CH%20301%20Chapter%206%20notes%20part%203 - CH301 Notes...

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HCl (g) Æ H (g) + Cl (g) H = +432 kJ/mol CH301 Notes Chapter 6 part 3 Bond Energy: amount of energy required to break a bond and separate the atoms in the gas phase. Breaking bonds always requires energy ! ALL B.E. VALUES ARE POSTIVE!! Bigger B.E. = more stable bond = harder to break. BE can be measured directly in some cases. BE can be measured via Hess’ Law using f (see below) BE can come from heat or light Bond energies can be calculated from other H f ° values Example 1: Calculate the bond energy for hydrogen fluoride, HF . 2. Calculate the average N-H bond energy in ammonia, NH 3 AVERAGE Bond Energies have been measured and tabulated. . Here’s ours for HW and exams: Bond Energy Table Notes : See your book: pages 234, 235: Tables 6.7 and 6.8 Exact values of BE differ slightly for same bond in different compounds - can be overlooked Single bonds weaker (lower BE) than double bonds Double bonds weaker (lower BE) than than triple bonds. Order (and thus strength) of the bond varies due to overlap of different atomic orbitals. Exact values of BE differ slightly between tables! READ VALUES CAREFULLY!!! Note this above figure is for an exothermic reaction. The endothermic one would have the reactants lower in energy than the products. .
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USING Bond Energies: For GAS PHASE REACTIONS ONLY: H o rxn = Σ BE reactants - Σ BE products Uses: If all BE’s known: can calculate H o rxn If H o rxn is known: can calculate BE from other BE’s, or, H o f ‘s HINT : You must count ALL of the bonds that are breaking and forming. It really helps to draw the Lewis structures of the gas molecules.
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This note was uploaded on 09/09/2009 for the course CH 301 taught by Professor Fakhreddine/lyon during the Spring '07 term at University of Texas.

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CH%20301%20Chapter%206%20notes%20part%203 - CH301 Notes...

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