Chapter 11 States of Matter and Intermolecular Forces General Chemistry: An Integrated Approach Hill, Petrucci, 4th Edition Chapter 11 States of Matter and Intermolecular Forces Mark P. Heitz State University of New York at Brockport © 2005, Prentice Hall, Inc.

Intermolecular Forces Intramolecular forces determine such molecular properties as molecular geometries and dipole moments Intermolecular forces determine the macroscopic physical properties of liquids and solids EOS Chapter 11: States of Matter and Intermolecular Forces

Intermolecular Forces Gases – fill container, random rapid motion, never coming to rest or clumping together Motion is mainly translational Liquids – fixed volume, flow and assume shape of container, only slightly compressible, stronger forces hold molecules together Motion is mainly translational EOS Solids – fixed volume, definite shape, generally less compressible than liquids, forces hold particles in a fixed shape Motion is mainly vibrational Chapter 11: States of Matter and Intermolecular Forces

Vaporization and Condensation Vaporization is the conversion of a liquid to a gas The enthalpy of vaporization (DHvapn) is the quantity of heat that must be absorbed to vaporize a given amount of liquid at a constant temperature H2O(l)  H2O(g) = +44 kJ mol–1 EOS Condensation (DHcondn) is the change of a gas to a liquid H2O(g)  H2O(l) = –44 kJ mol–1 Chapter 11: States of Matter and Intermolecular Forces

Enthalpies of Vaporization Because enthalpy is a function of state, the total enthalpy change between vaporization and condensation at constant temperature must be zero EOS Chapter 11: States of Matter and Intermolecular Forces

Chapter 11: States of Matter and Intermolecular Forces Vapor Pressure The vapor pressure of a liquid is the partial pressure exerted by the vapor when it is in dynamic equilibrium with a liquid at a constant temperature. EOS As T  vapor pressure  Chapter 11: States of Matter and Intermolecular Forces

Vapor Pressure of Water EOS Chapter 11: States of Matter and Intermolecular Forces

Vapor Pressure as a Function of Temperature Illustration EOS Chapter 11: States of Matter and Intermolecular Forces

Chapter 11: States of Matter and Intermolecular Forces Boiling Point The boiling point of a liquid is the temperature at which its vapor pressure becomes equal to the external pressure The normal boiling point is the boiling point at 1 atm – e.g., water boils at 100 oC At reduced pressure, the normal boiling temperature lowers EOS Chapter 11: States of Matter and Intermolecular Forces

Chapter 11: States of Matter and Intermolecular Forces Critical Point The critical temperature, Tc, is the highest temperature at which a liquid and vapor can coexist in equilibrium as physically distinct states of matter The critical pressure, Pc, is the vapor pressure at the critical temperature The condition corresponding to a temperature of Tc and a pressure of Pc is called the critical point EOS Chapter 11: States of Matter and Intermolecular Forces

Chapter 11: States of Matter and Intermolecular Forces The Critical Point EOS Chapter 11: States of Matter and Intermolecular Forces

Critical Temperature and Pressure of Various Substances Substances existing beyond the critical point are termed supercritical fluids EOS Chapter 11: States of Matter and Intermolecular Forces

Phase Changes Involving Solids The conversion of a solid to a liquid is called melting, or fusion, and the temperature at which a solid melts is its melting point The enthalpy of fusion, DHfusion, is the quantity of heat required to melt a given amount of solid Sublimation is the process of a molecule’s passing directly from the solid to the vapor state EOS DHsubln = DHfusion + DHvap Chapter 11: States of Matter and Intermolecular Forces

Some Enthalpies of Fusion EOS Chapter 11: States of Matter and Intermolecular Forces

Heating and Cooling Curves EOS Chapter 11: States of Matter and Intermolecular Forces

Chapter 11: States of Matter and Intermolecular Forces Phase Diagrams A phase diagram is a graphical representation of the temperature and pressure conditions under which a substance exists as a solid, liquid, gas, or some combination of these in equilibrium EOS critical point fusion curve vapor pressure curve triple point sublimation curve Chapter 11: States of Matter and Intermolecular Forces

Chapter 11: States of Matter and Intermolecular Forces Phase Diagram for HgI2 EOS Chapter 11: States of Matter and Intermolecular Forces

Chapter 11: States of Matter and Intermolecular Forces Phase Diagram for CO2 EOS Chapter 11: States of Matter and Intermolecular Forces

Chapter 11: States of Matter and Intermolecular Forces Phase Diagram for H2O EOS Chapter 11: States of Matter and Intermolecular Forces

Chapter 11: States of Matter and Intermolecular Forces Dispersion Forces Dispersion forces are forces of attraction between an instantaneous dipole and an induced dipole … also called London forces after Fritz London who offered a theoretical explanation in 1928 The polarizability of an atom or molecule is a measure of the ease with which electron charge density is distorted by an external electrical field EOS Dipoles can be induced in molecules Chapter 11: States of Matter and Intermolecular Forces

Chapter 11: States of Matter and Intermolecular Forces Dispersion Forces The greater the polarizability of molecules, the stronger the intermolecular forces between them EOS Chapter 11: States of Matter and Intermolecular Forces

Chapter 11: States of Matter and Intermolecular Forces Dipole–Dipole Forces Dipole–dipole forces arise when permanent dipoles align themselves with the positive end of one dipole directed toward the negative ends of neighboring dipoles EOS A permanent dipole in one molecule can induce a dipole in a neighboring molecule, giving rise to a dipole–induced dipole force Chapter 11: States of Matter and Intermolecular Forces

Predicting Physical Properties of Molecular Substances Dispersion forces (always present) become stronger with increasing molar mass and elongation of molecules In comparing nonpolar substances, molar mass and molecular shape are the essential influencing factors EOS Dipole–dipole and dipole–induced dipole forces are found in polar substances. The more polar the substance (dipole moment), the greater the intermolecular force is expected to be Chapter 11: States of Matter and Intermolecular Forces

Chapter 11: States of Matter and Intermolecular Forces Hydrogen Bonds A hydrogen bond is an intermolecular force in which a hydrogen atom covalently bonded to a nonmetal atom in one molecule is simultaneously attracted to a nonmetal atom of a neighboring molecule EOS The strongest hydrogen bonds are formed if the nonmetal atoms are small and highly electronegative – e.g., N, O, F Chapter 11: States of Matter and Intermolecular Forces

Hydrogen Bonding in Ice EOS Chapter 11: States of Matter and Intermolecular Forces

Hydrogen Bonding Effects Solid water is less dense than liquid water due to hydrogen bonding EOS The structures of proteins, substances essential to life, are determined partly by hydrogen bonding Hydrogen bonding is also the reason for the unusually high boiling point of water Proteins Chapter 11: States of Matter and Intermolecular Forces

Hydrogen Bonding Effects Many organic acids can form dimers due to hydrogen bonding Certain organic molecules can also form an intramolecular hydrogen bond EOS Chapter 11: States of Matter and Intermolecular Forces

Chapter 11: States of Matter and Intermolecular Forces Surface Tension Surface tension (g) is the amount of work required to extend a liquid surface and is usually expressed in unit of J m–2 Adhesive forces are intermolecular forces between unlike molecules Cohesive forces are intermolecular forces between like molecules EOS Chapter 11: States of Matter and Intermolecular Forces

Another Surface Phenomenon A meniscus is the interface between a liquid and the air above it EOS Capillary action results from intermolecular interactions Chapter 11: States of Matter and Intermolecular Forces

Chapter 11: States of Matter and Intermolecular Forces Viscosity Viscosity is a measure of a liquid’s resistance to flow Liquids which flow easily are said to be mobile EOS Viscosity units are poise (P) Typical liquids have viscosities measured in cP Chapter 11: States of Matter and Intermolecular Forces

Characteristics of Crystalline Solids EOS Chapter 11: States of Matter and Intermolecular Forces

Network Covalent Solids These substances contain a network of covalent bonds that extend throughout a crystalline solid, holding it firmly together Allotropes are two or more forms of an element that differ in basic molecular structure EOS Sulfur and carbon are examples of elements that form allotropes Chapter 11: States of Matter and Intermolecular Forces

Chapter 11: States of Matter and Intermolecular Forces Allotropes of Carbon Diamond has each carbon bonded to four other carbons in a tetrahedral arrangement using sp3 hybridization EOS Graphite has each carbon bonded to three other carbons in the same plane using sp2 hybridization Chapter 11: States of Matter and Intermolecular Forces

Chapter 11: States of Matter and Intermolecular Forces Crystal Lattices Crystals have repeating units … … in multidimensional space The repeating unit of the lattice is called the unit cell EOS Chapter 11: States of Matter and Intermolecular Forces

Chapter 11: States of Matter and Intermolecular Forces Unit Cells The simple cubic cell is the simplest unit cell and has structural particles centered only at its corners The body-centered cubic (bcc) structure has an additional structural particle at the center of the cube EOS The face-centered cubic (fcc) structure has an additional structural particle at the center of each face Chapter 11: States of Matter and Intermolecular Forces

Unit Cells – Coordination Number The coordination number is the number of nearest neighbor particles to a single particle in the crystal EOS In a simple cubic structure, the coordination number is 6 Chapter 11: States of Matter and Intermolecular Forces

Chapter 11: States of Matter and Intermolecular Forces Packing in Crystals “Open” packing has larger voids in between particles compared to close-packed crystals EOS Chapter 11: States of Matter and Intermolecular Forces

Close-Packed Structures Tetrahedral holes are located above a sphere in the bottom layer Octahedral holes are located above a void in the bottom layer EOS Chapter 11: States of Matter and Intermolecular Forces

Close-Packed Structures Hexagonal close-packed (hcp) arrangements occur when the third layer covers the tetrahedral holes. These produce two-layer repeating units EOS Cubic close-packed (ccp) arrangements occur when the third layer covers the octahedral holes. These produce three-layer repeating units Chapter 11: States of Matter and Intermolecular Forces

Ionic Crystal Structures Smaller cations can fill the voids between the larger anions Tetrahedral hole filling occurs when the radii ratio is: 0.225 < rc/ra < 0.414 Octahedral hole filling occurs when the radii ratio is: 0.414 < rc/ra < 0.732 EOS The arrangement is cubic if rc/ra > 0.732 Chapter 11: States of Matter and Intermolecular Forces

Chapter 11: States of Matter and Intermolecular Forces Two Examples EOS Chapter 11: States of Matter and Intermolecular Forces

Experimental Determination of Crystal Structures EOS nl = 2d sinq Chapter 11: States of Matter and Intermolecular Forces

Chapter 11: States of Matter and Intermolecular Forces Summary of Concepts Vaporization is the passage of molecules from the liquid to the gaseous state A phase diagram is a plot showing the phases of a substance at various temperatures and pressures Fluctuations in electrical charge produce an instantaneous dipole, which induces a dipole in neighboring atoms and are called dispersion forces EOS A strong intermolecular force called the hydrogen bond is formed when a hydrogen atom is attracted to a N, O, or F atom on a neighboring molecule Chapter 11: States of Matter and Intermolecular Forces

Chapter 11: States of Matter and Intermolecular Forces Summary of Concepts The intermolecular forces in a liquid determine its viscosity, or resistance to flow In some solids, covalent bonds extend throughout a crystal In ionic solids, interionic attractions bind all the ions together in a crystal The structure of a crystal is described by a three-dimensional pattern called a lattice EOS Unit cell properties and dimensions can be used to find atomic radii and the densities of crystalline substances Chapter 11: States of Matter and Intermolecular Forces