Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Chapter Eleven 1 States of Matter and Intermolecular Forces Chapter Ten

Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Chapter Eleven 2 Intramolecular forces (bonds) govern molecular properties such as molecular geometries and dipole moments. Intermolecular forces determine the macroscopic physical properties of liquids and solids. This chapter: –describes changes from one state of matter to another. –explores the types of intermolecular forces that underlie these and other physical properties of substances. Chapter Preview

Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Chapter Eleven 3 Molecular Forces Compared

Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Chapter Eleven 4 States of Matter Compared Intermolecular forces are of little significance; why? Intermolecular forces must be considered. Intermolecular forces are very important.

Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Chapter Eleven 5 Vapor Pressure The vapor pressure of a liquid is the partial pressure exerted by the vapor when it is in dynamic equilibrium with the liquid at a constant temperature. vaporization LiquidVapor condensation

Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Chapter Eleven 6 Liquid–Vapor Equilibrium More vapor forms; rate of condensation of that vapor increases … … until equilibrium is attained.

Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Chapter Eleven 7 Vapor Pressure Curves Which of the five compounds has the strongest intermolecular forces? How can you tell? CS 2 CH 3 OH CH 3 CH 2 OH H2OH2O C 6 H 5 NH 2 What is the vapor pressure of H 2 O at 100 °C, according to this graph? What is the significance of that numeric value of vapor pressure?

Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Chapter Eleven 8 Boiling Point and Critical Point Boiling point: the temperature at which the vapor pressure of the liquid equals the external pressure. Normal boiling point: boiling point at 1 atm. Critical temperature (T c ): the highest temperature at which a liquid can exist. The critical pressure, P c, is the vapor pressure at the critical temperature. The condition corresponding to a temperature of T c and a pressure of P c is called the critical point.

Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Chapter Eleven 9 Phase Changes Involving Solids Melting (fusion): transition of solid  liquid. Melting point: temperature at which melting occurs. –Same as freezing point! Enthalpy of fusion,  H fusion, is the quantity of heat required to melt a set amount (one gram, one mole) of solid. Sublimation: transition of solid  vapor. –Example: Ice cubes slowly “disappear” in the freezer. Enthalpy of sublimation,  H subln, is the sum of the enthalpies of fusion and vaporization. Triple point: all three phases—solid, liquid, vapor—are in equilibrium.

Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Chapter Eleven 10 Phase Diagrams A phase diagram is a graphical representation of the conditions of temperature and pressure under which a substance exists as a solid, liquid, a gas, or some combination of these in equilibrium. A—B, solid-vapor equilibrium. A—D, solid-liquid equilibrium. A—C, liquid-vapor equilibrium. Triple point

Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Chapter Eleven 11 Phase Diagram for HgI 2 HgI 2 has two solid phases, red and yellow. As the vessel is allowed to cool, will the contents appear more red or more yellow?

Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Chapter Eleven 12 Phase Diagram for CO 2 Note that at 1 atm, only the solid and vapor phases of CO 2 exist.

Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Chapter Eleven 13 Phase Diagram for H 2 O

Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Chapter Eleven 14 Intermolecular Forces … are forces between molecules. They determine melting points, freezing points, and other physical properties. Types of intermolecular forces include: dispersion forces Dipole–dipole forces hydrogen bonding

Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Chapter Eleven 15 Dispersion Forces … exist between any two particles. Also called London forces (after Fritz London, who offered a theoretical explanation of these forces in 1928). Dispersion forces arise because the electron cloud is not perfectly uniform. Tiny, momentary dipole moments can exist even in nonpolar molecules.

Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Chapter Eleven 16 Dispersion Forces Illustrated (1) At a given instant, electron density, even in a nonpolar molecule like this one, is not perfectly uniform.

Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Chapter Eleven 17 Dispersion Forces Illustrated (2) The region of (momentary) higher electron density attains a small (–) charge … When another nonpolar molecule approaches … … the other end of the molecule is slightly (+).

Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Chapter Eleven 18 Dispersion Forces Illustrated (3) … this molecule induces a tiny dipole moment … … in this molecule. Opposite charges ________.

Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Chapter Eleven 19 Strength of Dispersion Forces Dispersion force strength depends on polarizability: the ease with which the electron cloud is distorted by an external electrical field. The greater the polarizability of molecules, the stronger the dispersion forces between them. Polarizability in turn depends on molecular size and shape. Heavier molecule => more electrons => a more- polarizable molecule. As to molecular shape …

Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Chapter Eleven 20 Molecular Shape and Polarizability Long skinny molecule … … can have greater separation of charge along its length. Stronger forces of attraction, meaning … … higher boiling point. In the compact isomer, less possible separation of charge … … giving weaker dispersion forces and a lower boiling point.

Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Chapter Eleven 21 Dipole–Dipole Forces A polar molecule has a positively charged “end” (δ+) and a negatively charged “end” (δ–). When molecules come close to one another, repulsions occur between like-charged regions of dipoles. Opposite charges tend to attract one another. The more polar a molecule, the more pronounced is the effect of dipole–dipole forces on physical properties.

Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Chapter Eleven 22 Dipole–Dipole Interactions Opposites attract!

Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Chapter Eleven 23 Predicting Physical Properties of Molecular Substances Dispersion forces become stronger with increasing molar mass and elongation of molecules. In comparing nonpolar substances, molar mass and molecular shape are the essential factors. Dipole–dipole and dipole-induced dipole forces are found in polar substances. The more polar the substance, the greater the intermolecular force is expected to be. Because they occur in all substances, dispersion forces must always be considered. Often they predominate.

Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Chapter Eleven 24 Homology A series of compounds whose formulas and structures vary in a regular manner also have properties that vary in a predictable manner. This principle is called homology. Example: both densities and boiling points of the straight-chain alkanes increase in a continuous and regular fashion with increasing numbers of carbon atoms in the chain. Trends result from the regular increase in molar mass, which produces a fairly regular increase in the strength of dispersion forces.

Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Chapter Eleven 25 Hydrogen Bonds A hydrogen bond is an intermolecular force in which: –a hydrogen atom that is covalently bonded to a (small, electronegative) nonmetal atom in one molecule … –is simultaneously attracted to a (small, electronegative) nonmetal atom of a neighboring molecule. Y ––– H Z ~~~~ When Y and Z are small and highly electronegative (N, O, F) … … this force is called a hydrogen bond; a special, strong type of dipole– dipole force.

Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Chapter Eleven 26 Hydrogen Bonds in Water

Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Chapter Eleven 27 Hydrogen Bonding in Ice Hydrogen bonding arranges the water molecules into an open hexagonal pattern. “Hexagonal” is reflected in the crystal structure. “Open” means reduced density of the solid (vs. liquid).

Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Chapter Eleven 28 Hydrogen Bonding in Acetic Acid Hydrogen bonding occurs between molecules.

Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Chapter Eleven 29 Hydrogen Bonding in Salicylic Acid Hydrogen bonding occurs within the molecule.

Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Chapter Eleven 30 Intermolecular Hydrogen Bonds Intermolecular hydrogen bonds give proteins their secondary shape, forcing the protein molecules into particular orientations, like a folded sheet …

Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Chapter Eleven 31 Intramolecular Hydrogen Bonds … while intramolecular hydrogen bonds can cause proteins to take a helical shape.

Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Chapter Eleven 32 Liquids and Intermolecular Forces Much behavior and many properties of liquids can be attributed to intermolecular forces. Surface tension (  ) is the amount of work required to extend a liquid surface and is usually expressed in J/m 2. Adhesive forces are intermolecular forces between unlike molecules. Cohesive forces are intermolecular forces between like molecules. A meniscus is the interface between a liquid and the air above it. Viscosity is a measure of a liquid’s resistance to flow.

Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Chapter Eleven 33 Surface Tension To create more surface, the molecules at the surface must be separated from one another.

Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Chapter Eleven 34 Adhesive and Cohesive Forces The liquid spreads, because adhesive forces are comparable in strength to cohesive forces. The liquid “beads up.” Which forces are stronger, adhesive or cohesive?

Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Chapter Eleven 35 Meniscus Formation Water wets the glass (adhesive forces) and its attraction for glass forms a concave- up surface. What conclusion can we draw about the cohesive forces in mercury?

Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Chapter Eleven 36 Capillary Action The adhesive forces wet more and more of the inside of the tiny tube, drawing water farther up the tube.

Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Chapter Eleven 37 Comparing Viscosity Which oil flows more readily? Which oil has stronger intermolecular forces between its molecules? Oil is mostly hydrocarbons; what kind of forces are these?

Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Chapter Eleven 38 Types of Solids Amorphous solids have no significant long-range order. Crystalline solids have atoms/ions/molecules arranged in a regular pattern. Types of crystalline solids include: –Molecular solids, containing molecules held to one another by dispersion/dipole–dipole/ hydrogen bonding forces. –Network (covalent) solids. –Ionic solids. –Metallic solids (metals).

Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Chapter Eleven 39 Network Covalent Solids Network solids have a network of covalent bonds that extend throughout the solid, holding it firmly together. The allotropes of carbon provide good examples. –Diamond has each carbon bonded to four other carbons in a tetrahedral arrangement using sp 3 hybridization. –Graphite has each carbon bonded to three other carbons in the same plane using sp 2 hybridization. –Fullerenes are roughly spherical collections of carbon atoms in the shape of a soccer ball. –A nanotube can be thought of as a plane of graphite rolled into a tube.

Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Chapter Eleven 40 Crystal Structure of Diamond Three-dimensional network is extremely strong, rigid. What kind of forces must be overcome to melt diamond?

Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Chapter Eleven 41 Crystal Structure of Graphite Forces between layers are relatively weak. Hexagons of sp 2 - hybridized carbon atoms.

Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Chapter Eleven 42 Structure of a Buckyball C 60 molecule

Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Chapter Eleven 43 Structure of a Nanotube A nanotube can be thought of as a sheet of graphite, rolled into a tube, capped with half of a buckyball.

Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Chapter Eleven 44

Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Chapter Eleven 45 Ionic Bonds as “Intermolecular” Forces There are no molecules in an ionic solid, and therefore there can’t be any intermolecular forces. The attractions are electrostatic interionic attractions. Lattice energy (Chapter 9) is a measure of the strength of interionic attraction. The attractive force between a pair of oppositely charged ions increases: –as the charges on the ions increase. –as the ionic radii decrease. Lattice energies increase accordingly.

Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Chapter Eleven 46 Interionic Forces of Attraction Melting point of NaCl is about 801 o C. Mg 2+ and O 2– have much stronger forces of attraction for one another than do Na + and Cl –. Melting point of MgO is about 2800 o C.

Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Chapter Eleven 47 Crystal Lattices To describe crystals, three-dimensional views must be used. The repeating unit of the lattice is called the unit cell. There are a number of different types of unit cell; hexagonal, rhombic, cubic, etc. The three types of cubic unit cells are: simple cubic, body-centered cubic (bcc), face-centered cubic (fcc).

Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Chapter Eleven 48 Cubic Unit Cells The unit cell is a cube in each case. Whole atoms shown for clarity.

Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Chapter Eleven 49 Close-Packed Structures A close packed structure in two dimensions.

Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Chapter Eleven 50 Close Packing in Three Dimensions Two layers, stacked, give two different locations for the third layer … Third layer directly above first layer: HCP Third layer over the octahedral holes in the second layer: CCP

Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Chapter Eleven 51 Ionic Crystal Structures Ionic crystals have two different types of structural units— cations and anions. The cations and anions ordinarily are different sizes. Smaller cations can fill the voids between the larger anions. Where the cations go depends on the size of the cations and on the size of the voids. The smallest voids are the tetrahedral holes, then the octahedral holes, and finally the holes in a cubic structure. Therefore …

Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Chapter Eleven 52 Ionic Crystal Structures Tetrahedral hole filling occurs when the cations are small; when the radii ratio is: < r c /r a < Octahedral filling occurs with larger cations, when the radii ratio is: < r c /r a < The arrangement is cubic if r c /r a >

Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Chapter Eleven 53 Unit Cell of Cesium Chloride How many cesium ions are inside this unit cell? How many chloride ions?

Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Chapter Eleven 54 Unit Cell of Sodium Chloride How many sodium ions are inside this unit cell? How many chloride ions?

Hall © 2005 Prentice Hall © 2005 General Chemistry 4 th edition, Hill, Petrucci, McCreary, Perry Chapter Eleven 55 Experimental Determination of Crystal Structures X rays Angle of diffraction can be used to find distance d, using simple trigonometry.