Real Gases

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Real Gases

• Under some conditions, real gases do not follow
  the ideal gas law.
• Intermolecular forces of attraction cause the
  measured pressure of a real gas to be less than
  expected.
• When molecules are close together, the volume of
  the molecules themselves becomes a significant
  fraction of the total volume of a gas.

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Real Gases

• Ideal gas equation (ideal gases)
• P (V ) nRT
• van der Waals equation (real gases)
• P (n2a)/V2 (V - nb) nRT
• a term is related to intermolecular force
  strength.
• b term is related to volume of the gas
  molecules (in liters per mole).
• Both a and b are empirical constants, determined
  by experiment.

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van der Waals ConstantsFor Selected Gases
Force of attraction
Molecular volume
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Volume of 1 mole of Gas at STP
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Ionic Bonds AsIntermolecular Forces

• There are no molecules in an ionic solid, and
  therefore there cant be any intermolecular
  forces.
• There are simply inter-ionic attractions.
• Lattice energy is a measure of the strength of
  inter-ionic attraction.
• The attractive force between a pair of oppositely
  charged ions increases as the charges on the ions
  increase and as the ionic radii decrease. Lattice
  energies increase accordingly.

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Interionic Forces of Attraction
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9.3
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Ion-Dipole Forces in Dissolution
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Hydrogen Bonds

• A hydrogen bond is an intermolecular force in
  which a hydrogen atom covalently bonded to a
  non-metal atom in one molecule is simultaneously
  attracted to a non-metal atom of a neighboring
  molecule.
• The strongest hydrogen bonds are formed if the
  non-metal atoms are small and highly
  electronegative.
• Usually occurs with nitrogen, oxygen, and
  fluorine atoms.
• Dotted lines are used to represent hydrogen bonds.

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Hydrogen Bonds in Water
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Intermolecular Hydrogen Bonds
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Intramolecular Hydrogen Bonds
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Dispersion Forces

• A dispersion force is the force of attraction
  between an instantaneous dipole and an induced
  dipole.
• Also called a London force after Fritz London who
  offered a theoretical explanation of these forces
  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.
• The greater the polarizability of molecules, the
  stronger the intermolecular forces between them.

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Volume of 1 mole of Gas at STP
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Molecular Shapeand Polarizability
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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 molecular substances,
  dispersion forces must always be considered.
  Often they predominate.

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Liquid-Vapor Equilibrium
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Vapor Pressure Of Water
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Vapor Pressure Curves

• Carbon disulfide CS2
• Methanol CH3OH
• Ethanol CH3CH2OH
• Water H2O
• Aniline C6H5NH2
• The temperature of the line P 760 mmHg with a
  vapor pressure curve is the normal boiling point.

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super-saturated
saturated (equilibrium)
unsaturated
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Equilibrium Vapor Pressure (mm Hg) T(oC) P T(oC
) P T(oC) P T(oC) P 0 4.58 21 18.65
35 42.2 92 567.0 5 6.54 22 19.83
40 55.3 94 610.9 10 9.21 23 21.07
45 71.9 96 657.6 12 10.52 24 22.38
50 92.5 98 707.3 14 11.99 25 23.76
55 118.0 100 760.0 16 13.63
26 25.21 60 149.4 102 815.9 17 14.53
27 26.74 65 187.5 104 875.1
18 15.48 28 28.35 70 233.7 106 937.9
19 16.48 29 30.04 80 355.1
108 1004.4 20 17.54 30 31.82
90 525.8 110 1074.6
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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.
• Condensation (DHcondn) is the change of a gas to
  a liquid.
• Because enthalpy is a function of state, the
  total enthalpy change between vaporization and
  re-condensation at constant temperature must be
  zero.
• DHvapn DHcondn 0
• DHcondn - DHvapn

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Some Enthalpies of Vaporization
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Clausius-Clapeyron Equation ln(P) -?Hvap/RT
C ln(P1/P2) (?Hvap/R)(1/T2 - 1/T1)
Dependent on substance
atm
K
kJ/mol
0.0083143 kJ/molK
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ln(P) -?Hvap/RT C
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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 molecules passing
  directly from the solid to the vapor state.
• Enthalpy of sublimation, DHsubln, is the sum of
  the enthalpies of fusion and vaporization.
• The triple point is the point at which the vapor
  pressure curve and the sublimation curve meet.

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Some Enthalpies of Fusion
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Generalized Phase Diagram
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Phase Diagram For H2O
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Freeze Dried Coffee
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Phase Diagram for Water
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Specific Heats of Water
ice (-3oC) 2.092 J/gK ?ice
liquid(14.5oC) 4.184 J/gK ?liq
steam(100oC) 1.841 J/gK ?gas
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Specific Heat Capacities Table
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• Properties of Water
• Density 0.99987 g/ml _at_ 0oC Heat of
  Fusion 6.008 kJ/mol _at_ 0oC
• 1.00000 g/ml _at_ 4oC Heat of Vaporization 44.94
  kJ/mol _at_ 0oC
• 0.99707 g/ml _at_ 25oC 44.02 kJ/mol _at_ 25oC
• 0.95838 g/ml _at_ 100oC 40.67 kJ/mol _at_ 100oC
• Ion-Product, Kw 1.14 x 10-15 _at_ 0oC
  Specific Heat ice (-3oC) 2.092 J/gK
• 1.01 x 10-14 _at_ 25oC
  liquid(14.5oC) 4.184 J/gK
• 5.47 x 10-14 _at_ 50oC
  steam(100oC) 1.841 J/gK
• Equilibrium Vapor Pressure (mm Hg)
• T(oC) P T(oC) P T(oC) P T(oC) P
• 0 4.58 21 18.65 35 42.2 92 567.0
• 5 6.54 22 19.83 40 55.3 94 610.9
• 10 9.21 23 21.07 45 71.9 96 657.6
• 12 10.52 24 22.38 50 92.5 98 707.3
• 14 11.99 25 23.76 55 118.0 100 760.0
• 16 13.63 26 25.21 60 149.4 102 815.9
• 17 14.53 27 26.74 65 187.5 104 875.1

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Surface Tension

• Surface tension (g) is the amount of work
  required to extend a liquid surface and is
  usually expressed in J / m2.
• 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 liquids resistance
  to flow.
• Liquids which flow easily are said to mobile.

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Intermolecular Forces in a Liquid
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Adhesive and Cohesive Forces
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Meniscus Formation
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Capillary Action
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Measuring Viscosity
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