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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

S

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