Meetings%2031%2C%2032%20F%202010

Meetings%2031%2C%2032%20F%202010 - Chapter 9 Coordinate...

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1 Meetings 31, 32 – Chapter 9 Coordinate covalent bonds Example H 3 N: + BF 3 H 3 N-BF 3 The bonding pair comes completely from one of the reactants. Compounds with incomplete octets can complete them by bonding with another compound with a lone pair to spare. Sometimes written as H 3 N BF 3 Lewis Acid, Lewis Base Lewis acid: electron pair acceptor. Lewis base: electron pair donor The bond formed is a coordinate covalent bond. In the reaction of NH 3 with BF 3 ammonia acts as a Lewis base and boron trifluoride acts as a Lewis acid.
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2 Bond Lengths and Bond energies Bond lengths depend on bond strength and the type of bond. Other things equal, stronger bonds are shorter than weaker ones Other things equal, single bonds are weaker than double bonds which are weaker than triple bonds, etc. Bond Length & Bond strength Single > Double > Triple > Quadruple Example : the single C-C bond in ethane is weaker (and longer, 152 pm) than the double C=C bond in ethylene (133 pm); the triple bond in acetylene is even shorter (120 pm) and stronger.
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3 Ethane Ethylene C-HΞC-H A cetylene Bond Energy H 2 requires 436 KJ/mol Polyatomic molecules are more complex First O-H bond dissociation in H 2 O requires 499 KJ/mol (center) Second O-H bond requires 428 KJ/mol
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4 Compare the bond lengths and bond energies (bond strengths) for single double and triple bonds. Note also that the covalent radius for homonuclear diatomic species is ½ the bond distance.
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5 Calculations involving bond energies Claim : we can calculate an approximate reaction enthalpy from bond energies (BE) Procedure: 1. Break all bonds in reactants to obtain gaseous reactants 2. Remake new bonds for the products from the gaseous atoms. 3. ΔH r = ΔH bonds broken - ΔH bonds formed 4. ΔH bonds broken ≈ ∑BE(reactants) 5. ΔH bonds formed ≈ -∑BE(products) 6. ΔH r ≈ ∑BE(reactants) – ∑BE(products) Example
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6 Using bond energies, estimate the enthalpy of formation for hydrazine, N
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Meetings%2031%2C%2032%20F%202010 - Chapter 9 Coordinate...

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