Chap. 12 INTERMOLECULAR FORCES

Transcription

1 Chap. 12 INTERMOLECULAR FORCES Know how energy determines physical properties and how phase changes occur as a result of heat flow. Distinguish between bonding (intermolecular) and nonbonding (intermolecular) forces. Know how interparticle forces contribute to physical properties such as surface tension, viscosity, capillarity, and crystal structure. Understand how the macroscopic properties of water arise from its molecular structure Physical States & Phase Changes DENSITY mass volume SAPE PROPERTY COMPRESSIBILITY V as ƒ(pressure) TERMAL EXPANSION V as ƒ(temperature) SOLID LIQUID GAS high definite moderate to high indefinite (fluid) low indefinite (fluid) small small large very small small moderate A Kinetic-Molecular View Types of Phase Changes SOLIDS Particles vibrate around fixed positions Strong cohesive (attractive) forces 100 vaporization vap = 40.7 kj/mol 2 O(l) + 2 O(g) 2 O(g) 75 LIQUIDS Particles in constant, random motion Cohesive forces dominate, particles see each other O(l) GASES Particles in constant, random motion Disruptive (kinetic energy) forces dominate, particles move independently of each other 0 2 O(s) melting fus = 6.02 kj/mol 2 O(s) + 2 O(l) eat Added, kj Phase Changes as Equilibria E sys Gas VAPORIZATION CONDENSATION SUBLIMATION MELTING Liquid FREEZING Solid endothermic exothermic DEPOSITION Some of the molecules in a liquid may contain sufficient energy to escape its surface As a result, some of the liquid vaporizes Some of the vapor loses energy and condenses In a closed system, the pressure exerted by the vapor in equilibrium with its liquid is the VAPOR PRESSURE Page 12-1

2 1.0 Mol Fraction T 1 E K for escape T 2 > T P vap, 0.6 atm 0.4 n-eptane 100 n-exane n-pentane O Butanol 74 E K NORMAL BOILING POINT T where where P vap = vap 1 atm atm Clausius Clapeyron Equation P vap, 1.00 atm 0.75 P vap, atm O(g) bp: P vap = 1 atm ln(p vap ) ln(p) = vap /RT + C n-pentane O(s) mp 2 O(l) Butanol 2 O P 2 vap 1 vap 1 ln ln = P 1 R T 2 T /T, 10 3 K 1 New York Phase Change Summary P, 0.7 atm 0.6 Denver La Paz 1 2 Altitude, 3 km 4 VAPOR PRESSURE P when l g Increases with temperature Decreases with mass (homologous series) Increases with weaker intermolecular forces Mount Everest 5 6 VAPORIZATION/CONDENSATION Significant ƒ(t) Boiling Point, C BOILING POINT P vap = vap P atm atm liquid gas gas Boiling point = T where P vap = P external Normal bp observed at 1 std atm (760 torr, etc) Page 12-2

3 Phase Change Summary MELTING (FUSION)/FREEZING Melting point = T where s l Little or no ƒ(p) SUBLIMATION/DEPOSITION Weak intermolecular forces CO 2 Naphthalene (C 10 8 ) Significant ƒ(t) Phase Diagrams Graphical representations of phases as ƒ(p,t) Phase transitions shown as lines between regions Identify CRITICAL POINT: ighest T (critical temperature) at which a substance can exist as a liquid; P (critical pressure) necessary to effect liquefaction at critical point TRIPLE POINT: T, P where 3 phases are in equilibrium P, atm O(s) 2 O(l) VAPORIZATION CRITICAL POINT P, atm CO 2 (s) a b CO 2 (l) DEPOSITION TRIPLE POINT 2 O(g) 1.00 SUBLIMATION POINT CO 2 (g) Intermolecular Forces Physical properties depend on physical state Physical properties of CONDENSED PASES (s and l) depend on particle-particle interactions. INTRAMOLECULAR (bonding) INTERMOLECULAR (nonbonding, interparticle) BONDING FORCES are relatively strong Charges operating at short distances INTERMOLECULAR FORCES are relatively weak Low-strength charges operating at greater distances Intermolecular Forces and Distance Bond Length Radius ½ distance between bonded atoms (= ½ bond length) O van der Waals Radius ½d between adjacent nonbonded atoms F Cl Br I Page 12-3

4 Comparison of Bonding Forces Comparison of Nonbonding Forces Force Ionic Metallic Basis of Attraction Cation Anion Nuclei Shared e pair Cation Delocalized e Energy, kj/mol Example NaCl 2 g Force Ion-Dipole bond Dipole-Dipole Ion- Induced dipole Dipole- Induced dipole Dispersion Basis of Attraction Ion charge Dipole charge X- bond Dipole charge Dipole charge Dipole charge Ion charge Polarizable e cloud Dipole charge Polarizable e cloud Polarizable e cloud Polarizable e cloud Energy, kj/mol Example Na + O 2 O- O 2 I-Cl I-Cl Fe 2+ O 2 2 O I-I Br-Br Br-Br DIPOLE INTERACTIONS Electrons that make up some bonds are not shared equally ( EN) Charged ends attract other charged species or species in which charges can be induced Cl δ Cl δ δδ Br Br These interactions can be moderately strong in certain cases δ δ O δ Na + O δ Example: Polarity and Boiling Point LiCl bp, C 500 NCN C 3C 2O CON 2 0 C 3OC 3 C 3C 2C μ, D SPECIAL CASE: YDROGEN BONDING Bonds between and electronegative elements (X = N, O, F, Cl) are particularly strong dipoles The attraction between the atom of one dipole and the X atom of another is called a YDROGEN BOND δ ydrogen Bonding: Implications Responsible for certain characteristics of 2 O, a small molecule which has relatively igh surface tension, capillarity igh heat capacity, heat of vaporization Great solvent power Complex density profile (max at 4 C) Page 12-4

5 bp, C O 7A 7A ydrides ydrides 6A 6A ydrides ydrides 5A 5A ydrides ydrides 4A 4A ydrides F ydrides N 3 C Molecular Mass DISPERSION (LONDON) FORCES There is a small chance that the electrons in any bond will be unevenly distributed for a short period of time (INSTANTANEOUS DIPOLES) In these instants, a covalent bond would have dipole character and attract other temporary dipoles The averages of many such weak, short-lived attractions are called DISPERSION FORCES δ Cl Cl δδ Cl Cl δδ Cl Cl δ The ease with which a molecule s charge distribution can be distorted is its polarizability Larger molecules have greater polarizabilities than small molecules C C C C C Molecules of linear molecules have greater interparticle contact than spherical molecules of equal mass The magnitude of dispersion forces increases with molecular weight [this is why polymers have unique solution properties] The magnitude of dispersion forces depends in molecular shape Dispersion forces operate between all molecules PENTANE bp 36.1 C C C C C C Branching reduces dispersion-force interactions lower bp bp NEOPENTANE bp 9.5 C NO DISPERSION FORCES Polar Molecules Only? NO YES NO -X Bonding? INTERACTING PARTICLES YES Ions? -BONDING YES NO IONIC IONIC BONDING Polar Molecules + Ions? YES DIPOLE- DIPOLE ION- ION- DIPOLE The Liquid State SURFACE TENSION Energy required to increase a liquid s surface area (J/m 2 ) Minimizes surface area and net intermolecular forces Balance between cohesive and adhesive forces CAPILLARITY (CAPILLARY ACTION) Flow arising from an imbalance of adhesive and cohesive forces Magnitude depends on the relation between liquid and container surface areas VISCOSITY Resistance to flow (N s/m 2 ) Magnitude depends on strength of intermolecular forces Decreases with increasing temperature Page 12-5

6 COESIVE FORCES Among particles of a substance 2 O J/m 2 C 3 C 2 OC 2 C J/m 2 ADESIVE FORCES Between particles of different substances 12.6 The Solid State X Glass Si-O bonds broken Quartz (SiO 2 ) AMORPOUS No orderly structure Solids with structural constraints to ordering Macromolecules CRYSTALLINE Ordered, repeating structural unit ighly regular shapes QUASICRYSTALLINE Multiple structural units with long-range repetition Unit Cells The 3-D system of points representing the centers of the elements (atoms, molecules) in a crystal is its LATTICE The UNIT CELL is the smallest repeating unit of the lattice The total of all elements in the unit cell should equal the cation anion ratio corner: 1/8 Na face: 1/2 Cl center: 1 Na NaCl unit cell edge: 1/4 Cl Cubic Packing When crystals form, atoms or molecules are arranged in a way that maximizes interparticle attractions. Elements touch 4 others in each layer; 2nd layer lies directly over 1st layer: SIMPLE CUBIC (52% OCCUPIED) Elements touch 4 others in each layer; layers fit into gaps (depressions) in lower layers: BODY-CENTERED CUBIC (68% OCCUPIED) Close Packing (74% OCCUPIED) An arrangement that minimizes the empty space between elements is called CLOSE PACKING 2nd layer fits into depressions of 1st layer EXAGONAL CLOSE PACKING ABAB In a closed-packed layer, elements touch 6 others. Where elements (usually ions as in Li 2 O) are of unequal size, the larger element assumes this arrangement 3rd layer can fit into 2nd layer depressions directly above 1st layer 3rd layer can also fit into 2nd layer depressions directly above gaps between 1st layer elements CUBIC CLOSE PACKING (FACE-CENTERED CUBIC) ABCABC Page 12-6

7 Molecular/Atomic Solids Repeating units made up of MOLECULES (atoms if solid noble gases) Network Solids Repeating units of atoms held together by COVALENT BONDS Water, 0 C carbon QUESTION: What holds the molecules together? Diamond Ionic Solids Repeating lattice of ions held together by IONIC BONDS Metallic Solids Metal atoms with inner electrons in a sea of outer electrons that flow from one atom to another (METALLIC BONDS) CsCl cesium chloride E Conduction Band (unoccupied) Conduction Band (unoccupied) face-centered cubic Valence Band (occupied) conductor contiguous Valence Band (occupied) semiconductor Characteristics of Crystalline Solids Solid Molecular/ Atomic Network Ionic Unit Particles Molecules or atoms Atoms Anions + cations Interparticle Forces Properties Examples Dispersion Dipole-dipole -bond bonds Ionic bonds Soft Lower mp Poor conductor ard igh mp Poor conductor ard, brittle igh mp Good conductor (l) Ar(s), CO 2 (s), cane sugar [C O 11 ] Diamond [C], quartz [SiO 2 ] NaCl E, kj/mol Metallic Atoms Metallic bonds Soft to hard Malleable, Ductile Varying mp Good conductor Cu, Fe 500 Ionic Bonding Bonding Metallic Bonding Page 12-7

8 E, kj/mol Ion- Dipole Bond Other Dispersion Dipole Page 12-8