chapter 10 the shapes of molecules lecture notes by k. marr (silberberg 3 rd edition) 10.1 depicting...
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Chapter 10 The Shapes of Molecules Lecture Notes by K. Marr
(Silberberg 3rd Edition)
10.1 Depicting Molecules and Ions with Lewis Structures
10.2 Using Lewis Structures and Bond Energies to Calculate Heats of Reaction
10.3 Valence-Shell Electron-Pair Repulsion (VSEPR) Theory and Molecular Shape
10.4 Molecular Shape and Molecular Polarity
Lewis Structures…..
1. Indicate the kind of bonding and which atoms are bonded in molecules and polyatomic ions
2. Do NOT indicate the molecular shape or structure. However….• VSEPR theory uses Lewis structures to
predict 3-D structure
Guidelines for Writing Lewis Structures
1. Decide which atoms are bonded2. Count all valence electrons (account for the charge of ions!!)
3. Place 2 electrons in each bond4. Complete the octets of the atoms attached to the central
atom by adding electrons in pairs5. Place any remaining electrons on the central atom in
pairs6. If the central atom does not have an octet, form double
bonds, or if necessary, a triple bond.7. Write the Lewis Structures for ClF5, TeF4, CO3
2-, CH3COO1-
The Octet Rule is Often Violated
1. H, Be, B, Al violate the octet rule (< 8 valence electrons)
e.g. BeCl2, BH3, AlCl3
2. Nonmetals with a valence shell greater than n = 2 (e.g. P, Cl, Br, I, etc.)
» May violate the octet rule when they are the CENTRAL atom (e.g. ClF5 )
– How can they do this?– Why doesn’t Fluorine violate the octet rule?
Lewis Structures for Organic Compounds
1. Alkanes: CnH2n+2
» Methane, Ethane, Propane, Butane, Pentane, Hexane– What are isomers?
2. Alkenes: CnH2n have double bond(s)
» One double bond: Ethene (ethylene), Propene (propylene)
3. Alcohols: CnH2n+1OH have hydroxyl group(s)
» methanol, ethanol4. Carboxylic Acids: CnH2n+1COOH have carboxyl
group(s)
» Methanoic acid (formic acid), HCOOH» Ethanoic acid (acetic acid, CH3COOH
Using Formal Charge to Select the Favored Lewis Structure
Sometimes more than one Lewis Structure is possible for a compound e.g. sulfuric acid, H2SO4; phosphate ion, PO4
-3
Formal Charge• Apparent charge on a bonded atom• An atom “owns” all of its nonbonding electrons and half of its bonding
electrons.• The Lewis Structure with the lowest total formal charge is favored
Formal charge of atom =
[# valence e-] – [# unshared e- + 1/2 # shared e-]OR
F.C. = [# of valence e-] - [# of unshared + # bonds formed ]
Use of Formal Charge to Select the Favored Lewis Structure
Formal Charge• Apparent charge on a bonded atom• An atom “owns” all of its nonbonding electrons and half of its
bonding electrons.• The Lewis Structure with the lowest total formal charge is
favored
Formal charge of atom =
[# valence e-] – [# unshared e- + 1/2 # shared e-]OR
F.C. = [# of valence e-] - [# of unshared + # bonds formed ]
Use of Formal Charge to Select the Favored Lewis Structure
Use formal charge to determine the correct Lewis structure for
a) sulfuric acid, H2SO4
b) phosphate ion, PO4-3
Recall:
F.C. = [# valence e-] – [# unshared e- + 1/2 # shared e-]OR
F.C. = [# of valence e-] - [# of unshared + # bonds formed ]
Formal Charge Three criteria for choosing the more important
structure
1. Smaller formal charges (either positive or negative) are preferable to larger charges;
2. Avoid like charges (+ + or - - ) on adjacent atoms;
3. A more negative formal charge should exist on an atom with a larger EN value.
ResonanceWhen Lewis Structures Fail.....
1. Write the Lewis Structure for the nitrate ion, NO3-
» Based on your Lewis structure, what kind of bonding would be expected ?
2. Experimental measurements indicate....
» All bond lengths and energies are the same!! (B.O. = 1.33)
3. The NO3- is a Resonance Hybrid of 3 different Lewis
structures....
» Just as mule is neither a horse or a donkey, none of the 3 structures represent NO3
-
Resonance Hybrids
1. Each resonance structure does not actually exist!!
2. The actual molecule or ion is a hybrid or average of each resonance structure
3. Electron-Pair Delocalizationa) Each bonding electron pair is delocalized or spread over the
entire molecule or ion.
b) Results in identical bonds with extra stability since electron repulsions reduced
Resonance Structures: Practice Makes Perfect?
1. Draw the resonance structures for the nitrite ion, NO2
- and the phosphite ion, PO3-3
2. How do you know when to use resonance?3. How do you know how many resonance structures are
possible?4. Draw the Lewis structures for ......
» The oxalate ion, C2O4-2
» Benzene, C6H6
– Benzene has a hexagonal ring structure
Using Bond Energies to Calculate Heats of Reaction, Hrxn
Lewis structures can be used to calculate Hrxn
For a reaction to occur….» Bonds within the reactants must be broken (endothermic)» Bonds within the reactants must be made (exothermic)
Hrxn = Hreactant bonds broken + Hproduct bonds formed
Reactants and products must be in gaseous state!! Why??
Using Bond Energies to Calculate Heats of Rxn
Hrxn = Hreactant bonds broken + Hproduct bonds formed
e.g. CH4 (g) + 2 O2 (g) CO2 (g) + 2 H2O (g) H0rxn= -818 kJ/mol
Figure 10.3 Using bond energies to calculate H0comb. of Methane, CH4
Ent
halp
y,H
BOND BREAKAGE
4BE(C-H)= +1652kJ
2BE(O2)= + 996kJ
H0(bond breaking) = +2648kJBOND FORMATION
4[-BE(O-H)]= -1868kJ
H0(bond forming) = -3466kJ
H0rxn= -818 kJ/mol
2[-BE(C O)]= -1598kJ
Examples: Using Bond Energies to Calculate Heats of
Reaction, Hrxn
Use bond energies (see table 9.2, page 340 3rd ed) to calculate in kJ/mole the
1. Standard heat of formation of water (compare your answer with Appendix B—they should be the same)
2. Standard heat of combustion of propane, C3H8 (ans.
= -2042 kJ/mol)
a) Now use standard heats of formation, Hof, to calculate
the heat of combustion of propane, C3H8 (ans. = - 2043 kJ/mol)
Predicting the Shapes of Molecules: VSEPR Theory
1. Valence Shell Electron Pair Repulsion Theory
» In order to limit electrostatic repulsion, electron pairs in the orbitals around the central atom stay as far apart as possible
VSEPR: A balloon analogy for the mutual repulsion of electron groups.
Figure 10.4
Linear Trigonal Planar Tetrahedral
Trigonal Bipyramidal
Octahedral
VSEPR TheoryThe Number of Electron Pairs around the Central
Atom Determine Molecular Geometry.... 2 bonding pairs linear (Bond angle = 180o)
3 bonding pairs planar triangle (Bond angle = 120o)
4 bonding pairs tetrahedral (Bond angle = 109.5o)
5 bonding pairs trigonal bipyramidal (Bond angles = 90o and 120o )
6 bonding pairs octahedral (Bond angle = 90o)
Figure 10.5
Predicting Molecular Geometry
Use Lewis structures and VSEPR Theory to explain the following molecular geometries....
a) H2O and SnCl2
Are they Bent or V-shaped molecules?
b) BeCl2 and CO2
Bent or linear molecules?
Treat double bonds as if only one pair...Why?
Predicting Molecular Geometry
Use Lewis structures and VSEPR Theory to predict the following molecular geometries....
1. BH3
2. NH3
3. ClF3
4. ClF3: T-Shaped and NOT trigonal planar. Why??
a) Nonbonding pairs take up more space than bonding electrons......why?
b) Therefore, nonbonding pairs need to be separated as much as possible.
Predicting Molecular Geometry
Use Lewis structures and VSEPR Theory to predict the following molecular geometries....
1. CH4 and PO43- (Ans. Tetrahedral)
2. XeF4 (Ans. Square planar. Why not tetrahedral?)
3. PCl5 (Ans. Trigonal bipyramidal)
4. BrF5 (Ans. Square pyramidal)
SAMPLE PROBLEM 10.9 Predicting Molecular Shapes with More Than One Central Atom
SOLUTION:
PROBLEM: Determine the shape around each of the central atoms in acetone, (CH3)2C=O.
PLAN: Find the shape of one atom at a time after writing the Lewis structure.
C C C
OH
H
H
HH
H
tetrahedral tetrahedral
trigonal planar
C
O
HC
HHH
CH
H>1200
<1200
Figure 10.13
The tetrahedral centers of ethanol.
Predicting Molecular Shapes with More Than One Central Atom
Figure 10.9 Lewis structures and molecular shapes
Molecular Polarity
1. Influences Chemical and Physical Properties
2. Polar molecules have higher MP’s and BP’s than nonpolar molecules.....Why?
• Magnitude of Dipole moment influences MP and BP
e.g. H2O vs H2S
3. Solubility: Like dissolves Like• Polar solutes dissolve in polar solvents
• Nonpolar solutes dissolve in nonpolar solvents
Nonpolar Molecules
1. Any Molecule with only nonpolar bonds
e.g. F2 and C8H18
2. Symmetrical Molecules with Polar Bonds of equal dipole moment.....
a) CO2 , BCl3, and CCl4
b) PCl5 and SF6
Polar Molecules
1. Asymmetrical Molecules with Polar Bondsa) H2O and NH3
b) HCl
2. Symmetrical Molecules with Polar Bonds of unequal dipole moment
e.g. CHCl3 and CF2Cl2
Note: CCl2F2 CFC-12 once used in refrigerators Ozone depletion
Electronegativities of the Elements
Figure 10.14
The orientation of polar molecules in an electric field.
Electric field OFF Electric field ON
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