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Chapter 11States of Matter and
Intermolecular Forces
General Chemistry: An Integrated Approach
Hill, Petrucci, 4th Edition
Mark P. HeitzState University of New York at Brockport
© 2005, Prentice Hall, Inc.
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Chapter 11: States of Matter and Intermolecular Forces 2
Intermolecular Forces
Intramolecular forces determine such molecular properties as molecular geometries and dipole moments
EOS
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 ForcesGases – fill container, random rapid motion, never coming to rest or clumping togetherMotion is mainly translational
Liquids – fixed volume, flow and assume shape of container, only slightly compressible, stronger forces hold molecules togetherMotion is mainly translational
EOS
Solids – fixed volume, definite shape, generally less compressible than liquids, forces hold particles in a fixed shapeMotion is mainly vibrational
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Chapter 11: States of Matter and Intermolecular Forces 4
Vaporization and CondensationVaporization is the conversion of a liquid to a gas
The enthalpy of vaporization (Hvapn) 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 (Hcondn) is the change of a gas to a liquid
H2O(g) H2O(l) = –44 kJ mol–1
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Chapter 11: States of Matter and Intermolecular Forces 5
Enthalpies of VaporizationBecause enthalpy is a function of state, the total enthalpy change between vaporization and condensation at constant temperature must be zero
EOS
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Chapter 11: States of Matter and Intermolecular Forces 6
Vapor PressureThe 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
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Chapter 11: States of Matter and Intermolecular Forces 7
Vapor Pressure of Water
EOS
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Chapter 11: States of Matter and Intermolecular Forces 8
Vapor Pressure as a Function of Temperature
EOS
Illustration
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Chapter 11: States of Matter and Intermolecular Forces 9
Boiling PointThe boiling point of a liquid is the temperature at which its vapor pressure becomes equal to the external pressure
EOS
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
Critical PointThe 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
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Chapter 11: States of Matter and Intermolecular Forces 11
The Critical Point
EOS
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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
EOS
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Chapter 11: States of Matter and Intermolecular Forces 13
Phase Changes Involving SolidsThe 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, Hfusion, 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
Hsubln = Hfusion + Hvap
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Chapter 11: States of Matter and Intermolecular Forces 14
Some Enthalpies of Fusion
EOS
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Chapter 11: States of Matter and Intermolecular Forces 15
Heating and Cooling Curves
EOS
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Chapter 11: States of Matter and Intermolecular Forces 16
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
Phase Diagrams
fusion curve
triple point
EOS
critical point
vapor pressure curve
sublimation curve
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Chapter 11: States of Matter and Intermolecular Forces 17
Phase Diagram for HgI2
EOS
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Chapter 11: States of Matter and Intermolecular Forces 18
Phase Diagram for CO2
EOS
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Chapter 11: States of Matter and Intermolecular Forces 19
Phase Diagram for H2O
EOS
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Chapter 11: States of Matter and Intermolecular Forces 20
Dispersion ForcesDispersion 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
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Chapter 11: States of Matter and Intermolecular Forces 21
Dispersion ForcesThe greater the polarizability of molecules, the stronger the intermolecular forces between them
EOS
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Chapter 11: States of Matter and Intermolecular Forces 22
Dipole–Dipole ForcesDipole–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
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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 moleculesIn 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
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Chapter 11: States of Matter and Intermolecular Forces 24
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
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Chapter 11: States of Matter and Intermolecular Forces 25
Hydrogen Bonding in Ice
EOS
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Chapter 11: States of Matter and Intermolecular Forces 26
Hydrogen Bonding EffectsSolid water is less dense than liquid water due to hydrogen bonding
Hydrogen bonding is also the reason for the unusually high boiling point of water
EOS
The structures of proteins, substances essential to life, are determined partly by hydrogen bonding Proteins
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Chapter 11: States of Matter and Intermolecular Forces 27
Hydrogen Bonding EffectsMany organic acids can form dimers due to hydrogen bonding
Certain organic molecules can also form an intramolecular hydrogen bond
EOS
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Chapter 11: States of Matter and Intermolecular Forces 28
Surface Tension
Surface tension () is the amount of work required to extend a liquid surface and is usually expressed in unit of J m–2
EOS
Adhesive forces are intermolecular forces between unlike moleculesCohesive forces are intermolecular forces between like molecules
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Chapter 11: States of Matter and Intermolecular Forces 29
Another Surface PhenomenonA meniscus is the interface between a liquid and the air above it
EOS
Capillary action results from intermolecular interactions
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Chapter 11: States of Matter and Intermolecular Forces 30
Viscosity
Liquids which flow easily are said to be mobile
EOS
Viscosity units are poise (P)
Typical liquids have viscosities measured in cP
Viscosity is a measure of a liquid’s resistance to flow
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Chapter 11: States of Matter and Intermolecular Forces 31
Characteristics of Crystalline Solids
EOS
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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
EOS
Sulfur and carbon are examples of elements that form allotropes
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Chapter 11: States of Matter and Intermolecular Forces 33
Allotropes of CarbonDiamond 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
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Chapter 11: States of Matter and Intermolecular Forces 34
Crystal Lattices
The repeating unit of the lattice is called the unit cell
EOS
Crystals have repeating units …
… in multidimensional space
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Chapter 11: States of Matter and Intermolecular Forces 35
Unit CellsThe 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
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Chapter 11: States of Matter and Intermolecular Forces 36
Unit Cells – Coordination NumberThe 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
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Chapter 11: States of Matter and Intermolecular Forces 37
Packing in Crystals
“Open” packing has larger voids in between particles compared to close-packed crystals
EOS
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Chapter 11: States of Matter and Intermolecular Forces 38
Close-Packed StructuresTetrahedral holes are located above a sphere in the bottom layer
EOS
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 StructuresHexagonal 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
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Chapter 11: States of Matter and Intermolecular Forces 40
Ionic Crystal StructuresSmaller 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
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Chapter 11: States of Matter and Intermolecular Forces 41
Two Examples
EOS
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Chapter 11: States of Matter and Intermolecular Forces 42
Experimental Determinationof Crystal Structures
EOS
n = 2d sin
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Chapter 11: States of Matter and Intermolecular Forces 43
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
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Chapter 11: States of Matter and Intermolecular Forces 44
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