Chem 111 102102

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Chem 111 10-21-02

• CAPA set 20 due today
• Discussion session today at 400, Peale 50
• Any seminar reports ??
• Chapter 9
• Lewis 2D structures gt 3D VSEPR
• Valence Shell Electron Pair Repulsion

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Release lattice energy by bringing ions together
at distance d
Energetics of ionic bond formation Born - Haber
cycle
411 kJ 108 kJ Na(g) 122 kJ
Cl(g) 496 kJ Nag) 349 kJ
Cl(g) ?Hlattice
?Hlattice 788 kJ is the energy to separate
NaCl(s) into gas phase ions
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Another example

• Combustion of acetylene, C2H2
• C2H2(g) 5/2 O2(g) ? 2 CO2(g) H2O(g)
• Use Lewis structures to get bond types
• Hesss Law and bond enthalpy estimate ?Hrxn
• Breaking bonds requires energy
• Making bonds releases energy
• The energy difference is ?Hrxn

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Bond Enthalpy 348 kJ/mol 614 kJ/mol 839 kJ/mol
Multiple bonds, more e density, shorter, stronger
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Lewis structures

• Connections between atoms, bonding electrons
• Single, double or triple bonds
• Number of unshared, non-bonding electrons
• Drawn in two dimensions
• The real world is three dimensional
• The Lewis model helps to predict the shape of
  many simple molecules

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Some generic simple molecules

• Why generic ?
• Look for patterns !!
• Simple !!
• Single central atom with two to six bonded atoms
• Central atom is A (from the p block) and bonded
  atoms are B
• ABn where n is from 2 to 6
• H2O, CO2, NH3, SO3, CH4, CCl4, SF4
• Only five fundamental molecular geometries

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AB2
AB3
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AB4
AB5
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AB6

• Eight faces w/equilateral triangles
• Six vertices

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Why these, not other random shapes

• The behavior of the bonded and non-bonded
  electron pairs gt referred to as electron domain
• Electron domain
• Non-bonding pair located on a single atom
• Single bond shared between two atoms
• Multiple bond shared between two atoms
• The best arrangement for a given number of
  electron domains minimizes repulsions - the
  domains want to be far apart !! VSEPR

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And these arrangements are

• For 2 domains, 180 gt linear
• For 3 domains, 120 gt trigonal planar
• For 4 domains, 109.5 gt tetrahedral
• For 5 domains, 120 90 gt Triganol
  bipyramid
• For 6 domains, 90 gt octahedral
• Same shapes as the ABn geometries

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VSEPR and Lewis structure

• Use Lewis structure to get total electron domains
  around central atom
• Each non-bonding pair is a domain
• Each bond type (single, double or triple) is
  counted as one domain

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VSEPR and two types of geometry

• Electron domain geometry
• Arrange all electron domains around the central
  atom to minimize repulsions
• Molecular geometry
• Arrangement of only bonded atoms around the
  central atom is used to determine the molecular
  geometry
• The exact molecular geometry is affected by the
  non-bonding domains

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The non-bonding pair takes up more space so that
it has a greater repulsion effect on bonded pairs
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• The Effect of Nonbonding Electrons and Multiple
  Bonds on Bond Angles
• The larger non-bonding orbitals decrease the
  expected bond angle
• Multiple bonds with greater electron density have
  a similar effect in that they take up more space
  and have a greater repulsion

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Geometries for expanded octets

• For 5 domains, 120 90 gt Triganol
  bipyramid

Non-bonding pairs will always go in the
equatorial position because they see only two
domains at 90. In axial they would see three
domains at 90, which would increase repulsions.
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Geometries for expanded octets

• For 5 domains, 120 90 gt Triganol
  bipyramid
• 4 bonding is seesaw
• 3 bonding is T-shaped
• 2 bonding is linear
• For 6 domains, 90 gt octahedral
• 5 bonding is square pyramid
• 4 bonding is square planar

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Geometries for larger molecules
Triganonal planar
Tetrahedral
Bent
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Molecular shape and polarity

• Each bond can be viewed as a dipole
• A vector quantity with magnitude and direction
• Molecule polarity is the sum of all the bond
  vectors
• Equal and opposite vectors cancel
• Symmetry around the central atom will lead to a
  non-polar molecule, even if bonds are polar
• Other vector combinations yield polar molecules

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Molecular shape and polarity
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Molecular shape and polarity
Net polarity for the molecule
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Polar
Polar
Polar
Non-polar
Non-polar