1. 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.

2. 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
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Chapter 11: States of Matter and Intermolecular Forces

3. 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
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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

4. 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
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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

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

6. 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.
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As T  vapor pressure 
Chapter 11: States of Matter and Intermolecular Forces

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

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

9. 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
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Chapter 11: States of Matter and Intermolecular Forces

10. 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
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Chapter 11: States of Matter and Intermolecular Forces

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

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

13. 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
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DHsubln = DHfusion + DHvap
Chapter 11: States of Matter and Intermolecular Forces

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

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

16. 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
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critical point
fusion curve
vapor pressure curve
triple point
sublimation curve
Chapter 11: States of Matter and Intermolecular Forces

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

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

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

20. 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
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Dipoles can be induced in molecules
Chapter 11: States of Matter and Intermolecular Forces

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

22. 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
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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

23. 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
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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

24. 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
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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

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

26. Hydrogen Bonding Effects
Solid water is less dense than liquid water due to hydrogen bonding
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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

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

28. 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
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Chapter 11: States of Matter and Intermolecular Forces

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

30. 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
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Viscosity units are poise (P)
Typical liquids have viscosities measured in cP
Chapter 11: States of Matter and Intermolecular Forces

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

32. 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
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Sulfur and carbon are examples of elements that form allotropes
Chapter 11: States of Matter and Intermolecular Forces

33. 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
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Graphite has each carbon bonded to three other carbons in the same plane using sp2 hybridization
Chapter 11: States of Matter and Intermolecular Forces

34. 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
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Chapter 11: States of Matter and Intermolecular Forces

35. 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
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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

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

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

38. 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
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Chapter 11: States of Matter and Intermolecular Forces

39. Close-Packed Structures
Hexagonal close-packed (hcp) arrangements occur when the third layer covers the tetrahedral holes. These produce two-layer repeating units
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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

40. 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
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The arrangement is cubic if rc/ra > 0.732
Chapter 11: States of Matter and Intermolecular Forces

41. Chapter 11: States of Matter and Intermolecular Forces
Two Examples
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Chapter 11: States of Matter and Intermolecular Forces

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

43. 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
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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

44. 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
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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