Intramolecular Forces

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Intramolecular Forces
Forces (chemical bonds) within a
molecule Typical value 350 kJ/mol C-C bond
Intermolecular Forces
Forces between molecules Typical value 20
kJ/mol H-bond 1 kJ/mol van der Waals
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Dipole-dipole Forces If present
usually dominate intermolecular interactions.
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Water Held together by O-HH hydrogen
bonds
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Hydrogen bond donors O-H, N-H, S-H, X-H
Hydrogen bond acceptors O, N, X, S
For example The amide hyrdrogen bond is the
dominate intermolecular force in proteins.
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London Dispersion Forces or van der Waals
Forces
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What determines the B.P. of a liquid?
DGvap DHvap - T DSvap
0 at the B.P.
DHvap T DSvap
For simple liquids, all Dsvap values are
about the same. So B.P. is proportional to DHvap
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Four General Classifications for Solids 1.
Metals Fe, Co, Ag 2. Covalent Network
Solids Diamond, SiO2 3. Ionic Solids
NaCl ZnS 4. Molecular Solids I2 Sugar
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Braggs Law
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Determination of Crystal Structures using X-Ray
diffraction.
Diffraction of any wave will take place when you
have a grid with a spacing similar to the
wavelength of the wave.
X-Rays have wavelengths on the order of 1
Ångstrom. Typical value 0.71Å
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Four General Classifications for Solids 1.
Metals Fe, Co, Ag 2. Covalent Network
Solids Diamond, SiO2 3. Ionic Solids
NaCl ZnS 4. Molecular Solids I2 Sugar
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Close Packing of Spheres
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Hexagonal Close Packed
Co, Ti, Mg
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Body Centered Cubic
Fe, Cr, V
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Cubic Close Packed - Face Centered Cubic
Ni, Cu, Au
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Efficiency of Close Packing What fraction of the
volume is occupied?
fv volume spheres in unit cell / volume of
cell
0.740
fv
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The radius of Ag atom is 1.44 Å. Calculate the
density. The Ag structure is fcc (ccp).
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The radius of Ag atom is 1.44 Å. Calculate the
density. The Ag structure is fcc (ccp).

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Close packing of spheres fills 0.74 of the
available space
The remaining space can be allotted to
three types of holes that occur between the
spheres.
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rtet 0.225 roct 0.414 rcub 0.732
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Sodium Chloride
Na .95 Å Cl- 1.81Å Table 13.7
Ratio .95 / 1.81 .52
rtet 0.225 roct 0.414 rcub 0.732
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Zinc Sulfide
Ratio Zn2/S-2 .35
rtet 0.225 roct 0.414 rcub 0.732
The Zinc atoms occupy 1/2 of the tetrahedral holes
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Glass Quartz
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Molecular Solids C60
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Iodine I2
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Acetylene HCCH
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Oxalic Acid
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P
DG DH - TDS - RT ln K
ln K -(DH/R) /T DS/R
ln P -(DH/R)(1/T) DS/R
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Triple point 4.588 torr 0.0098C
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