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March 11, 2009 - Reaction Initial and final molecules have...

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March 11, 2009
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Reaction Initial and final molecules have there own potential energies independent of path Energy of reaction lost (-) or added (+) is outside If A → B ΔH r = -100,000 A → D → C → B ΔH r = -100,000
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Hess’ Law Find enthalpy (or energy) for any reaction via a path where enthalpies are known A → 2 C ΔH r = ? Know A 2B ΔH r = +20KJ B C ΔH r = -30KJ 2B → 2C ΔH r = -60KJ Add 1 + 3 A → 2C +20 -60 = - 40KJ
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C(gr) → C(dia) ΔH r = ? Path burn both C(gr) + O 2 CO 2 ΔH r = -400KJ C(dia) + O 2 CO 2 ΔH r = -402KJ • CO 2 C(dia) + O 2 ΔH r = +402KJ Add 1 + 3 • C(gr) → C(dia) ΔH r = +402 -400 = +2KJ
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2A 1B ΔH r = -20,000 @ 300K What is ΔH r at 400K ? 2A B 400K | | 2A B 300K
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3 step path 1. Cool A from 400 to 300 ΔH = 2C p (300 -400) = -2C p (100) If 2C p (A) = 60, ΔH = -6000 2. React at 300K 2A B ΔH r = -20,000J
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3. Heat B from 300K to 400K ΔH = C p (B) (400-300) = C p (B) (100) If C p (B) = 20 ΔH = +20 (100) = +2000 Reaction at 400K (1) + (2) +(3) = -6000 -20,000 +2,000 = -24,000J
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Work Non-path Work
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