Liquids, Solids, and Phase Changes

Polar Bonds and Polar Molecules Chapter 10 z z Liquids, Solids, and Phase Changes Draw Lewis Structures for CCl4 and CH3Cl. What’s the same? What’s different? 1 Polar Covalent Bonds and Dipole Moments z z z Bonds are polar if electrons are shared unequally (differences in electronegativity) (e.g., CH3Cl) Individual bonds can be polar but can cancel each other out to yield a nonpolar molecule (e.g., CO2) Molecules with a lone pair of electrons or atoms with different electronegativities are polar (have a dipole moment) Polar or Nonpolar Molecules? z z z z z z Draw Lewis Structures and Dipole Moments along each bond: ● BF3 HCl ● CH3Cl CCl4 ● H2 NH3 H2O Worked Ex. 10.2, Problems 10.1 - 3 Polarity 3 Dipole Moments Intermolecular Forces (IMF) z z z Bonds: attractive forces within molecules Intermolecular Forces (IMF): attractive forces between molecules Intermolecular forces are WEAKER than bonds (intramolecular forces), but have profound effects on the properties of liquids 431 kJ/mol 16 kJ/mol 5 1 Intermolecular Forces z z z Dispersion Forces What is the difference between bonds and intermolecular forces? IMF affect boiling points, melting points, and solubilities As a group, intermolecular forces are called van der Waals forces: z z  London Dispersion  Dipole-dipole  Hydrogen bonds  Ion-dipole I2 induced dipoles London dispersion forces: attractive forces that result from temporary shift of electrons in atoms or molecules; present in all molecules Explains why certain nonpolar substances benzene, bromine, etc.) are liquids at room temperature. Figure 10.5 8 Dispersion Forces z z Boiling Points Only force in non-polar molecules and in unbonded atoms (e.g., CO2, Ne) Larger atoms or molecules have stronger London forces (and higher boiling points) z Boiling point increases with the size of molecules because of increases in London forces with larger electron clouds  Larger electron clouds are easier to deform (“squishier”, more polarizable) and tend to have more electrons z Found in mixtures or pure substances 9 Dipole-Dipole Forces z z z 10 Dipole-Dipole Forces Pure substance or mixture made up of polar molecules Opposite dipoles (charges) attract; like dipoles repel Strength of these forces depends on the polarity of the molecules z  It takes 2000 mL of H2O to dissolve 1 mL of CCl4  It takes 50 mL of H2O to dissolve 1 mL of CH2Cl2 z dipole Figure 10.4 Like dissolves like (refers to polarity): polar liquids are more soluble in polar liquids 11 Which member of each pair has the stronger intermolecular forces? CCl4 or CHCl3 CO2 or SO2 12 2 Boiling Points – which is higher? Hydrogen Bonds (aka bridges) z Hydrogen bridge: special type of dipole-dipole force; attractive force between a hydrogen atom bonded to a very small, electronegative atom (F, O, N) and lone p e- pair z Ice melting HF 14 Hydrogen Bridges z z Hydrogen Bridges Hydrogen bridges are seen in HF, H2O, NH3, but not CH4 or H2S Requirements for hydrogen bridging: z Rasmol 2.6.2. Molecules hydrogen bridge to themselves or to other molecules bridge bridge  H attached tt h d tto a small, ll hi highly hl electronegative l t ti element in one molecule  Small, highly electronegative element with one or more unshared electron pairs in the other molecule z Observed for the elements: F, O, N (rarely S and Cl – too large, not electronegative enough) 15 Hydrogen Bridging Trends 16 Hydrogen Bridging z Which of the following pure substances will experience hydrogen bridging? H2O H2Se S HBr HF NH3 NF3 DNA 18 3 Ion-Dipole Forces z z Ion-Ion Forces NaCl In H2O Ions have full charges that are attracted to the partial charge on polar molecules (dipoles) Explains solubility of ionic salts in polar solvents (e.g., NaCl in water) z Strength of bonds in ionic compounds (already covered in Lattice Energy section!). NaCl CaF2 Figure 10.3 19 Strengths of Intermolecular Forces z Intermolecular Forces z Intermolecular forces generally increase in strength as: London < Dipole-Dipole < H-bridging < Ion-Dipole < Ion-ion forces (ionic bonding) z Summary:  Nonpolar molecules: London Dispersion  Polar molecules: Dipole-dipole  Polar with H-N, H-O, or H-F: Hydrogen bridging  Ionic compound: Ion-ion forces ● For each substance below, indicate the strongest type of intermolecular force observed. H2O CO CH4 NH3 HCN CH3OH CO2 CH3NH2 Rasmol 2.6.2 F2 N2 Worked Ex. 10.3, Problem 10.5 21 Trends in Intermolecular Forces z Surface Tension Which member of each pair has stronger intermolecular forces (and higher boiling point)? CH3OH or CH3SH CH4 or CH3CH2CH3 CO or F2 CH3Cl or HF CO2 or NH3 NH3 or N2 Boiling point curves Problem 10.6 z z Rasmol 2.6.2.l ● 22 23 Surface tension: attraction of molecules to each other on a liquid’s liquid s surface Molecules must break IMF in order to move to the surface and increase the surface area (large IMF, high surface tension) 24 4 Properties of Liquids z z z z Phase Changes Properties (e.g., IMF) in between those of gases and solids Viscosity: resistance to flow D Depends d on iintermolecular t l l forces and sizes z z z fusion (melting): s Æ l vaporization: l Æ g sublimation: s Æ g freezing: l Æ s condensation: g Æ l deposition: g Æ s Ethanol vs Glycerol Viscosity glycerol vs ethanol 25 Phase Changes - note structures 10.11 Phase Diagram: Water z Low Temp Boil Phase diagram: plot of temperature vs. pressure; solid lines are phase equilibria Phase Diagram 10.5 Vapor Pressure z Vaporizing liquid Liquid evaporates, gaseous molecules exert a pressure (vapor pressure) that can be measured as shown below Phase Diagram z Atomic Phase diagrams Features of a phase diagram:  Triple point: temperature and pressure at which all three phases (s, l, and g) are in equilibrium  Vaporization curve: equilibrium between liquid and gas  Melting curve: equilibrium between solid and liquid  Normal melting point: melting point at 1 atm  Normal boiling point: boiling point at 1 atm Figure 10.11 Label Phase diagrams 30 5 Critical Temperature and Pressure z z z Phase Diagram: Carbon DioxidePhase DiaWorked Ex. 10.11, Problems 10.17, 10.18 Critical point: liquid and gas phases are indistinguishable Critical temperature, Tc : highest temperature at which a substance can exist as a liquid (cannot be liquified) no matter how much pressure is applied Critical pressure, Tp : minimum pressure that must be applied to bring about liquefaction at the critical temperature Grams: H2O and CO2 32 10.6 Kinds of Solids Kinds of Solids Amorphous: random arrangement (rubber) Ionic: ordered arrangement of ions (salt) z z z See Figure 10.14 z z Molecular (left): covalent molecules in an ordered arrangement (sucrose, ice); intermolecular forces hold molecules together Covalent network (right): atoms connected by covalent bonds in 3D array (quartz) Cubic Crystal Systems z z Cubic is the most common Three forms of cubic crystals simple, body-centered, face-centered Cubic unit cells SC - build BCC –CsCl FCC – CaF2 35 6