 = 2 x [4/3(πr 3 )]/[4r/(3) 1/2 ] 3 = 0.68 = 68% Face-centered cubic: a = (8) 1/2 r 4 atom/unit cell occupancy = 4 x [4/3(πr 3 )]/a 3 = 4 x [4/3(πr 3 )]/[(8) 1/2 r] 3 Closest packed = 0.74 = 74% Chapter Eight Slide 54 of 99

55 Using desity to identify structure The atomic radius of copper is 128 pm, mass number is 63.54, and the density of copper is 8.93g/cm 3. Is copper metal close packed? Density = M/V, V= M/D The volume of a Cu atom occupied in the lattice = 63.54g/mol 8.93g/cm x atom/mol = 1.18 x cm 3 /atom = 1.18 x 10 7 pm 3 /atom Occupancy = [4/3(πr 3 )]/[1.18 x 10 7 ] = [4/3 π( )]/[1.18 x 10 7 ] = = 74.4% Close-packed structure Chapter Eight Slide 55 of 99

56 Closest packed a ab aba unoccupied holes abc Chapter Eight Slide 56 of 99

57 abab abcabc = Face-centered cubic Chapter Eight Slide 57 of 99

58 Close-packing of Spheres in Three Dimensions Chapter Eight Slide 58 of 99

59 Close Packed Structures Hexagonal close-packed (hcp) arrangements occur when the third layer covers the tetrahedral holes. These produce two-layer repeating units. ABABAB.. Cubic close-packed (ccp) arrangements occur when the third layer covers the octahedral holes. These produce three-layer repeating units. ABCABC. Chapter Eight Slide 59 of 99

60 Holes in Close Packed Structures Tetrahedral holes are located above a sphere in the bottom layer. Octahedral holes are located above a void in the bottom layer. Chapter Eight Slide 60 of 99

61 Tetrahedral holes Octahedral holes Cubic holes Close packed structure Chapter Eight Slide 61 of 99

62 r h /r = r h /r = r h /r = Chapter Eight Slide 62 of 99

63 Chapter Eight Slide 63 of 99

64 Metallic bond Sea of Electrons surrounding a giant lattice of positive ions Chapter Eight Slide 64 of 99

65 Band Theory This is a quantum-mechanical treatment of bonding in metals. The spacing between energy levels is so minute in metals that the levels essentially merge into a band. When the band is occupied by valence electrons, it is called a valence band. A partially filled or low lying empty band of energy levels, which is required for electrical conductivity, is a conduction band. Band theory provides a good explanation of metallic luster and metallic colors. Chapter Eight Slide 65 of 99

66 Benzene E Chapter Eight Slide 66 of 99

67 Antibonding bonding Chapter Eight Slide 67 of 99

68 The 2s Band in Lithium Metal Anti-bonding Conduction band e- e- Bonding Valence band Chapter Eight Slide 68 of 99

69 Band Overlap in Magnesium Conduction band Valence band Chapter Eight Slide 69 of 99

70 Alloy An alloy is a combination, either in solution or compound, of two or more elements, at least one of which is a metal, and where the resulting material has metallic properties. Alloys are usually designed to have properties that are more desirable than those of their components. For instance, steel is stronger than iron, one of its main elements. Chapter Eight Slide 70 of 99

71 Shape memory alloy (SMA) A shape memory alloy (SMA) (also known as memory metal or smart wire) is a metal that remembers its geometry. After it is deformed, it regains its original geometry by itself during heating (one-way effect), at higher ambient temperatures, or simply during unloading (pseudo-elasticity). These extraordinary properties are due to a temperaturedependent martensitic phase transformation from a lowsymmetry to a highly symmetric crystallographic structure. Three main types of SMA are the Cu-Zn-Al, Cu-Al-Ni, and Ni-Ti alloys. Chapter Eight Slide 71 of 99

72 starting from martensite (a), adding a reversible deformation for the one-way effect or severe deformation with an irreversible amount for the two-way (b), heating the sample (c) and cooling it again (d). Chapter Eight Slide 72 of 99

73 Shape memory alloy (SMA) ξ (xi) represents the martensite fraction. Ms : temperature at which the structure starts to change from austenite to martensite upon cooling Mf : temperature at which the transition is finished. Chapter Eight Slide 73 of 99

74 Figure 1. Different phases of an SMA. Chapter Eight Slide 74 of 99

75 Figure 2. Temperature-induced phase transformation of an SMA without mechanical loading. Chapter Eight Slide 75 of 99

76 Band Structure of Insulators and Semiconductors Chapter Eight Slide 76 of 99

77 Intrinsic and Extrinsic Semiconductors n-type p-type Chapter Eight Slide 77 of 99

78 p-type n-type Forward-bias Reverse-bias p-n junction Chapter Eight Slide 78 of 99

79 Type of Polymers - based on synthesis Addition polymers Condensation polymers Chapter Eight Slide 79 of 99

80 Condensation Polymerization Polyamide Chapter Eight Slide 80 of 99

81 Polyester Dacron Chapter Eight Slide 81 of 99

82 Kevlar Chapter Eight Slide 82 of 99

83 Addition Polymerization Initiation Propagation Chapter Eight Slide 83 of 99

84 Termination Chapter Eight Slide 84 of 99

85 Molecular Models of a Segment of a Polyethylene Molecule Chapter Eight Slide 85 of 99

86 2-methyl-1,3-butadiene Chapter Eight Slide 86 of 99

87 Conducting Polymers Chapter Eight Slide 87 of 99

88 Physical Properties Of Polymers A thermoplastic polymer is one that can be softened by heating and then formed into desired shapes by applying pressure. Thermosetting polymers become permanently hard at elevated temperatures and pressures. High-density polyethylene (HDPE) consists primarily of linear molecules and has a higher density, greater rigidity, greater strength, and a higher melting point. Low-density polyethylene (LDPE) has branched chains and is a waxy, semi-rigid, translucent material with a low melting point. Chapter Eight Slide 88 of 99

89 Chapter Eight Slide 89 of 99

90 Organization of Polymer Molecules LDPE HDPE Chapter Eight Slide 90 of 99

91 Chapter Eight Slide 91 of 99

92 Chapter Eight Slide 92 of 99

93 Tacticity isotactic syndiotactic atactic Chapter Eight Slide 93 of 99

94 Elastomers Elastomers are flexible, elastic materials. Natural rubber is soft and tacky when hot. It can be made harder in a reaction with sulfur, called vulcanization. Several kinds of synthetic rubber were developed during and after World War II. Neoprene (polychloroprene) is one example of this. Copolymerization is a process in which a mixture of two different monomers forma a product in which the chain contains both monomers as building blocks. Chapter Eight Slide 94 of 99

95 Rubber Natural rubber was first introduced to Europe in the mid. 18th century - and is an example of an elastomer -an elastic polymer. A problem was that matural rubber is a very weak, soft thermoplastic when heated - but very brittle when cold. A process, vulcanization, was invented by Goodyear, where rubber heated with sulphur produces a harder, less tacky elastic material. n Chapter Eight Slide 95 of 99

96 Extent of Cross-linking in Rubber Products Chapter Eight Slide 96 of 99

97 Copolymers Copolymers are polymers made by polymerizing a mixture of two or more monomers. alternating block random graft Chapter Eight Slide 97 of 99

98 Copolymers An example is styrene-butadiene rubber (SBR) - which is a copolymer of butadiene and styrene. Most is vulcanized and used in tire production - though some is used for bubble-gum (unvulcanised form). Chapter Eight Slide 98 of 99

99 ABS - Poly(Acrylonitrile, Butadiene, Styrene) ABS is a copolymer of Acrylonitrile, Butadiene, and Styrene. ABS is often used as the cost and performance dividing line between standard plastics (PVC, polyethylene, polystyrene, etc.) and engineering plastics (acrylic, nylon, acetal, etc.). ABS polymers can be given a range of properties, depending on the ratio of the monomeric constituents and the molecular level connectivity. Typically, a styreneacrylonitrile glassy phase is toughened by an amorphous butadiene/butadiene-acrylonitrile rubber phase. Chapter Eight Slide 99 of 99