1 Chapter Twelve Intermolecular Forces And Liquids and Solids
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Chapter Twelve
Intermolecular Forces And
Liquids and Solids
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States of Matter Gases
Low density No fixed volume or shape Readily compressed Atoms/molecules move easily
Liquids
High density Fixed volume Assumes shape of container Does not compress Molecules flow past each other
Solids High density Fixed shape and volume Does not compress Vibrational motion only
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Intermolecular Forces Types of forces
Intermolecular forces: attractive forces between molecules Intramolecular forces: hold atoms together in a molecule
Comparison: Intermolecular vs. intramolecular Intermolecular: 40 kJ to vaporize 1 mole of water Intramolecular: 460 kJ to break O-H bonds in 1 mole of water Intermolecular forces weaker than intramolecular forces
“Measure” of intermolecular force Boiling point: break forces holding liquid together Melting point: break forces holding solid together Enthalpy of these reactions: ∆Hvap, ∆Hfus, ∆Hsub
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Exist between molecules with permanent dipoles. Dipoles align themselves with the positive end of one dipole
directed toward negative ends of neighboring dipoles. Increases with molecular polarity
Dipole-Dipole Forces
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Ion-Dipole Forces Attractive forces between an ion and a polar molecule Electrostatic charges only No induced charges Hydration Cations/anions interact with water Dissolve ionic salts Polar H2O molecules surrounds ion
Higher charge Stronger force
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Attractive forces that result from temporary dipoles induced in atoms or molecules
Strength determined by polarizability Measure of how easy it is to distort electron density Ion induced dipole Dipole induced due to attraction to nearby ion Dipole induced dipole
Pairs form and fade away. Net attraction holds molecules together
Dispersion Forces
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Polarizability High polarizability
Form strong intermolecular forces: molecules stick together
High molecular mass Larger area to spread out electrons; less repulsion Many electrons
Large surface area Long and thin more polarizable : charge separation
Molecule Length BP CH3CH2CH2CH2CH2CH3 Long 69 °C (CH3)2CHCH(CH3)2 Branched 50 °C CCI4 Heavy 171°C CH4 Light -182 °C
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Bond between H atom (δ+) bonded to a nonmetal atom (δ-) and attracted another non-metal atom (δ-)
O,N,F must be bonded to Hive Creates linear group of 3 atoms Strength > dipole-dipole
Partial covalent bond between H and lone pair on other atom
Hydrogen Bonds
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Partial covalent bond between H and lone pair on other atom
Hydrogen Bonds in DNA
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HBr HBr is polar: dipole-dipole & dispersion forces CH4 CH4 is nonpolar: dispersion forces only SO2 SO2 is polar: dipole-dipole & dispersion forces NH3 NH3 is matches H-bond criteria: H-bonding Dispersion forces negligible
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What type(s) of intermolecular forces exist between each of the following molecules?
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Properties of Liquids Surface Tension Energy required to extend a liquid surface Top of liquid has tighter bonds than in liquid Results: Water bugs don’t sink Cohesion Attraction between like molecules Adhesion Attraction between unlike molecules An “adhesive” bonds things together Capillary Action Adhesive forces: liquid sticks to glass Cohesive forces: molecules stick together Viscosity: Measure of resistance to flow
H2O:A>C Liquid higher than surface
Hg: C>A Liquid lower than surface
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Crystal Structure
13 Summary of Crystalline Solids ( good study sheet)
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Crystal Structure Crystal Solid that possesses a rigid order Atoms, molecules or ions occupy predictable positions Can have several different arrangements Unit cell Basic repeating structural unit of a crystalline solid Can exist as 3 basic types Amorphous solid Does not possess a well-defined arrangement
lattice point
Unit cell
Lattice points: Atoms Molecules Ions
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Ionic Crystals Lattice points occupied by anions (sometimes cations) Held together by electrostatic attraction Hard, brittle, high melting point Poor conductor of heat and electricity electrons locked in place due to connection to anions
Simple Cubic CsCl
Face Centered Cubic ZnS
Face Centered Cubic CaF2
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Covalent Crystals Lattice points occupied by atoms Held together by covalent bonds Hard, high melting point Usually poor conductor of heat and electricity graphite conducts electricity due to pi bonding
diamond graphite
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Lattice points occupied by molecules Held together by intermolecular forces Nonpolar: Dispersion forces Polar: Dipole-dipole or H-bonding Soft, low melting point, often a gas Doesn’t want to be a solid! Poor conductor of heat and electricity No electron movement S8→
Molecular Crystals
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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 shifting of electrons between metal atoms
nucleus & inner shell e-
mobile “sea” of electrons
Metallic Crystals
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Phase Changes
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Liquid-Vapor Equilibrium Vaporization: Conversion of liquid to vapor Fast molecules leave liquid surface Remaining molecules lower in energy Endothermic: Temperature (l) decreases Measure gas pressure using gas laws Condensation: Conversion of vapor to liquid Slower molecules drop out of gas Exothermic: liquid more stable than gas Enthalpy Conversions (∆Hvapn) = - ∆Hcondn Boiling point: temp. where vapor pressure= atm. Pressure
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Supercritical Fluid As temperature is raised in a sealed container:
Liquid vaporizes above critical T (Tc): the vapor gets dense Pressure can’t be relieved to go to a gas, below Pc Critical temp (Tc): can’t be liquid Critical pressure (Pc): can’t be gas Liquid & vapor meld into one fluid
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Melting and Freezing Solid Liquid Melting Order of the solid collapses Endothermic: requires energy (heat) Freezing Solid settles into a crystalline form Exothermic: Solid more stable Determined by melting / freezing point:
Temperature at melting (or freezing) Normal melting point: MP at 1 atm
Molar Heat of Fusion/Melting (∆H°fus) Heat absorbed/released when 1 mole solid melts/freezes at constant T & P
Supercooling: A pure liquid cooled slowly may exist below
its freezing point
23 Sublimation and Deposition: Solid Vapor Sublimation
Solid converted directly to gas Endothermic: Need heat to increase molecular movement Breaks intermolecular forces
Heat of Sublimation (∆H°sub ) Summation of solid to liquid Then liquid to gas (∆H°sub ) = ∆H°fus + ∆H°vap
Deposition: Opposes sublimation Gas to solid
Exothermic
Triple Point Solid, liquid and gas exist simultaneously
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At 1 atm CO2 (s) CO2 (g)
Phase Diagrams
A phase diagram summarizes the conditions at which a substance exists as a solid, liquid, or gas.
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Heating Curve