Chapter%2013%20Student%20Notes%20Part%201%20PHW - Chapter...

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Unformatted text preview: Chapter 13 Objectives 13.1. Explain the basic concepts surrounding bonding including the Lennard Jones potential, electronegativity, ionization, and electron affinity. 13.2. Illustrate the spectrum of bonding based upon percent ionic character. 13.3. Discuss Lewis Structures. 13.4. Describe resonance and formal charges. 13.5. Outline the characteristics of polar bonds and polar molecules. 13.6. Discuss VSEPR theory. 9/15/2008 Zumdahl Chapter 13 1 "What we represent when we draw a molecule is what we want to represent: We abstract a piece of reality to show it to another person." Roald Hoffmann Chemistry Nobel Laureate Cornell University 9/15/2008 Zumdahl Chapter 13 2 1 Preliminaries Our knowledge of atomic structure, electron configurations, & periodic properties is a foundation for understanding bonding. Electrons can be divided into (e- in a filled shell) (e- in an unfilled shell; outermost electrons) Valence electrons participate in bonding through Sharing of e- by atoms: Transfer of e- from one atom to another: 9/15/2008 Zumdahl Chapter 13 3 Ionic Bonds Ionic substances are formed when an atom that loses electrons relatively easily reacts with an atom that has a high affinity for electrons Na e- + Cl Na+ 9/15/2008 Loss of a valence electron Na+ + e- .. : Cl : Gain of a valence electron .. .. Combination to form the NaCl : Cl : + ionic compound NaCl .. Zumdahl Chapter 13 4 2 The Coulomb Potential The energy of interaction between a pair of ions can be calculated by using Coulomb's law: If Q1 and Q2 have opposite signs, V is negative ( interaction). If Q1 and Q2 have the same sign, V is positive ( interaction). 9/15/2008 Zumdahl Chapter 13 5 The Lennard - Jones Potential The potential energy of interaction between neutral atoms and/or molecules is conveniently described using the Lennard - Jones potential: potential: V = 4 {(r0/r)12 - (r0/r)6} Well Depth r0 Separation at which V = 0 re 9/15/2008 Separation at the well minimum = 21/6 r0 6 Zumdahl Chapter 13 3 Atoms or molecules approach at large distance (V 0 as r ). V goes negative as intermolecular forces come into play Minimum energy at r with maximum . (r) V goes positive as forces become dominant at small r. 9/15/2008 Zumdahl Chapter 13 7 Covalent Bonds Covalent Bonding: Whenever possible, the valence electrons in a compound are distributed in such a way that each main-group element in a molecule (except hydrogen) is surrounded by electrons (an should have of electrons). Hydrogen electrons in such a structure. H + Cl: .. 9/15/2008 .. H:Cl: .. Zumdahl Chapter 13 .. or HCl: 8 .. .. 4 The Dipole Moment Polar Covalent Bonds Bonded atoms share electrons unequally, whenever the atoms differ in Example - HF: The F atom carries a slightly electric charge and the H atom a slightly charge of equal magnitude. Aligns in an electric field. Polar molecules posses a 9/15/2008 Zumdahl Chapter 13 , . 9 Electronegativity (EN) vs. Electron Affinity (EA) Electron affinity is a measure of the energy required to detach an electron from an atom or molecule: A- A + e- , E EA (a large EA means a strong attraction of electrons) Electronegativity is a measure of the ability of an atom in a molecule to attract shared electrons to itself. 9/15/2008 Zumdahl Chapter 13 10 5 The Person Behind the Science Linus Pauling (1901-1994 ) Moments in a Life 1954 Nobel Prize in chemistry Chemical Bonding, molecular biology 1962 Nobel Peace Prize Health (Vitamin C advocate) and Nuclear testing (banning atmospheric tests ) Electronegativity Scale: concept of partially ionic bonds. fluorine with x = 4 is the most electronegative element, cesium with x = 0.7 the least. Percent Ionic Character x values can also be used to estimate the dipole moment and ionic character of bonds. Pauling: Electronegativity is the power of an atom in a molecule to attract electrons to itself. x differs from the electron affinity of the free atom although the two run roughly parallel. 9/15/2008 Zumdahl Chapter 13 11 The Pauling Electronegativity Values as Updated by A.L. Allred in 1961 Note: 9/15/2008 Zumdahl Chapter 13 12 6 Expected H-X bond energy = [(H-H bond energy) (X-X bond energy)]1/2 (Geometric Mean of H-H and X-X Bond Energies) Bond energy = expected bond energy if = EN(H) -EN(X) = 0. The larger is, the stronger the H-X bond is. H- Ionic vs. Covalent Bonding 9/15/2008 Zumdahl Chapter 13 13 Ionic Character = 100% if = Q r where r = bond length ( & r det'd experimentally) Q = electron charge 9/15/2008 Zumdahl Chapter 13 14 7 Non-Polar Covalent Bonding 9/15/2008 Zumdahl Chapter 13 15 Ca: O: 9/15/2008 [Ar]4s2 Lose 2 electrons Ca2+: [Ar] or [Ne] 2- 2 6 [He] 2s22p4 Gain 2 electrons O : [He] 2s 2p Zumdahl Chapter 13 16 8 Sizes of ions related to positions of elements in the periodic table. Atomic Radii In picometers Cations: Anions: Isoelectronic series: O2FNa+ Mg2+ 9/15/2008 Zumdahl Chapter 13 Al3+ 17 Formation of Binary Ionic Compounds Lattice Energy (5) can be calculated using a modified version of Coulomb's Law (see text, p. 600) 9/15/2008 Zumdahl Chapter 13 18 9 Partial Ionic Character of Covalent Bonds Percent Ionic Character Covalent e.g., H2, Cl2 N2 Polar Covalent e.g., HF, H2O The relationship between the ionic character of a covalent bond and the electronegativity difference between the bonded atoms Ionic Bond e.g., LiF, NaCl 9/15/2008 Zumdahl Chapter 13 19 Covalent Bond Energy and Enthalpy Bond Enthalpy (H) is the enthalpy change in a reaction in which a chemical bond is broken in the gas phase. Bond Energy (E) is the energy needed to break a chemical bond. Energy is released when bonds are formed ( Energy must be supplied when bonds are broken ( ) ) Gas Phase: H = Condensed Phase: H = 9/15/2008 Zumdahl Chapter 13 20 10 The breaking of chemical bonds in stable substances often generates highly reactive products (or intermediates, i.e., radicals) CH4 CH3 + H (H = + 439 kJ mol-1) Bond Enthalpy 9/15/2008 Zumdahl Chapter 13 21 Average Bond Enthalpies C2H6 C2H5 + H CHF3 CF3 + H CHCl3 CCl3 + H CHBr3 CBr3 + H H = + 410 kJ mol-1 H = + 429 kJ mol-1 H = + 380 kJ mol-1 H = + 377 kJ mol-1 average H(C-H) ~ + 412 kJ mol-1 9/15/2008 Zumdahl Chapter 13 22 11 9/15/2008 Zumdahl Chapter 13 23 Estimate the Standard Enthalpy of Reaction for the gas-phase reaction that forms methanol from methane and water CH4(g) + H2O(g) CH3OH(g) + H2(g) 9/15/2008 Zumdahl Chapter 13 24 12 CH4(g) + H2O(g) CH3OH(g) + H2(g) H C H H H + O H H H H C H O H + H H Broken 4 C-H = 4 x 413 C- 2 O-H = 2 x 467 O- Formed 3 C-H or 3 x (-413) C- (- 1 O-H or 1 x (-467) O- (- 1 C-O or 1 x (-358) C- (- 1 H-H or 1 x (-432) H- (- -2496 kJ mol-1 Exothermic +2586 kJ mol-1 Endothermic H = 9/15/2008 = Zumdahl Chapter 13 25 CCl2F2 + 2H2 CH2Cl2 + 2HF Hr = ? = H products H reactants = 114 kJ mol-1 Application of Bond Enthalpy (Endothermic) Given a reaction 1st Step is break all bonds to give free atoms in the gas phase 2nd Step is form new bonds for the products. (Exothermic) 9/15/2008 Zumdahl Chapter 13 26 13 9/15/2008 Zumdahl Chapter 13 27 14 ...
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This note was uploaded on 09/04/2009 for the course CHEM 1310 taught by Professor Cox during the Spring '08 term at Georgia Institute of Technology.

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