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Intermolecular Forces, Liquid, and Solid

Intermolecular Forces, Liquid, and Solid. Kinetic-Molecular View of Liquid and Solid Intermolecular Forces Liquid Properties Crystal Structure Phase Changes and Phase Diagram. Gas. Liquid. Solid. T P. T. Gas Assume volume and shape of the container

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Intermolecular Forces, Liquid, and Solid

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  1. Intermolecular Forces, Liquid, and Solid • Kinetic-Molecular View of Liquid and Solid • Intermolecular Forces • Liquid Properties • Crystal Structure • Phase Changes and Phase Diagram

  2. Gas Liquid Solid T P T • Gas • Assume volume and shape of the container • Low density, very compressible, free motion • Liquid • Has definite volume, assume shape of the container • High density, slightly compressible, flow readily • Solid • Definite volume, definite shape • High density, incompressible I. Kinetic–Molecular View

  3. intramolecular forces > intermolecular forces II. Intermolecular Forces • Intermolecular force: attractive force between molecules • Intramolecular force: hold atom together in a molecule Ex: 41 kJ to vaporize 1 mole of water (inter) 930 kJ to break all O-H bonds in 1 mole of water (intra) • Measure of intermolecular forces • Boiling point, melting point, DHvap DHsub, DHfus

  4. Types of Intermolecular Forces • Ion-Dipole Forces • Attractive forces between an ion and a polar molecule • Dipole-Dipole Forces • Attractive forces between polar molecules • Dispersion Forces • Attractive forces that arise as a result of temporary dipoles induced in atoms or molecules • Hydrogen Bond • Special dipole-dipole forces involving H and F, O, N • van der Waals Forces • includes dipole-dipole, dipole-induced dipole and dispersion forces

  5. Orientation of Polar Molecules in a Solid II. Intermolecular Forces A. Dipole-Dipole Forces Attractive forces between polar molecules

  6. B. Ion-Dipole Forces Ion-Dipole Interaction II. Intermolecular Forces Attractive forces between an ion and a polar molecule

  7. C. Dispersion Forces II. Intermolecular Forces Attractive forces that arise as a result of temporary dipoles induced in atoms or molecules (exist in all molecules) Resulted from electron cloud distortion ion-induced dipole interaction dipole-induced dipole interaction

  8. C. Dispersion Forces < < II. Intermolecular Forces • (3) Related to the polarizability of the molecule • Polarizability is the ease with which the electron distribution in the atom or molecule can be distorted. • Polarizability generally increases with: • greater number of electrons • more diffuse electron cloud • larger molar mass Ex: Arrange boiling point CH3CH3 CH3CH2CH3 He Ne Kr <

  9. C. Dispersion Forces II. Intermolecular Forces (4) Depend on the molecular shape Ex: C5H12 two isomers H H H CH3 H3C - C - C - C - C H3 H3C - C - CH3 H H H CH3 >

  10. D. Hydrogen Bond or … … H H B A A A II. Intermolecular Forces The hydrogen bond is a special dipole-dipole interaction between they hydrogen atom in a polar N-H, O-H, or F-H bond and an electronegative O, N, or F atom. A & B are N, O, or F

  11. D. Hydrogen Bond Decreasing molar mass Decreasing boiling point II. Intermolecular Forces

  12. O O S Only (a) CH3OH CH3 O H …. O CH3 H Lets try these: (1) What type(s) of intermolecular forces exist between each of the following molecules? HBr is a polar molecule: dipole-dipole forces. There are also dispersion forces between HBr. HBr CH4 CH4 is nonpolar: dispersion forces. SO2 SO2 is a polar molecule: dipole-dipole forces and dispersion forces between SO2 molecules. (2) Which molecule can form H bond with each other? (a) CH3OH (b) CH3OCH3 (c) F2

  13. One more example: Predict the trend in boiling point for the following species: BaCl2, H2, CO, HF, Ne BaCl2 > HF > CO > Ne > H2 BaCl2 : ionic bonding HF: HB + dispersion CO: dipole-dipole + dispersion Ne: dispersion H2 : dispersion Stronger intermolecular forces, higher BP, MP

  14. III. Properties of Liquids A. Surface Tension The amount of energy required to stretch or increase the surface of a liquid by a unit area. Unit: J/m2 Stronger intermolecular forces, higher surface tension

  15. III. Properties of Liquids A. Surface Tension (2) Capillary action Cohesion is the attraction between like molecules Adhesion is an attraction between unlike molecules Capilliary rise: adh > coh coh > adh

  16. III. Properties of Liquids B. Viscosity - fluid’s resistance to flow Viscosity increases as interaction increases Viscosity decreases as T increases Viscosity depends on molecular shape larger for long flexible molecule

  17. III. Properties of Liquids C. Water: a unique substance due to HB Maximum Density 40C Density of Water Each H2O has 4 HB, hexagonal arrangement

  18. lattice point Unit Cell Unit cells in 3 dimensions IV. Crystal Structure A crystalline solid possesses rigid and long-range order. In a crystalline solid, atoms, molecules or ions occupy specific (predictable) positions. An amorphoussolid does not possess a well-defined arrangement and long-range molecular order (no sharp MP) A. Unit Cell the basic repeating structural unit of a crystalline solid. • At lattice points: • Atoms • Molecules • Ions

  19. IV. Crystal Structure B. Cubic Cells

  20. B. Cubic Cell a. Simple cubic lattice (scc) • No. of atom per unit cell 8 x 1/8 =1 • No. of nearest neighbors (coordination number): 6 • In a unit cell, 52% space is occupied by atom • Size of unit cell to size of atom r = radius of atom a = edge length of unit cell 2 r = a

  21. B. Cubic Cell b. body-centered cubic (bcc) • No. of atom per unit cell 8 x 1/8 +1 =2 • No. of nearest neighbors (coordination number): 8 • In a unit cell, 68% space is occupied by atom • Size of unit cell to size of atom r = radius of atom a = edge length of unit cell b2 = a2 + a2 C2 = a2 +b2 = 3 a2

  22. B. Cubic Cell c. face-centered cubic (fcc) • No. of atoms per unit cell 8 x 1/8 + 6 x 1/2 =4 • No. of nearest neighbors (coordination number): 12 • In a unit cell, 74 % space is occupied by atom • Size of unit cell to size of atom r = radius of atom a = edge length of unit cell b2 = a2 + a2 b = 4 r 2 a2 = 16 r2

  23. d = d = m m V V x = x 1 mole Ag 107.9 g 7.17 x 10-22 g 6.022 x 1023 atoms mole Ag 6.83 x 10-23 cm3 Question: When silver crystallizes, it forms face-centered cubic cells. The unit cell edge length is 409 pm. Calculate the density of silver. V = a3 = (409 pm)3 = 6.83 x 10-23 cm3 4 atoms/unit cell in a face-centered cubic cell m = 4 Ag atoms = 7.17 x 10-22 g = 10.5 g/cm3

  24. V. Bonding in Solids Non-crystalline quartz glass • Amorphous solid • Crystalline solid Crystalline quartz (SiO2)

  25. A. Ionic Crystals • Lattice points occupied by cations and anions • Held together by electrostatic attraction • Hard, brittle, high melting point • Poor conductor of heat and electricity in solid form • Conduct electricity when melted CsCl ZnS CaF2

  26. B. Molecular Crystals • Held together by intermolecular forces • Soft, low melting point • Poor conductor of heat and electricity • Examples: • CH4, Ar, P4, S8

  27. C. Covalent Crystals • Form 3-D network • Held together by covalent bonds • Hard, high melting point • Poor conductor of heat and electricity • Examples include C, B, Si, SiO2 (quartz) graphite diamond

  28. nucleus & inner shell e- mobile “sea” of e- D. Metallic Crystal • Lattice points occupied by metal atoms • Held together by metallic bonds • Soft to hard, low to high melting point • Good conductors of heat and electricity Cross Section of a Metallic Crystal

  29. VI. Phase Changes A substance can be converted into different phases by adding Or removing heat Gas Vaporization Condensation Temperature Endothermic Sublimation Deposition Liquid Melting Freezing Solid

  30. A. Liquid-Vapor Equilibrium At Equilibrium Before Evaporation

  31. Rate of evaporation Rate of condensation = a. as T b. as intermolecular forces A. Liquid-Vapor Equilibrium The equilibrium vapor pressure is the vapor pressure measured when a dynamic equilibrium exists between condensation and evaporation 1. Dynamic Equilibrium 2. Equilibrium vapor pressure

  32. ln P = - DHvap + C RT 3. Molar heat of vaporization(DHvap) is the energy required to vaporize 1 mole of a liquid. Clausius-Clapeyron Equation P = (equilibrium) vapor pressure T = temperature (K) R = gas constant (8.314 J/K•mol)

  33. P2=230 mmHg Question: The vapor pressure (VP) of ethanol is 100 mmHg at 34.9oC. What is the VP a 52.6oC? (DHvap = 39.3 kJ/mol) T1=34.9oC = 308.1 K T2=52.6oC = 325.8 K P1=100 mmHg P2=?

  34. 4. Boiling point The boiling point is the temperature when the (equilibrium) vapor pressure = the external pressure. The normal boiling point is the temperature at which a liquid boils when the external pressure is 1 atm.

  35. 5. Critical point The critical temperature(Tc) is the temperature above which the gas cannot be made to liquefy, no matter how great the applied pressure. The critical pressure(Pc) is the minimum pressure that must be applied to bring about liquefaction at the critical temperature.

  36. B. Liquid-Solid Equilibrium Molar heat of fusion (DHfus) is the energy required to melt 1 mole of a solid substance.

  37. C. Solid-Vapor Equilibrium Molar heat of sublimation (DHsub) is the energy required to sublime 1 mole of a solid. Gas DHvap Sublimation, Deposition Liquid DHfus DHsub = DHfus + DHvap DHsub Solid

  38. Heating (cooling curve)

  39. Question: How much heat (in kJ) is needed to convert 80.0 g of ethanol at -130oC to vapor at 96oC? The specific heat of solid, liquid, and vapor ethanol are 0.97, 2.4, and 1.2 J/g.oC, respectively. DHfus= 5.02 kJ/mol, DHvap= 39.3 kJ/mol. The mp is -114oC and bp is 78oC. Step 1: heat solid up to mp (-114oC), remember q=smDT q1=0.97 J/g.oC x 80.0 g x (-114oC -(-130oC))x 1 kJ/1000J=1.24 kJ Step 2: melt solid to liquid q2= 80.0g x 1 mol C2H5OH/46.07 g x 5.02 kJ/mol=8.72 KJ Step 3: heat liquid from mp (-114oC) to bp (78oC) q3=2.4 J/g.oC x 80.0 g x (78oC -(-114oC))x 1 kJ/1000J=36.9 kJ Step 4: convert liquid to vapor q4= 80.0g x 1 mol C2H5OH/46.07 g x 39.3 kJ/mol=68.2 KJ Step 5: heat vapor from 78oC to 96oC q5=1.2 J/g.oC x 80.0 g x (96oC -78oC)x 1 kJ/1000J=1.73 kJ Total heat=q1 +q2 + q3+ q4 + q5 =117 kJ

  40. sublimation melting vaporization deposition freezing condensation VII. Phase diagram – summarizes the conditions at which a substance exists as a solid, liquid, or gas. Critical point Liquid Solid Pressure, P Gas Triple point Temperature, T

  41. Phase Diagram of Water Phase Diagram of CO2

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