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C1403.2006.EXAM3._Review

C1403.2006.EXAM3._Review - Valence Bond Theory Molecules...

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Valence Bond Theory • Molecules are constructed by electrons in valence shells pairing up as atoms bond together. ELECTRON PAIRS ARE LOCALIZED between the atoms they bond. RESONANCE is a refinement of the theory for more complex bonding situations. VSEPR uses valence bond theory for predicting molecular geometries VALENCE SHELL EXPANSION makes use of d orbitals to form extended structures
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Simple Molecules of Second Row Elements Cl—Be—Cl F— O F :
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Structure Number 3
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Structure Number 4
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Electron Groups SN number Nonbonding Pairs Examples Geometry 2 0 BeCl 2 Linear 3 0 BCl 3 Trigonal planar 3 1 SnCl 2 Bent 4 0 CH 4 Tetrahedral 4 1 NH 3 Trigonal pyramidal 4 2 H 2 O Bent 5 0 PCl 5 Trigonal bipyramidal 5 1 SF 4 Seesaw shaped 5 2 IF 3 T-shaped 5 3 XeF 2 Linear 6 0 SF 6 Octahedral 6 1 IF 5 Square pyramidal 6 2 XeF 4 Square planar
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Expanding the Valence Shell and Multiple Bonds Making use of d-orbitals by valence shell expansion PCl 3 and PCl 5 SF 2 ; SF 4 ; and SF 6 Double and triple bonds C 2 H 6 ; C 2 H 4 ; C 2 H 2 and C 2 2- C 6 H 6 (benzene) HCN and CN - CO; H 2 CO (formaldehyde)
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Expanding the valence shell PCl 3 PCl 5 SF 4 SF 4
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Expanding the valence shell ClF 3
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Expanding the valence shell SF 6 XeF 4
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Hydrogen Peroxide 2 H 2 O 2 2 H 2 O + O 2 Δ H = -210 kJ/mol dihedral angle dihedral angle
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Hydroxylamine Hydrazine
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The Hydrogen Molecule
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Hybrid sp 3 orbitals in CH 4
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Hybrid sp 3 orbitals in H 2 O
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Hybrid sp 3 orbitals in NH 3
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Hybrid sp 2 orbitals in BF 3
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Hybrid sp orbitals in BeF 2
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Valence Bond Theory and Multiple Bonds Hybrid orbitals on carbon • Methane and ethane CH 4 and C 2 H 6 • Ethene (ethylene) C 2 H 4 • Ethyne (acetylene) C 2 H 2 • Butadiene C 4 H 6 • Benzene C 6 H 6 • Formaldehyde
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When No Valence Bond Structure is Satisfactory
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When No Valence Bond Structure is Satisfactory
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Ethene (ethylene) C 2 H 4
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Ethyne (acetylene) C 2 H 2
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Acetylene A colorless gas at room temperature (bp=-84°C), acetylene is prepared from coke and lime in an electric furnace: 3 C(coke) + CaO(lime) CaC 2 (calcium carbide) + CO CaC 2 + 2 H 2 O HC CH + Ca(OH) 2 It can also be prepared directly from natural gas (methane): 2 CH 4 C 2 H 2 + 3H 2 HEATS OF COMBUSTION data suggest ethane, not acetylene, releases the greatest amount of energy, yet acetylene is the gas of choice for lamps and torches. WHY? C 2 H 6 = 373 kcal/mol C 2 H 4 = 337 kcal/mol C 2 H 2 = 317 kcal/mol HEATS OF HYDROGENATION: special case for resonance stabilization in benzene (C 6 H 6 ).
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When No Valence Bond Structure is Satisfactory
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Resonance structures in Benzene
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Molecular Orbital Theory There are orbitals in molecules that are equivalent to orbitals in atoms. That is, a molecular orbital is not much different from an
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C1403.2006.EXAM3._Review - Valence Bond Theory Molecules...

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