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16-Bond Energies+Bond Enthalpies

# 16-Bond Energies+Bond Enthalpies - 5.111 Lecture Summary#16...

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5.111 Lecture Summary #16 October 16, 2009 16.1 Reading for today: Sections 6.13, 6.15-6.18, 6.20 Reading for Lecture 17: Sections 7.1, 7.2, 7.8, 7.12, 7.13, 7.15, 7.16 BOND ENERGIES/BOND ENTHALPIES Bond energies are important conceptually, but not that useful in practice. More useful are bond enthalpies, Δ H. The enthalpy change in a reaction is the heat input (or output) at constant pressure. This can be measured easily! CH 3 + H Δ E d CH 4 H = Δ E + Δ (PV) = q rxn For gases, Δ H & Δ E differ by 1-2%. For liquids and solids, negligible difference. Bond enthalpies for C—H bonds … all gases CH 4 CH 3 + H Δ H° = +438 kJ/mole C 2 H 6 C 2 H 5 + H Δ H° = +410 kJ/mole CHF 3 CF 3 + H Δ H° = +429 kJ/mole CHCl 3 CCl 3 + H Δ H° = +380 kJ/mole CHBr 3 CBr 3 + H Δ H° = +377 kJ/mole Δ H° is positive – endothermic The values are within 8% of average value. (See Table 6.8 in text.) What is the little circle superscript? In order to compare bond enthalpies and determine reaction thermodynamics, we need to define a standard state - in this case defined as a pressure of 1 bar = 10 5 Pa = 10 5 kg/m•s 1 atm = 1.013 bar Δ H° means Δ H measured at 1 bar We’ll use mostly values determined at room T. Δ H depends weakly on T. If it’s measured at 1 bar and a different T, it’s still called Δ H° (with T specified).

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5.111 Lecture Summary #16 October 16, 2009 16.2 Importance of bond enthalpies:
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16-Bond Energies+Bond Enthalpies - 5.111 Lecture Summary#16...

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