States of matter Chapter 11 Intermolecular Forces Liquids and Solids By changing the T and P, any matter can exist as solid, liquid or gas. Forces of attraction determine physical state Phase homogeneous part of system in contact with other parts of system, separated by well-defined boundary e.g., ice in water, subliming dry ice, evaporating isopropanol Kinetic Molecular Theory of Liquids and Solids Liquids and solids = condensed states Liquids Molecules are close together w/ little empty space difficult to compress Molecules held together by attractive forces Liquid has definite volume Molecules can move past each other freely, flow into shape of container Kinetic Molecular Theory of Solids Solids Molecules are held rigidly in place vibrate about a fixed point Less compressible than liquids. Usually solid is denser than liquid important exception = water Definite shape and volume 11.2 Intermolecular Forces Attractive forces between molecules. As T of gas drops, intermolecular forces overcome thermal motion Condensation gas molecules slow until attraction pulls them together into liquid Intermolecular forces < intramolecular forces (bonds) Stronger intermolecular force d higher boiling and melting points
Types of Intermolecular Forces Ion - dipole Dipole - dipole * Dipole - induced dipole * Dispersion (London) forces * Hydrogen bond * van der Waals force Ion-dipole interactions Cations generally have stronger interactions because they are smaller, with more concentrated charge than anions. Dipole-Dipole Pairs of cation-anion attractions align polar molecules Ion-Induced Dipole & Dipole-Induced Dipole Non-polarized atom Polarized atoms Induced dipole = separation of + and - charges in atom or nonpolar molecule caused by proximity of ion or polar molecule Induced Dipoles Polarizability - ease with which e - distribution of atom can be distorted More e - s d greater polarizability Often anions, with more diffuse charge, are more polarizable than cations Molecules w/ dipoles can be polarized, causing them to become more polar Instantaneous dipole induced dipole attraction Instantaneous dipole any atom or non-polar molecule is briefly polar because of separation between e - and + nucleus Instantaneous dipole can induce dipole in adjacent molecule Attraction between instantaneous and induced dipole is very weak London or dispersion forces
Melting Points of Nonpolar CX 4 MELTING POINT COMPOUND ( O C) CH 4-182.5 CF 4-150.0 CCl 4-23.0 CBr 4 90.0 CI 4 171.0 Larger molecule Why? d higher melting point Dispersion forces Occur in all substances (polar and non-polar) Allow non-polar molecules to condense Increase w/ polarizability, which increases with molar mass Polar molecules have dipole-dipole attractions as well as dispersion forces For polar molecules, larger dipole moment d stronger attraction. For any molecule, larger molar mass (more e - ) d greater intermolecular attraction For a large molecule, dispersion force can exceed dipole-dipole attraction Hydrogen Bond Special dipole-dipole interaction between H on N, O or F and a nearby N, O or F weak H-bonds with other elements such as S and P H-bonds are strong intermolecular attractions Large effects on intermolcular organization higher b.p. and m.p. alignment of protein molecules Approximate energies in kj/mol Compare to covalent single bonds, 200-600 kj/mol 25 5 15-20 5-35 weak up to 45 F H F - HO H OH - 169 85 Strong hydrogen bonds Weak or no hydrogen bonds Identify intermolecular forces H 2 O CH 2 Cl 2 KBr F - + H 2 O I 2 CH 3 OH PCl 3 C 6 H 6 SiH 4 Fe N(CH 3 ) 3 CS 2 BCl 3 Na + + NH 3
11.3 Properties of Liquids Surface Tension amount of energy required to increase surface area of liquid by a unit area Interior molecules are attracted in all directions Surface molecules are attracted selectively into liquid Anisotropic forces differ with direction Isotropic forces same in all directions Capillary action Cohesion intermolecular attraction between like molecules (pure liquid) Adhesion intermolecular attraction between unlike molecules (liquid and its container) Adhesion > cohesion H 2 O in glass Concave meniscus Cohesion > adhesion Hg in glass Convex meniscus Viscosity Measure of a fluid s resistance to flow High viscosity ( thick ) = slow flow Strong intermolecular forces d high viscosity glucose (typical sugar) is viscous because of intermolecular hydrogen bonding O HC O H C H H C OH OH C H O H C H CH2 O H Newtonian Fluids viscosity is independent of shear rate Non-Newtonian Fluids (examples) Thixotropic viscosity decreases with time under constant shear (gel-flow paint) Dilatant viscocity increases with shear rate (Silly Putty) Viscosity Liquid Viscosity (N s/m 2 ) Water (H 2 O) 1.01 x 10-3 Ethanol (C 2 H 5 OH) 1.20 x 10-3 Glycerol (HOCH 2 CHOHCH 2 OH) 1.49 Blood 4 x 10-3 More hydrogen bonds d more viscous liquid Viscosity Liquid Viscosity (N s/m 2 ) Water (H 2 O) 1.01 x 10-3 Ethanol (C 2 H 5 OH) 1.20 x 10-3 Glycerol (HOCH 2 CHOHCH 2 OH) 1.49 Blood 4 x 10-3 Water Large specific heat bodies of water moderate climate by absorbing and releasing heat Liquid is denser than solid (ice floats) H 2 O can form 2 H-bonds per molecule, leading to a very open solid structure. More hydrogen bonds d more viscous liquid
Large open cavities in solid ice are filled in liquid water, increasing density Ice 11.4 Crystal Structure Crystalline solid has rigid, longrange order Amorphous solid lacks welldefined arrangement and long range order Unit cell basic repeating structural unit of a crystalline solid Lattice point atom, molecule, or fixed arrangement of atoms Coordination number (CN) number of atoms (ions) surrounding an atom (ion) in a crystal lattice CN of an atom is a simple cubic lattice is 6 1/8 of each corner atom is inside the unit cell
How many atoms are in a body-centered cubic unit cell? Corner atom is shared by 8 cells. Edge atom is shared by 4 cells Face-centered atom is shared by two cells. 8 corner atoms: 8 x 1/8 = 1 1 central atom: 1 x 1 = 1 d 1+1 = 2 atoms in body-centered cubic cell (a) Most efficient arrangement of spheres (b) 2nd layer (c) Hexagonal closest packing ABABA... (d) Cubic closest packing ABCABC... Closest packing (a) Hexagonal close packing ABABA... (b) Cubic close packing ABCABC... Cubic close packing is identical to facecentered cubic packing Hexagonal vs. cubic close packing A B A A B C A X-Ray Diffraction X-Ray diffraction by a crystal Scattering of X-rays by the atoms of a crystalline solid Atoms in a crystal absorb then re-emit X-radiation When scattered X-rays from adjacent layers of atoms are in phase, constructive interference occurs at certain point in space
X-rays of wavelength 0.0900 nm are used to analyze a metallic crystal. 1st-order diffraction (n = 1) occurs at 15.2. What is the layer spacing? What is the 2ndorder angle (n = 2)? nλ = 2d sinθ When additional distance traveled by an X-ray (BC + CD = 2d sinθ) equals integral number of wavelengths (nλ), waves are in phase nλ = 2d sinθ (n = 1, 2, 3...) 11.6 Types of Crystals Ionic Crystals array of anions and cations high-melting brittle NaCl, Li 2 O, CaF 2, MgO Ionic bonds become stronger but more covalent as charges increase Covalent crystals 3-D network of covalent bonds high-melting, often hard diamond, graphite and quartz (SiO 2 ) 154 pm Diamond: sp 3 carbon 142 pm Graphite: sp 2 carbon Types of Crystals Molecular crystals lattice points occupied by molecules, held together by van der Waals forces and/or H-bonding low -melting (<100 o C) Metallic crystals metal atoms at lattice points dense, conductive, shiny etc. metal ions in sea of electrons Amorphous solids Lack a regular, three-dimensional arrangement of atoms Glass (optically transparent) fusion product of (inorganic) materials cooled to non-crystalline rigid state Common glass is mainly SiO 2 (quartz), w/ B 2 O 3 (Pyrex) or CaO, Na 2 O (lime glass) Transition metal oxides color glass
11.8 Phase Changes Transformation between phases Energy (usually heat) is added or removed Liquid-vapor equilibrium Liquid-solid equilibrium Solid-vapor equilibrium Liquid - Vapor Equilibrium Evaporation (vaporization) process in which a liquid is transformed into a gas Occurs when molecules have enough energy to escape from the liquid s surface Higher T d more KE d faster evaporation Low T d fewer energetic molecules High T d more energetic molecules (Equilibrium) Vapor Pressure Pressure exerted by evaporated gas molecules above a liquid When rate of evaporation = rate of condensation, dynamic equilibrium is reached P at this point = (equilibrium) vapor pressure Vacuum Hg Heat of Vaporization, H vap E required to vaporize 1 mole of a liquid X(l) d X(g) H vap Stronger intermolecular forces d larger H vap Table 11.6 Everyday experience evaporating water or alcohol makes you feel cold heat required to vaporize liquid comes from your skin Quantitative relationship between vapor pressure (P) and temperature (T) Clausius-Clapeyron Equation ln P = - H vap + C RT at a single T; C is a constant plot log P vs. 1/T to find H vap P 1 ln = Hvap 1 1 - P 2 R T 2 T 1 Form to compare two temperatures
Boiling Point T at which vapor pressure of a liquid = applied pressure Stronger intermolecular forces d higher boiling point Normal boiling point defined at 1 atm Critical Points Critical Temperature (T c ) T above which gas cannot be liquefied at any pressure Critical Pressure (P c ) minimum P needed to liquefy a gas at T c Supercritical fluids (phase above T c ) are industrially important supercritical CO 2 is used to decaffeinate coffee, extract oils from grain, dry-clean clothing, etc. Liquid-Solid Equililbrium Freezing phase change from liquid to solid Melting or Fusion phase change from solid to liquid Melting point (freezing point) T at which the solid and liquid phases coexist in equilibrium Normal m.p. (f.p.) defined at 1 atm (Molar) Heat of Fusion, H fus E required to melt one mole of a solid Table 11.8 H fus < H vap molecules are closely packed in the liquid and solid states, but widely separated in the gas state Compare slopes: s (solid > liquid > gas) How much heat is required to convert 425 g of ice at -15 C to steam at 125 C? (specific heats: ice = 2.03 J/g C, water = 4.18 J/g C, steam = 1.99 J/g C; H fus = 6.01 kj/mol; H vap = 40.79 kj/mol) Σ heats of 126 o C segments q 5 = msdt = total heat 100 o C q 4 = ndh vap Compare lengths: H fus < H vap 0 o C -10 o C q 1 = msdt q 2 = ndh fus q 3 = msdt
Solid-Vapor Equilibrium Sublimation conversion of solid directly to vapor Deposition vapor to solid (Molar) heat of sublimation, H sub = E required to sublime one mole of solid H sub = H fus + H vap Evaporations (up) are endothermic Condensations (down) are exothermic 11.9 Phase Diagrams H 2 O CO 2 Triple point condition in which all 3 phases are in equilibrium Slope of curve (line) between phases d P dependence of transition mp ice decreases at higher P mp CO 2 increases at higher P