Liquids, Solids, and Phase Changes

Transcription

1 C h a p t e r 10 Liquids, Solids, and Phase Changes KMT of Liquids and Solids 01 Gases have little or no interactions. Liquids and solids have significant interactions. Liquids and solids have well-defined volume. Liquid molecules flow, while solids are held rigid. Dipole Moments 02 Dipole Moments 03 Polar covalent bonds form between atoms of different electronegativity. This is described as a bond dipole. Dipole Moment (µ): The measure of net molecular polarity or charge separation. µ = Q r r = distance between charges δ+ = Q, δ = Q Dipole moments are expressed in debyes (D) where 1 D = x C m A proton and electron separated by 100 pm have µ = 4.80 D 1

2 Dipole Moments 04 Dipole Moments 05 Polarity can be illustrated with an electrostatic potential map. These show electron-rich groups as red and electron-poor groups as blue-green. Dipole Moments 06 Intermolecular Forces 07 The dipole moment of HCl is 1.03 D, and the distance between the atoms is 127 pm. Calculate the percent ionic character of the HCl bond. Which of the following compounds will have a dipole moment? Show the direction of each. a. SO 2 b. NH 3 c. CF 4 d. TeH 4 Attractive forces between molecules and ions. Determine bulk properties of matter. Much weaker than intramolecular forces. Several types of forces: Ion dipole Dipole dipole Instantaneous induced dipole (dispersion forces) Hydrogen bonds. 2

3 Intermolecular Forces 08 Intermolecular Forces 09 Ion Dipole: Between polar molecules and ions. Dipole Dipole: Between polar molecules. Intermolecular Forces 10 Intermolecular Forces 11 London Dispersion Forces: Attraction is due to instantaneous, temporary dipoles formed due to electron motions. Hydrogen Bond: Molecules containing N H, O H, or F H groups, and an electronegative O, N, or F. 3

4 Intermolecular Forces 12 Intermolecular Forces 13 Effect on Boiling Point: CH K H 2 O 373 K SiH K H 2 S 215 K GeH K H 2 Se 225 K SnH K H 2 Te 270 K Intermolecular Forces 14 Intermolecular Forces 15 Surface Tension is the resistance of a liquid to spread out and increase its surface area. Surface tension results from intermolecular force differences between molecules in the interior of a liquid and those on the surface. Viscosity is the measure of a liquid s resistance to flow and is related to the ease with which molecules move around, and thus to the intermolecular forces. 4

5 X-Ray Crystallography 16 X-Ray Crystallography 17 Diffraction is the scattering of radiation by an object containing regularly spaced lines, with a spacing that is equivalent to the wavelength of radiation. Diffraction is due to interference between two waves passing through the same region of space at the same time. X Ray Crystallography 18 Crystal Structure 19 Solids are divided into two categories: 1. Crystalline: Possesses rigid and long-range order. 2. Amorphous: Lacks well-defined arrangement. Structure of a crystalline solid is based on the unit cell, a basic repeating structural unit. nλ = 2d sin θ 5

6 Crystal Structure 20 Crystal Structure 21 The number of particles touching each atom (the coordination number) and the number of atoms contained in each cell depends on the cell type. Simple Cube and Body-Centered Cube: Coord # # atoms/cell SC 6 1 BCC 8 2 FCC 12 4 Crystal Structure 22 Crystal Structure 23 Simple Cube and Body-Centered Cube: Face-Centered Cube: 6

7 Crystal Structure 24 Crystal Structure 25 Hexagonal Close Packing Arrangements: Hexagonal Close Packing Arrangements: Flash Animation - Click to Continue Crystal Structure 26 Types of Crystal 27 Hexagonal Close Packing Arrangements: Ionic Crystals: 1. Charged species. 2. Generally hard/brittle and high melting point. 3. Conduct ONLY as melt or in a solution. 7

8 Types of Crystal 28 Types of Crystal 29 Sodium Chloride: Covalent (Network) Crystals: 1. Held together by covalent bonds. 2. Generally hard & high melting. 3. Nonconducting. Types of Crystal 30 Types of Crystal 31 Carbon: Molecular Crystals: 1. Held together by intermolecular forces. 2. Soft and low melting point. 3. Nonconducting. 8

9 Types of Crystal 32 Types of Crystal 33 Water: Metallic Crystals: 1. BCC, FCC, or Hex arrangement. 2. Positive ions in a sea of electrons. 3. Variable hardness and melting point. 4. All conduct. Phase Changes 34 Phase Changes 35 Molar Heat of Fusion ( H fus ): The energy required to melt one mole of solid (in kj). Molar Heat of Vaporization ( H vap ): The energy (in kj) required to vaporize one mole of liquid. 9

10 Phase Changes 36 Phase Changes 37 Sublimation: The process in which molecules go directly from the solid into the vapor phase. Deposition: The process in which molecules go directly from the vapor into the solid phase. Molar heat of sublimation ( H sub ): The energy (kj) required to sublime one mole of solid. ΔH sub = Δ H fus + ΔH vap Phase Changes 38 Phase Changes 39 Calculate the amount of energy (kj) needed to heat 346 g of liquid water from 0 C to 182 C. Assume that the specific heat of water is J/g C over the entire liquid range and that the specific heat of steam is 1.99 J/g C. Vapor Pressure: The pressure exerted by gaseous molecules above a liquid. The molar heats of fusion and sublimation of molecular iodine are kj/mol and kj/mol, respectively. Estimate the molar heat of vaporization. 10

11 Phase Changes 40 Phase Changes 41 Phase changes depend on temperature. Clausius Clapeyron Equation: Provides a link between vapor pressure (P), temperature (T), and molar heat of vaporization ( H vap ). ΔHvap lnp = + C PT By taking measurements at two temps, we get: P1 ΔHvap 1 1 ln = P2 R T2 T1 Phase Changes 42 Phase Diagrams 43 Ethyl ether is a volatile, highly flammable organic liquid that is used mainly as a solvent. The vapor pressure of ethyl ether is 401 mm Hg at 18 C. Calculate its vapor pressure at 32 C. The vapor pressure of ethanol is 100 mm Hg at 34.9 C. What is its vapor pressure at 63.5 C? ( H vap for ethanol is 39.3 kj/mol) A Phase Diagram is a graphical display of the temperatures and pressures at which two phases of a substance are in equilibrium. Triple Point: The only condition under which all three phases can be in equilibrium with one another. Critical Temperature (T c ): The temperature above which the gas phase cannot be made to liquefy at any pressure. Critical Pressure (P c ): The minimum pressure required to liquefy a gas at its critical temp. 11

12 Phase Diagrams 44 Phase Diagrams 45 Heating Curves: By following the change in temperature on heating a solid compound up to the gas phase permits the construction of a phase diagram. Carbon Dioxide: Heating Curve for Water Temperature ( o C) Heat Added (kj) Phase Diagrams 46 Phase Diagrams 47 Water: Approximately, what is the normal boiling point and what is the normal melting point of the substance? What is the physical state when: i. T = 150 K, P = 0.5 atm ii. T = 325 K, P = 0.9 atm iii. T = 450 K, P = 165 atm 12