Lecturez12SHWeb - Mon Oct 3 2011 Todays Lecture Energy...

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1 Mon. Oct. 3, 2011 Today’s Lecture Energy, Enthalpy, and Thermochemistry 9.5 Hess’ Law 9.6 Enthalpies of Formation 9.7/8 Sources of Energy (Omit) Lecture 12
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2 Hess’s Law: – indirect determination of Δ H – H is a state function: path independent Δ H is the same whether a reaction occurs in one step or in a series of steps: N 2(g) + 2O 2(g) 2NO 2(g) Δ H 1 = 68 kJ – reaction may be carried out in two steps: N 2(g) + O 2(g) 2NO (g) Δ H 2 = 180 kJ 2NO (g) + O 2(g) 2NO 2(g) Δ H 3 = -112 kJ ———————————————————— N 2(g) + 2O 2(g) 2NO 2(g) Δ H 2 + Δ H 3 = 68 kJ • note that Δ H 1 = Δ H 2 + Δ H 3
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3 • characteristics of Δ H : – if a reaction is reversed, sign of Δ H is also reversed – magnitude of Δ H is directly proportional to the quantities of reactants and products in a reaction – If the coefficients of a balanced equation are multiplied by an integer, the value of Δ H is multiplied by the same number. See Example 9.6 in text
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4 Standard enthalpies of formation: can measure Δ H by calorimetry some reactions may not be easily investigated: e.g., C graphite(s) C diamond(s) – symbol: Δ H o f ( o indicates standard conditions) Δ H o f of a compound:
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