Chapter 8 Chemical Bonding

Of 1 o2 2 o3 3 h2o2 0 1 0 2 0 3 4 87 exceptions

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Unformatted text preview: he Octet Rule q Odd-Electron Molecules q Incomplete Octets (<8) qExpanded Octets (>8) 4 Exceptions to the Octet Rule q Odd-Electron Molecules q Write a Lewis formula for NO and for NO2 q Why does NO2 combine with itself to form N2O4? q Incomplete Octets q Write a Lewis formula for BH3 4 Exceptions to the Octet Rule qMolecules with too few electron pairs Molecules can bond with molecules with unshared electron pairs to form a new sharedelectron electron-pair bond. qBH3 + :NH3 → H3B:NH3 4 Xe(g) + F2(g) → XeF2(s) Why is this an Why unusual reaction? unusual 4 Exceptions to the Octet Rule q Expanded Valence Shells q What do you do if there are too many What electrons to be accommodated by octets? electrons q Write Lewis formulas for the following: SF4 SF6 IF4+ XeF4 XeF2 PF5 BrF3 BrF5 XeF qWhen do we find expanded octets? When 4 Group Work q Draw the Lewis structure for each q I3− q IF4- 5 Which of these molecules should not exist? NF3 NF5 PF5 SF6 O F6 1. NF5 2. PF5, SF6 3. NF3 4. OF6 5. NF5, OF6 0% 1 0% 2 0% 3 0% 4 0% 5 5 8.8 Strengths of Covalent Bonds q Bond Energy or Bond Dissociation Energy Bond - energy required to break a bond in a gaseous molecule gaseous Less stable q Reactions generally proceed to form Reactions compounds with more stable bonds (greater bond energy) (greater q Values in Table 8.4 More stable q Bond energy is the amount of energy to Bond break bond break 5 Average Bond Energy q Bond energy varies somewhat from one Bond molecule to another, or even within one molecule, so we use an average bond energy (D) (D) q H-OH 502 kJ/mol q H-O 427 kJ/mol q H-OOH 431 kJ/mol q Average = 459 kJ/mol for O-H 5 Bond Energies (Table 8.4) 5 Predicting ΔH q Consider the following reaction: q CH4 + Cl2 →CH3Cl + HCl q Which bonds break and which form as Which reactants convert to products? reactants 5 Break all reactant bonds, then make product bonds 5 Bond Energies and Heats of Reaction q ∆ Hrxn = Σ Dbroken - Σ Dmade reactants products q Use only when heats of formation are not Use available, since bond energies are average values for gaseous molecules. values q This is only an approximate ∆ Hrxn This 5 Heats of Reaction q Use bond energies to calculate the enthalpy Use change for the following reaction: change N2(g) + 3H2(g) → 2NH3(g) (g) q ∆ Hrxn = DN ≡ N + 3DH-H - 6DN-H rxn ∆ Hrxn = 942 + 3(432) - 6(386) = -78 kJ measured value = -92.2 kJ q Why are the values different? 5 Sample Problem q Use bond energies to calculate the Use enthalpy change for the following reaction: reaction: 2CO(g) + O2(g) → 2CO2(g) (g) DC ≡ O = 1072 kJ DO = O = 492 kJ DC = O = 799 kJ q ∆ Hrxn = 2DC ≡ O + DO = O - 4DC = O 2D rxn ∆ Hrxn = 2(1072) + 492 - 4(799) = -560 kJ 5 Bond Energy and Bond Length q The distance between the nuclei of the atoms The involved in a bond is called the bond length. involved q Multiple bonds are shorter than single Multiple bonds. bonds. q Multiple bonds are also stronger than single Multiple bonds. bonds. q As the number of bonds between two atoms As increases, the atoms are held closer and more tightly together. more 6 Bond Lengths 6...
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