Chapter 11: Chemical Bonding I: Basic Concepts Chemistry 140 Fall 2005 General Chemistry Principles and Modern Applications Petrucci • Harwood • Herring 8th Edition Chapter 11: Chemical Bonding I: Basic Concepts Philip Dutton University of Windsor, Canada N9B 3P4 Prentice-Hall © 2002 (modified 2003 by Dr. Paul Root and 2005 by Dr. David Tramontozzi) Thermochemistry branch of chemistry concerned with heat effects accompanying chemical reactions. Direct and indirect measurement of heat. Answer practical questions: why is natural gas a better fuel than coal, and why do fats have higher energy value than carbohydrates and protiens. Prentice-Hall © 2002 General Chemistry: Chapter 9

General Chemistry: Chapter 9 Contents 11-1 Lewis Theory: An Overview 11-2 Covalent Bonding: An Introduction 11-3 Polar Covalent Bonds 11-4 Writing Lewis Structures 11-5 Resonance 11-6 Exceptions to the Octet Rule 11-7 The Shapes of Molecules 11-8 Bond Order and Bond Lengths 11-9 Bond Energies Focus on Polymers— Macromolecular Substances Prentice-Hall © 2002 General Chemistry: Chapter 9

Strategy for Writing Lewis Structures Prentice-Hall © 2002 General Chemistry: Chapter 9

Apparent charges on certain atoms in a Lewis structure. Chemistry 140 Fall 2005 Formal Charge Apparent charges on certain atoms in a Lewis structure. 1 FC = #valence e- - #lone pair e- - #bond pair e- 2 These can be used to help determine the most probable Lewis structure when there is more than one Lewis structure possible for a compound. The formal charge on an atom in a Lewis structure is the number of valence e- in the free atom minus the number of e- assigned to that atom in the Lewis structure. Prentice-Hall © 2002 General Chemistry: Chapter 9

General Chemistry: Chapter 9 Example 11-6 Writing a Lewis Structure for a Polyatomic Ion. Write the Lewis structure for the nitronium ion, NO2+. Step 1: Total valence e- = 5 + 6 + 6 – 1 = 16 e- Step 2: Plausible structure: O—N—O Step 3: Add e- to terminal atoms: O—N—O •• Step 4: Determine e- left over: 16 – 4 – 12 = 0 Prentice-Hall © 2002 General Chemistry: Chapter 9

General Chemistry: Chapter 9 Example 11-6 Step 5: Use multiple bonds to satisfy octets. •• •• •• O=N=O + O—N—O •• •• •• •• Step 6: Determine formal charges: FC(O) = 6 - 4 – (4) = 0 2 1 FC(N) = 5 - 0 – (8) = +1 2 1 Prentice-Hall © 2002 General Chemistry: Chapter 9

Alternative Lewis Structure •• •• •• O N O + + - O—N—O •• •• •• •• FC(O≡) = 6 - 2 – (6) = +1 2 1 FC(N) = 5 - 0 – (8) = +1 2 1 FC(O—) = 6 - 6 – (2) = -1 2 1 Prentice-Hall © 2002 General Chemistry: Chapter 9

Alternative Lewis Structures Sum of FC is the overall charge. FC should be as small as possible. Negative FC usually on most electronegative elements. FC of same sign on adjacent atoms is unlikely. + •• O≡N—O - Prentice-Hall © 2002 General Chemistry: Chapter 9

General Chemistry: Chapter 9 Example 11-7 Using the Formal Charge Concept in Writing Lewis Structures. Write the most plausible Lewis structure of nitrosyl chloride, NOCl, one of the oxidizing agents present in aqua regia. 2+ 2- - + Prentice-Hall © 2002 General Chemistry: Chapter 9

General Chemistry: Chapter 9 Chemistry 140 Fall 2005 11-5 Resonance Two or more plausible Lewis structures can be written but ‘correct’ structure itself cannot be written. + - - + •• •• •• •• •• •• O O O •• •• O O O •• •• •• •• One would expect either structure to have one bond (double bond) shorter than the other (single bond). In reality, both bonds are of equal length and true structure is a hybrid of the above structures. O •• + -½ Many Lewis structures may be written for a given structure.. Ozone has two good possibilities, but neither gives the correct structure that has two equivalent O-O bonds. In general, the more plausible Lewis structures that can be written, the more stable the molecule. Prentice-Hall © 2002 General Chemistry: Chapter 9

Resonance in the Nitrate Ion (an example) Chemistry 140 Fall 2005 Resonance in the Nitrate Ion (an example) Draw the Lewis structure for the nitrate ion, NO3- •• •• O The true structure of the nitrate ion is a hybrid of the three equivalent resonance structures. •• N •• O O •• •• •• •• •• - •• •• O •• •• •• O •• - •• O •• Many Lewis structures may be written for a given structure.. Ozone has two good possibilities, but neither gives the correct structure that has two equivalent O-O bonds. + N + N + N O •• O •• •• - •• O O •• •• •• - - •• •• •• O O •• •• - •• •• •• •• Prentice-Hall © 2002 General Chemistry: Chapter 9

11-6 Exceptions to the Octet Rule Chemistry 140 Fall 2005 11-6 Exceptions to the Octet Rule Odd e- species. N=O •• •• • •• H Free radicals are highly reactive •• • O—H H—C—H H • •• Prentice-Hall © 2002 General Chemistry: Chapter 9

Exceptions to the Octet Rule Incomplete octets. •• B F - + •• B F - •• + •• F •• B •• F F •• •• •• •• •• BF3 tendency is to form a coordinate covalent bond with species capable of donating an electron pair. These species are limited to some compounds of beryllium, boron and aluminum Prentice-Hall © 2002 General Chemistry: Chapter 9

Exceptions to the Octet Rule Expanded octets. S F •• •• P Cl •• •• Cl •• P •• •• Cl Cl •• •• •• •• •• Normal octet Expanded octets Prentice-Hall © 2002 General Chemistry: Chapter 9

Expanded Valence Shell Chemistry 140 Fall 2005 Expanded Valence Shell Expanded valence shell reduces the number of formal charges. Experimentally determined sulfur to oxygen bond length is shorter than a single bond It is not clear which is the more correct representation. Prentice-Hall © 2002 General Chemistry: Chapter 9

11-7 The Shapes of Molecules From the Lewis structure it appears that water should be linear in shape. The experimentally determined shape of water is bent. Does this matter? Why the difference? H O H Prentice-Hall © 2002 General Chemistry: Chapter 9

General Chemistry: Chapter 9 Terminology Bond length – distance between nuclei. Bond angle – angle between adjacent bonds. VSEPR Theory Electron pairs repel each other whether they are in chemical bonds (bond pairs) or unshared (lone pairs). Electron pairs assume orientatioins about an atom to minimize repulsions. Electron group geometry – distribution of e- groups (single, double, triple bonds, lone pairs or single electrons). Molecular geometry – distribution of nuclei. Prentice-Hall © 2002 General Chemistry: Chapter 9