chapter 12
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Chapter 12. Intermolecular Forces and the Physical Properties of Liquids and Solids. Polar Bonds and Polar Molecules. Draw Lewis Structures for CCl 4 and CH 3 Cl. What’s the same? What’s different?. Polar Covalent Bonds and Dipole Moments. - PowerPoint PPT PresentationTRANSCRIPT
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Chapter 12Intermolecular Forces and the Physical Properties
of Liquids and Solids
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Draw Lewis Structures for CCl4 and CH3Cl. What’s the same?
What’s different?
Polar Bonds and Polar Molecules
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Bonds are polar if electrons are shared unequally (differences in electronegativity) (e.g., CH3Cl)
Individual bonds can be polar but can cancel each other out to yield a nonpolar molecule (e.g., CO2)
Molecules with a lone pair of electrons or atoms with different electronegativities are polar (have a dipole moment)
Polar Covalent Bonds and Dipole Moments
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Polarity
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Draw Lewis Structures and Dipole Moments along each bond:
HCl CCl4 NH3 H2O
Polar or Nonpolar Molecules?
● BF3● CH3Cl● H2
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Dipole Moments
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Bonds: attractive forces within molecules Intermolecular Forces (IMF): attractive
forces between molecules Intermolecular forces are WEAKER than
bonds (intramolecular forces), but have profound effects on the properties of liquids
Intermolecular Forces (IMF)
431 kJ/mol
16 kJ/mol
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What is the difference between bonds and intermolecular forces?
IMF affect boiling points, melting points, and solubilities
As a group, intermolecular forces are called van der Waals forces: ◦ London Dispersion◦ Dipole-dipole◦ Hydrogen bonds◦ Ion-dipole
Intermolecular Forces
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London dispersion forces: attractive forces that result from temporary shift of electrons in atoms or molecules; present in all molecules
Explains why certain nonpolar substances benzene, bromine, etc.) are liquids at room temperature.
Dispersion Forces
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Figure 10.5
I2 induced dipoles
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Only force in non-polar molecules and in unbonded atoms (e.g., CO2, Ne)
Larger atoms or molecules have stronger London forces (and higher boiling points)◦ Larger electron clouds are easier to deform
(“squishier”, more polarizable) and tend to have more electrons
Found in mixtures or pure substances
Dispersion Forces
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Boiling point increases with the size of molecules because of increases in London forces with larger electron clouds
Boiling Points
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Pure substance or mixture made up of polar molecules
Opposite dipoles (charges) attract; like dipoles repel
Strength of these forces depends on the polarity of the molecules
Dipole-Dipole Forces
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dipole
Figure 10.4
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Like dissolves like (refers to polarity): polar liquids are more soluble in polar liquids◦ It takes 2000 mL of H2O to dissolve 1 mL of CCl4◦ It takes 50 mL of H2O to dissolve 1 mL of CH2Cl2
Which member of each pair has the stronger intermolecular forces?
CCl4 or CHCl3 CO2 or SO2
Dipole-Dipole Forces
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Boiling Points – which is higher?
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Hydrogen bridge: special type of dipole-dipole force; attractive force between a hydrogen atom bonded to a very small, electronegative atom (F, O, N) and lone e- pair
Ice melting
Hydrogen Bonds (aka bridges)
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HF
http://bio.winona.edu/berg/Illustr.htm
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Hydrogen bridges are seen in HF, H2O, NH3, but not CH4 or H2S
Requirements for hydrogen bridging:◦ H attached to a small, highly electronegative
element in one molecule◦ Small, highly electronegative element with one or
more unshared electron pairs in the other molecule Observed for the elements: F, O, N
(rarely S and Cl – too large, not electronegative enough)
Hydrogen Bridges
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Molecules hydrogen bridge to themselves or to other molecules
Hydrogen Bridges
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Rasmol 2.6.2.lnk
bridge bridge
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Hydrogen Bridging Trends
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Which of the following pure substances will experience hydrogen bridging?
H2O H2Se HBr HF NH3
NF3
DNA
Hydrogen Bridging
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Ions have full charges that are attracted to the partial charge on polar molecules (dipoles)
Explains solubility of ionic salts in polar solvents (e.g., NaCl in water)
Ion-Dipole Forces
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Figure 10.3
NaClIn H2O
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Strength of bonds in ionic compounds (already covered in Lattice Energy section!).
Ion-Ion Forces
NaCl CaF2
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Intermolecular forces generally increase in strength as:London < Dipole-Dipole < H-bridging < Ion-Dipole
< Ion-ion forces (ionic bonding)
Summary:◦ Nonpolar molecules: London Dispersion◦ Polar molecules: Dipole-dipole◦ Polar with H-N, H-O, or H-F: Hydrogen bridging◦ Ionic compound: Ion-ion forces
Strengths of Intermolecular Forces
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For each substance below, indicate the strongest type of intermolecular force observed.H2O COCH4 NH3
HCN CH3OHCO2 CH3NH2
F2 N2
Intermolecular Forces
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Rasmol 2.6.2.lnk
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Which member of each pair has stronger intermolecular forces (and higher boiling point)?
CH3OH or CH3SH CH4 or CH3CH2CH3
CO or F2 CH3Cl or HF
CO2 or NH3
NH3 or N2
Trends in Intermolecular Forces
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Boiling point curves
Rasmol 2.6.2.lnk
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Which member of each pair has stronger intermolecular force (boiling point, heat of vaporization)? Explain your reasoning.CH4 or CH3CH3
NH3 or NF3
CO2 or SO2
O2 or O3
I2 or Cl2
Group Quiz #16
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Surface tension: attraction of molecules to each other on a liquid’s surface
Molecules must break IMF in order to move to the surface and increase the surface area (large IMF, high surface tension)
Surface Tension
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Properties (e.g., IMF) in between those of gases and solids
Viscosity: resistance to flow Depends on intermolecular
forces and sizes Ethanol vs Glycerol
Properties of Liquids
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Viscosityglycerol vs ethanol
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fusion (melting): s lfreezing: l s vaporization: l g condensation: g
l sublimation: s g deposition: g s
Phase Changes
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Phase Changes - note structures
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Phase diagram: plot of temperature vs. pressure; solid lines are phase equilibria
10.11 Phase Diagram: Water Low Temp Boil
PhaseDiagram
http://www.chm.davidson.edu/ChemistryApplets/PhaseChanges/PhaseDiagram.html
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Liquid evaporates, gaseous molecules exert a pressure (vapor pressure) that can be measured as shown below
10.5 Vapor Pressure
Figure 10.11
Vaporizing liquid
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Features of a phase diagram:◦ Triple point: temperature and pressure at which
all three phases (s, l, and g) are in equilibrium◦ Vaporization curve: equilibrium between liquid
and gas◦ Melting curve: equilibrium between solid and
liquid◦ Normal melting point: melting point at 1 atm◦ Normal boiling point: boiling point at 1 atm
Phase Diagram
31Label Phase diagrams
Atomic Phasediagrams
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Critical point: liquid and gas phases are indistinguishable
Critical temperature, Tc : highest temperature at which a substance can exist as a liquid (cannot be liquified) no matter how much pressure is applied
Critical pressure, Tp : minimum pressure that must be applied to bring about liquefaction at the critical temperature
Critical Temperature and Pressure
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Phase Diagram: Carbon Dioxide
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Amorphous: random arrangement (rubber) Ionic: ordered arrangement of ions (salt)
10.6 Kinds of Solids
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Molecular (left): covalent molecules in an ordered arrangement (sucrose, ice); intermolecular forces hold molecules together
Covalent network (right): atoms connected by covalent bonds in 3D array (quartz)
Kinds of Solids
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Cubic is the most common
Three forms of cubic crystals - simple, body-centered, face-centered
Cubic Crystal Systems
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Cubic unit cells SC - build BCC –CsCl FCC – CaF2
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The End