Chapter 10 The Shapes of Molecules Lecture Notes by K 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….. Indicate the kind of bonding and which atoms are bonded in molecules and polyatomic ions Do NOT indicate the molecular shape or structure. However…. VSEPR theory uses Lewis structures to predict 3-D structure

Guidelines for Writing Lewis Structures Decide which atoms are bonded Count all valence electrons (account for the charge of ions!!) Place 2 electrons in each bond Complete the octets of the atoms attached to the central atom by adding electrons in pairs Place any remaining electrons on the central atom in pairs If the central atom does not have an octet, form double bonds, or if necessary, a triple bond. Write the Lewis Structures for ClF5, TeF4, CO32-, CH3COO1-

The Octet Rule is Often Violated H, Be, B, Al violate the octet rule (< 8 valence electrons) e.g. BeCl2, BH3, AlCl3 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 Alkanes: CnH2n+2 Methane, Ethane, Propane, Butane, Pentane, Hexane What are isomers? Alkenes: CnH2n have double bond(s) One double bond: Ethene (ethylene), Propene (propylene) Alcohols: CnH2n+1OH have hydroxyl group(s) methanol, ethanol 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 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 sulfuric acid, H2SO4 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 Smaller formal charges (either positive or negative) are preferable to larger charges; Avoid like charges (+ + or - - ) on adjacent atoms; A more negative formal charge should exist on an atom with a larger EN value.

Resonance When Lewis Structures Fail..... Write the Lewis Structure for the nitrate ion, NO3- Based on your Lewis structure, what kind of bonding would be expected ? Experimental measurements indicate.... All bond lengths and energies are the same!! (B.O. = 1.33) 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 Each resonance structure does not actually exist!! The actual molecule or ion is a hybrid or average of each resonance structure Electron-Pair Delocalization Each bonding electron pair is delocalized or spread over the entire molecule or ion. Results in identical bonds with extra stability since electron repulsions reduced

Resonance Structures: Practice Makes Perfect? Draw the resonance structures for the nitrite ion, NO2 - and the phosphite ion, PO3-3 How do you know when to use resonance? How do you know how many resonance structures are possible? 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, DHrxn Lewis structures can be used to calculate DHrxn For a reaction to occur…. Bonds within the reactants must be broken (endothermic) Bonds within the reactants must be made (exothermic) DHrxn = S DHreactant bonds broken + S DHproduct bonds formed Reactants and products must be in gaseous state!! Why??

Using Bond Energies to Calculate Heats of Rxn DHrxn = S DHreactant bonds broken + S DHproduct bonds formed e.g. CH4 (g) + 2 O2 (g)  CO2 (g) + 2 H2O (g) DH0rxn= -818 kJ/mol

Using bond energies to calculate DH0comb. of Methane, CH4 Figure 10.3 Using bond energies to calculate DH0comb. of Methane, CH4 BOND BREAKAGE 4BE(C-H)= +1652kJ 2BE(O2)= + 996kJ DH0(bond breaking) = +2648kJ BOND FORMATION 2[-BE(C O)]= -1598kJ Enthalpy,H 4[-BE(O-H)]= -1868kJ DH0(bond forming) = -3466kJ DH0rxn= -818 kJ/mol

Examples: Using Bond Energies to Calculate Heats of Reaction, DHrxn Use bond energies (see table 9.2, page 340 3rd ed) to calculate in kJ/mole the Standard heat of formation of water (compare your answer with Appendix B—they should be the same) Standard heat of combustion of propane, C3H8 (ans. = -2042 kJ/mol) Now use standard heats of formation, DHof, to calculate the heat of combustion of propane, C3H8 (ans. = - 2043 kJ/mol)

Predicting the Shapes of Molecules: VSEPR Theory 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

Linear Trigonal Planar Tetrahedral Trigonal Bipyramidal Octahedral VSEPR: A balloon analogy for the mutual repulsion of electron groups. Linear Trigonal Planar Tetrahedral Trigonal Bipyramidal Octahedral Figure 10.4

VSEPR Theory The 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.... H2O and SnCl2 Are they Bent or V-shaped molecules? 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.... BH3 NH3 ClF3 ClF3: T-Shaped and NOT trigonal planar. Why?? Nonbonding pairs take up more space than bonding electrons......why? 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.... CH4 and PO43- (Ans. Tetrahedral) XeF4 (Ans. Square planar. Why not tetrahedral?) PCl5 (Ans. Trigonal bipyramidal) BrF5 (Ans. Square pyramidal)

SAMPLE PROBLEM 10.9 Predicting Molecular Shapes with More Than One Central Atom 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. SOLUTION: tetrahedral tetrahedral trigonal planar >1200 <1200

The tetrahedral centers of ethanol. Predicting Molecular Shapes with More Than One Central Atom The tetrahedral centers of ethanol. Figure 10.13

Lewis structures and molecular shapes Figure 10.9 Lewis structures and molecular shapes

Molecular Polarity Influences Chemical and Physical Properties 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 Solubility: Like dissolves Like Polar solutes dissolve in polar solvents Nonpolar solutes dissolve in nonpolar solvents

Nonpolar Molecules Any Molecule with only nonpolar bonds e.g. F2 and C8H18 Symmetrical Molecules with Polar Bonds of equal dipole moment..... CO2 , BCl3, and CCl4 PCl5 and SF6

Polar Molecules Asymmetrical Molecules with Polar Bonds H2O and NH3 HCl 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

The orientation of polar molecules in an electric field. Figure 10.14 The orientation of polar molecules in an electric field. Electric field ON Electric field OFF