ก ก ก Intermolecular Forces: Liquids, Solids, and Phase Changes

Transcription

1 ก ก ก Intermolecular Forces: Liquids, Solids, and Phase Changes ก ก ก ก Mc-Graw Hill 1 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 2

2 12.1 An Overview of Physical States and Phase Changes ATTRACTIVE FORCES electrostatic in nature Intramolecular forces bonding forces These forces exist within each molecule. They influence the chemical properties of the substance. Intermolecular forces nonbonding forces These forces exist between molecules. They influence the physical properties of the substance. 3 Phase Changes sublimination exothermic melting vaporizing solid liquid gas freezing condensing endothermic 4

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 5 Heats of vaporization and fusion for several common substances. 6

4 Phase changes and their enthalpy changes Quantitative Aspects of Phase Changes A cooling curve for the conversion of gaseous water to ice. 8

5 Quantitative Aspects of Phase Changes Within a phase, a change in heat is accompanied by a change in temperature which is associated with a change in average E k as the most probable speed of the molecules changes. q = (amount)(molar heat capacity)( T) During a phase change, a change in heat occurs at a constant temperature, which is associated with a change in E p, as the average distance between molecules changes. q = (amount)(enthalpy of phase change) 9 Sample Problem 12.1 Finding the Heat of a Phase Change Depicted by Molecular Scenes PROBLEM: These molecular scenes represent a phase change of water. Select data from the previous text discussion to find the heat (in kj) lost or gained when 24.3 g of H 2 O undergoes this change. PLAN: The scenes show a disorderly, condensed phase (liquid) changing to separate molecules (gas) and represent the vaporization of water. Three endothermic stages: (1) heating liquid 85.0 to 100. o C, (2) liquid to gas at 100. o C, and (3) heating gas 100. to 117 o C. SOLUTION: mol H 2 O = 24.3 g H 2 O x mol H 2 O g H 2 O = 1.35 mol H 2 O q = n x C water(l) x T = (1.35 mol)(75.4 J/mol oc)( o C) = 1527 J = 1.53 kj q = n( H o vap ) = (1.35 mol)(40.7 kj/mol) = 54.9 kj q = n x C water(g) x T = (1.35 mol)(33.1 J/mol oc)( o C) = J = kj q total = kj = 57.2 kj 10

6 Liquid-gas equilibrium. 11 The effect of temperature on the distribution of molecular speed in a liquid. 12

7 Vapor pressure as a function of temperature and intermolecular forces. A linear plot of the relationship between vapor pressure and temperature. 13 The Clausius-Clapeyron Equation ln - Hvap P 1 = + C R T P ln P - H = R 1 T 2 vap T 1 14

8 Sample Problem 12.2 Using the Clausius-Clapeyron Equation PROBLEM: The vapor pressure of ethanol is 115 torr at 34.9 o C. If H vap of ethanol is 40.5 kj/mol, calculate the temperature (in o C) when the vapor pressure is 760 torr. PLAN: We are given 4 of the 5 variables in the Clausius-Clapeyron equation. Substitute and solve for T 2. SOLUTION: ln P2 - Hvap 1 1 = P R T T o C = K ln 760 torr 115 torr = x103 J/mol J/mol K 1 1 T K T 2 = 350. K = 77 C 15 Iodine subliming. test tube with ice iodine solid iodine vapor iodine solid 16

9 Phase diagrams for CO 2 and H 2 O. CO 2 H 2 O Types of Intermolecular Forces Covalent and van der Waals radii. van der Waal s distance bond length covalent radius van der Waal s radius 18

10 Periodic trends in covalent and van der Waals radii (in pm)

11 21 Polar molecules and dipole-dipole forces. solid liquid 22

12 Dipole moment and boiling point. 23 THE HYDROGEN BOND a dipole-dipole intermolecular force A hydrogen bond may occur when an H atom in a molecule, bound to small highly electronegative atom with lone pairs of electrons, is attracted to the lone pairs in another molecule. The elements which are so electronegative are N, O, and F..... F.. hydrogen bond donor.. H Ọ. hydrogen bond acceptor.. O.. H.. N hydrogen bond acceptor N.. H.. F.... hydrogen bond donor hydrogen bond acceptor hydrogen bond donor 24

13 Hydrogen bonding and boiling point. 25 Sample Problem 12.3 PROBLEM: Drawing Hydrogen Bonds Between Molecules of a Substance Which of the following substances exhibits H bonding? For those that do, draw two molecules of the substance with the H bond(s) between them. O (a) C 2 H 6 (b) CH 3 OH (c) CH 3 C NH 2 PLAN: Find molecules in which H is bonded to N, O, or F. Draw H bonds in the format B: H A. SOLUTION: (a) C 2 H 6 has no H bonding sites. H (b) H H C O H H H O H C H H (c) O CH 3 C N H H H H N CH 3 C O CH 3 C O N H H H N O CH 3 O 26

14 Polarizability and Charged-Induced Dipole Forces distortion of an electron cloud Polarizability increases down a group size increases and the larger electron clouds are further from the nucleus Polarizability decreases left to right across a period increasing Z eff shrinks atomic size and holds the electrons more tightly Cations are less polarizable than their parent atom because they are smaller. Anions are more polarizable than their parent atom because they are larger. 27 Dispersion forces among nonpolar particles. separated Ar molecules instantaneous dipoles 28

15 Molar mass and boiling point. 29 Molecular shape and boiling point. more points for dispersion forces to act fewer points for dispersion forces to act 30

16 Summary diagram for analyzing the intermolecular forces in a sample. ions present INTERACTING PARTICLES (atoms, molecules, ions) ions not present ions only IONIC BONDING (Section 9.2) ion + polar molecule ION-DIPOLE FORCES polar molecules only DIPOLE-DIPOLE FORCES H bonded to N, O, or F HYDROGEN BONDING nonpolar molecules only DISPERSION FORCES only polar + nonpolar molecules DIPOLE- INDUCED DIPOLE FORCES DISPERSION FORCES ALSO PRESENT 31 Sample Problem 12.4 Predicting the Types of Intermolecular Force PROBLEM: For each pair of substances, identify the dominant intermolecular force(s) in each substance, and select the substance with the higher boiling point. (a) MgCl 2 or PCl 3 (b) CH 3 NH 2 or CH 3 F (c) CH 3 OH or CH 3 CH 2 OH CH 3 (d) Hexane (CH 3 CH 2 CH 2 CH 2 CH 2 CH 3 ) or 2,2-dimethylbutane CH 3CCH 2 CH 3 PLAN: Use the formula, structure, Table 12.2 and Figure CH 3 Bonding forces are stronger than nonbonding (intermolecular) forces. Hydrogen bonding is a strong type of dipole-dipole force. Dispersion forces are decisive when the difference is molar mass or molecular shape. 32

17 Sample Problem 12.4 Predicting the Types of Intermolecular Force SOLUTION: (a) Mg 2+ and Cl are held together by ionic bonds while PCl 3 is covalently bonded and the molecules are held together by dipole-dipole interactions. Ionic bonds are stronger than dipole interactions and so MgCl 2 has the higher boiling point. (b) CH 3 NH 2 and CH 3 F are both covalent compounds and have bonds which are polar. The dipole in CH 3 NH 2 can H bond while that in CH 3 F cannot. Therefore CH 3 NH 2 has the stronger interactions and the higher boiling point. (c) Both CH 3 OH and CH 3 CH 2 OH can H bond but CH 3 CH 2 OH has more CH for more dispersion force interaction. Therefore CH 3 CH 2 OH has the higher boiling point. (d) Hexane and 2,2-dimethylbutane are both nonpolar with only dispersion forces to hold the molecules together. Hexane has the larger surface area, thereby the greater dispersion forces and the higher boiling point Properties of the Liquid State The molecular basis of surface tension. hydrogen bonding occurs across the surface and below the surface the net vector for attractive forces is downward hydrogen bonding occurs in three dimensions 34

18 Table 12.3 Surface Tension and Forces Between Particles Substance Formula Surface Tension (J/m 2 ) at 20 0 C Major Force(s) diethyl ether CH 3 CH 2 OCH 2 CH 3 1.7x10-2 dipole-dipole; dispersion ethanol CH 3 CH 2 OH 2.3x10-2 H bonding butanol CH 3 CH 2 CH 2 CH 2 OH 2.5x10-2 H bonding; dispersion water H 2 O 7.3x10-2 H bonding mercury Hg 48x10-2 metallic bonding 35 Shape of water or mercury meniscus in glass. 36

19 Table 12.4 Viscosity of Water at Several Temperatures viscosity resistance to flow Temperature ( o C) Viscosity (N s/m 2 )* x x x x10 3 *The units of viscosity are Newton-seconds per square meter The Uniqueness of Water The H-bonding ability of the water molecule. hydrogen bond donor hydrogen bond acceptor 38

20 The Unique Nature of Water great solvent properties due to polarity and hydrogen bonding ability exceptional high specific heat capacity high surface tension and capillarity density differences of liquid and solid states 39 The hexagonal structure of ice. 40

21 The expansion and contraction of water breaks rocks to sand and soil. 41 The macroscopic properties of water and their atomic and molecular roots. 42

22 12.6 The Solid State: Structure, Properties, and Bonding The striking beauty of crystalline solids. 43 The crystal lattice and the unit cell. lattice point unit cell unit cell portion of 3-D lattice 44

23 The three cubic unit cells. Simple Cubic 1/8 atom at 8 corners Atoms/unit cell = 1/8 x 8 = 1 Coordination number = 6 45 The three cubic unit cells. Body-centered Cubic 1/8 atom at 8 corners 1 atom at center Coordination number = 8 Atoms/unit cell = (1/8 x 8) + 1 = 2 46

24 The three cubic unit cells. Face-centered Cubic 1/8 atom at 8 corners 1/2 atom at 6 faces Coordination number = 12 Atoms/unit cell = (1/8 x 8) + (1/2 x 6) = 4 47 Packing identical spheres. simple cubic (52% packing efficiency) body-centered cubic (68% packing efficiency) 48

25 layer a layer a hexagonal closest packing layer b cubic closest packing layer c closest packing of first and second layers abab (74%) abcabc (74%) hexagonal unit cell expanded side views face-centered unit cell 49 Edge length and atomic (ionic) radius in the three cubic unit cells. 50

26 Sample Problem 12.5 Determining Atomic Radius from Crystal Structure PROBLEM: The crystal structure of copper adopts cubic closest packing and the edge length of the unit cell is pm What is the atomic radius of copper? PLAN: Copper has a face-centered cubic unit cell with edge length A = pm see Figure 12.29C. The diagonal of the cell s face is 4r and the Pythagorean theorem can be used to solve for r. SOLUTION: 2 2 C = A + B C= C = 4r 2 2 2A = 2(361.5 pm) = pm r = C/4 = pm/4 = pm 51 Cubic closest packing for frozen argon. Cubic closest packing of frozen methane. 52

27 Type Table 12.5 Particle(s) Characteristics of the Major Types of Crystalline Solids Interparticle Forces Physical Properties Examples (mp, o C) Atomic Molecular Atoms Molecules Dispersion Dispersion, dipole-dipole, H bonds Soft, very low mp, poor thermal & electrical conductors Fairly soft, low to moderate mp, poor thermal & electrical conductors Group 8A(18) [Ne(-249) to Rn(-71)] Nonpolar: O 2 [-219], C 4 H 10 [-138], Cl 2 [-101], C 6 H 14 [-95], P 4 [44.1] Polar: SO 2 [-73],CHCl 3 [-64], HNO 3 [-42], H 2 O [0.0], CH 3 COOH[17] Ionic Positive & negative ions Ion-ion attraction Hard & brittle, high mp, good thermal & electrical conductors when molten NaCl [801] CaF 2 [1423] MgO [2852] Metallic Atoms Metallic bond Soft to hard, low to very high mp, excellent thermal and electrical conductors, malleable and ductile Na [97.8] Zn [420] Fe [1535] Network Covalent Atoms Covalent bond Very hard, very high mp, usually poor thermal and electrical conductors SiO 2 (quartz) [1610] C (diamond) [~4000] 53 The sodium chloride structure. expanded view space-filling 54

28 The zinc blende structure. zinc sulfide 55 The fluorite (CaF 2 ) structure. 56

29 Crystal structures of metals. cubic closest packing hexagonal closest packing 57 58

30 Crystalline and amorphous silicon dioxide. Cristobalite (Silica) Glass 59 Tools of the Laboratory Bragg s equation: 2dsinθ = nλ Diffraction of x-rays by crystal planes. 60

31 Tools of the Laboratory Monochromator Formation of an x-ray diffraction pattern of the protein hemoglobin. 61 Tools of the Laboratory Scanning tunneling micrographs. Gold surface Cesium atoms on gallium arsenide surface 62