Copyright©2000 by Houghton Mifflin Company. All rights reserved.

A phase is a homogeneous part of the system in contact with other parts of the system but separated from them by a well-defined boundary. 2 Phases Solid phase - ice Liquid phase - water Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Schematic representations of the three states of matter.
Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Intermolecular Forces
Intermolecular forces are attractive forces between molecules. Intramolecular forces hold atoms together in a molecule. Intermolecular vs Intramolecular 41 kJ to vaporize 1 mole of water (inter) 930 kJ to break all O-H bonds in 1 mole of water (intra) “Measure” of intermolecular force boiling point melting point DHvap DHfus DHsub Generally, intermolecular forces are much weaker than intramolecular forces. Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Forces between (rather than within) molecules. dipole-dipole attraction: molecules with dipoles orient themselves so that “+” and “” ends of the dipoles are close to each other. hydrogen bonds: dipole-dipole attraction in which hydrogen is bound to a highly electronegative atom. (F, O, N) Copyright©2000 by Houghton Mifflin Company. All rights reserved.

The boiling point represents the magnitude and type of bonding
Why is the hydrogen bond considered a “special” dipole-dipole interaction? Why this sudden increase? The boiling point represents the magnitude and type of bonding Decreasing molar mass Decreasing boiling point Copyright©2000 by Houghton Mifflin Company. All rights reserved.

London Dispersion Forces
relatively weak forces that exist among noble gas atoms and nonpolar molecules. (Ar, C8H18) caused by instantaneous dipole, in which electron distribution becomes asymmetrical. the ease with which electron “cloud” of an atom can be distorted is called polarizability. Copyright©2000 by Houghton Mifflin Company. All rights reserved.

(a) An instantaneous polarization can occur on atom A, creating an instantaneous dipole. This dipole creates an induced dipole on neighboring atom B. (b) Nonpolar molecules such as H2 also can develop instantaneous and induced dipoles. Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Dispersion Forces Continued Polarizability is the ease with which the electron distribution in the atom or molecule can be distorted. Polarizability increases with: greater number of electrons more diffuse electron cloud Dispersion forces usually increase with molar mass. Copyright©2000 by Houghton Mifflin Company. All rights reserved.

What type(s) of intermolecular forces exist between each of the following molecules? HBr HBr is a polar molecule: dipole-dipole forces. There are also dispersion forces between HBr molecules. CH4 CH4 is nonpolar: dispersion forces. S O SO2 SO2 is a polar molecule: dipole-dipole forces. There are also dispersion forces between SO2 molecules. Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Properties of Liquids Surface tension is the amount of energy required to stretch or increase the surface of a liquid by a unit area. Or The resistance to an increase in its surface area (polar molecules). Strong intermolecular forces High surface tension Copyright©2000 by Houghton Mifflin Company. All rights reserved.

A molecule in the interior of a liquid is attracted by the molecules surrounding it, whereas a molecule at the surface of a liquid is attracted only by molecules below it and on each side. Copyright©2000 by Houghton Mifflin Company. All rights reserved.

More Properties of Liquids
Capillary Action: Spontaneous rising of a liquid in a narrow tube. Nonpolar liquid mercury forms a convex meniscus in a glass tube, whereas polar water forms a concave meniscus. Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Types of Solids Crystalline Solids: highly regular arrangement of their components [table salt (NaCl), pyrite (FeS2)]. Amorphous solids: considerable disorder in their structures (glass). Copyright©2000 by Houghton Mifflin Company. All rights reserved.

An amorphous solid does not possess a well-defined arrangement and long-range molecular order. A glass is an optically transparent fusion product of inorganic materials that has cooled to a rigid state without crystallizing Crystalline quartz (SiO2) Non-crystalline quartz glass Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Unit cells in 3 dimensions
A crystalline solid possesses rigid and long-range order. In a crystalline solid, atoms, molecules or ions occupy specific (predictable) positions. An amorphous solid does not possess a well-defined arrangement and long-range molecular order. A unit cell is the basic (smallest) repeating structural unit of a crystalline solid. At lattice points: Atoms Molecules Ions lattice point Copyright©2000 by Houghton Mifflin Company. All rights reserved. Unit Cell Unit cells in 3 dimensions

Bragg Equation Used for analysis of crystal structures. n = 2d sin  d = distance between atoms n = an integer  = wavelength of the x-rays Copyright©2000 by Houghton Mifflin Company. All rights reserved.

X rays of wavelength nm are diffracted from a crystal at an angle of Assuming that n = 1, what is the distance (in pm) between layers in the crystal? nl = 2d sin q n = 1 q = l = nm = 154 pm nl 2sinq = 1 x 154 pm 2 x sin14.17 d = = 77.0 pm Copyright©2000 by Houghton Mifflin Company. All rights reserved. 11.5

Types of Crystalline Solids
Ionic Solid: contains ions at the points of the lattice that describe the structure of the solid (NaCl). Molecular Solid: discrete covalently bonded molecules at each of its lattice points (sucrose, ice). Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Types of Crystals Ionic Crystals Lattice points occupied by cations and anions Held together by electrostatic attraction Hard, brittle, high melting point Poor conductor of heat and electricity CsCl ZnS CaF2 Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Types of Crystals Atomic Crystals Lattice points occupied by atoms Held together by covalent bonds Hard, high melting point Poor conductor of heat and electricity carbon atoms diamond Copyright©2000 by Houghton Mifflin Company. All rights reserved. graphite

Packing in Metals Model: Packing uniform, hard spheres to best use available space. This is called closest packing. Each atom has 12 nearest neighbors. hexagonal closest packed (“aba”) cubic closest packed (“abc”) Copyright©2000 by Houghton Mifflin Company. All rights reserved.

When spheres are closest packed so that the spheres in the third layer are directly over those in the first layer (aba), the unit cell is the hexagonal prism illustrated here in red. Copyright©2000 by Houghton Mifflin Company. All rights reserved.

When silver crystallizes, it forms face-centered cubic cells. The unit cell edge length is 409 pm. Calculate the density of silver. d = m V V = a3 = (409 pm)3 = 6.83 x cm3 4 atoms/unit cell in a face-centered cubic cell 107.9 g mole Ag x 1 mole Ag 6.022 x 1023 atoms x m = 4 Ag atoms = 7.17 x g d = m V 7.17 x g 6.83 x cm3 = = 10.5 g/cm3 Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Types of Crystals Molecular Crystals Lattice points occupied by molecules Held together by intermolecular forces Soft, low melting point Poor conductor of heat and electricity Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Types of Crystals Metallic Crystals
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 Cross Section of a Metallic Crystal nucleus & inner shell e- mobile “sea” of e- Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Bonding Models for Metals
Electron Sea Model: A regular array of metals in a “sea” of electrons. Band (Molecular Orbital) Model: Electrons assumed to travel around metal crystal in MOs formed from valence atomic orbitals of metal atoms. Copyright©2000 by Houghton Mifflin Company. All rights reserved.

The electron sea model for metals postulates a regular array of cations in a "sea" of valence electrons. (a) Representation of an alkali metal (Group 1A) with one valence electron. (b) Representation of an alkaline earth metal (Group 2A) with two valence electrons. Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Metal Alloys Substances that have a mixture of elements and metallic properties. 1. Substitutional Alloy: some metal atoms replaced by others of similar size. brass = Cu/Zn Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Metal Alloys (continued)
2. Interstitial Alloy: Interstices (holes) in closest packed metal structure are occupied by small atoms. steel = iron + carbon 3. Both types: Alloy steels contain a mix of substitutional (carbon) and interstitial (Cr, Mo) alloys. Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Network Solids Composed of strong directional covalent bonds that are best viewed as a “giant molecule”. brittle do not conduct heat or electricity carbon, silicon-based graphite, diamond, ceramics, glass Copyright©2000 by Houghton Mifflin Company. All rights reserved.

The p orbitals (a) perpendicular to the plane of the carbon ring system in graphite can combine to form (b) an extensive π-bonding network. Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Graphite consists of layers of carbon atoms.

Computer-generated model of silica.

Semiconductors A substance in which some electrons can cross the band gap. Conductivity is enhanced by doping with group 3a or group 5a elements. Copyright©2000 by Houghton Mifflin Company. All rights reserved.

(a) A silicon crystal doped with arsenic, which has one more valence electron than silicon. (b) A silicon crystal doped with boron, which has one less electron than silicon. Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Molecular Solids Discrete molecular units at each lattice position e.g. ice, solid CO2, S8, P4. Covalent bond within molecule while weak dipole-dipole or London dispersion forces e.g. Ice (Hydrogen bond), CO2, P4, S8 (LD) very reactive. As the size of the molecule increases, the LD become large causing many substances to be solids at 250C. Copyright©2000 by Houghton Mifflin Company. All rights reserved.

A “steaming” piece of dry ice CO2

(a) Sulfur crystals (yellow) contain S8 molecules. (b) White phosphorus (containing P4 molecules) is so reactive with the oxygen in air that it must be stored under water. Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Ionic Solids Ionic solids are stable, high melting point and held ions together by stron electrostatic forces between opposite ions. Structure of most binary ionic solids (NaCl) can be explained by the closest packing of spheres Anion (large ion) packed in one of the closest packing (hcp or ccp) Cation (small ion) fits in holes among the anions. Copyright©2000 by Houghton Mifflin Company. All rights reserved.

The holes that exist among closest packed uniform spheres. (a) The trigonal hole formed by three spheres in a given plane. (b) The tetrahedral hole formed when a sphere occupies a dimple formed by three spheres in an adjacent layer. (c) The octahedral hole formed by six spheres in two adjacent layers. Copyright©2000 by Houghton Mifflin Company. All rights reserved.

(a) The location (X) of a tetrahedral hole in the face-centered cubic unit cell. (b) One of the tetrahedral holes. (c) The unit cell for ZnS where the S2- ions (yellow) are closest packed with the Zn2+ ions (red) in alternating tetrahedral holes. Copyright©2000 by Houghton Mifflin Company. All rights reserved.

(a) The locations (gray X) of the octahedral holes in the face-centered cubic unit cell. (b) Representation of the unit cell for solid NaCl. Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Vapor Pressure . . . is the pressure of the vapor present at equilibrium. . . . is determined principally by the size of the intermolecular forces in the liquid. . . . increases significantly with temperature. Volatile liquids have high vapor pressures. Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Behavior of a liquid in a closed container.

The equilibrium vapor pressure is the vapor pressure measured when a dynamic equilibrium exists between condensation and evaporation H2O (l) H2O (g) Rate of condensation evaporation = Dynamic Equilibrium Copyright©2000 by Houghton Mifflin Company. All rights reserved.

(a) The vapor pressure of a liquid can be measured easily using a simple barometer of the type shown here. (b) The three liquids, water, ethanol (C2H5OH), and diethyl ether [(C2H5)2O], have quite different vapor pressures. Copyright©2000 by Houghton Mifflin Company. All rights reserved.

(a) The vapor pressure of water, ethanol, and diethyl ether as a function of temperature. (b) Plots of In(Pvap) versus 1/T (Kelvin temperature) for water, ethanol, and diethyl ether. Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Molar heat of vaporization (DHvap) is the energy required to vaporize 1 mole of a liquid. ln P = - DHvap RT + C Clausius-Clapeyron Equation P = (equilibrium) vapor pressure T = temperature (K) R = gas constant (8.314 J/K•mol) Copyright©2000 by Houghton Mifflin Company. All rights reserved.

The boiling point is the temperature at which the (equilibrium) vapor pressure of a liquid is equal to the external pressure. The normal boiling point is the temperature at which a liquid boils when the external pressure is 1 atm. Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Melting Point Molecules break loose from lattice points and solid changes to liquid. (Temperature is constant as melting occurs.) vapor pressure of solid = vapor pressure of liquid Copyright©2000 by Houghton Mifflin Company. All rights reserved.

H2O (s) H2O (l) The melting point of a solid or the freezing point of a liquid is the temperature at which the solid and liquid phases coexist in equilibrium Melting Freezing Copyright©2000 by Houghton Mifflin Company. All rights reserved. 11.8

The supercooling of water. The extent of supercooling is given by S.

H2O (s) H2O (g) Molar heat of sublimation (DHsub) is the energy required to sublime 1 mole of a solid. Sublimation Deposition DHsub = DHfus + DHvap ( Hess’s Law) Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Phase Diagram Represents phases as a function of temperature and pressure. critical temperature: temperature above which the vapor can not be liquefied. critical pressure: pressure required to liquefy AT the critical temperature. critical point: critical temperature and pressure (for water, Tc = 374°C and 218 atm). Copyright©2000 by Houghton Mifflin Company. All rights reserved.

The phase diagram for water.

The critical temperature (Tc) is the temperature above which the gas cannot be made to liquefy, no matter how great the applied pressure. The critical pressure (Pc) is the minimum pressure that must be applied to bring about liquefaction at the critical temperature. Copyright©2000 by Houghton Mifflin Company. All rights reserved.

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