# Exam04answers pg3 - Consider the reaction CH 4(g X 2(g →...

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Cheung, Anthony – Exam 4 – Due: Dec 6 2006, 1:00 pm – Inst: McCord 3 008 (part 1 of 1) 12 points Trapped on a desert island with a desper- ate need to know the boiling point of chlo- roform (CHCl 3 ), you come across some old thermodynamic tables that include the fol- lowing data: For CHCl 3 ( l ): Δ H 0 f = - 134 . 5 kJ / mol , S 0 = 202 J / mol · K For CHCl 3 (g): Δ H 0 f = - 103 . 1 kJ / mol , S 0 = 295 . 6 J / mol · K Using the data, what is the approximate boiling point of chloroform? 1. 273 K 2. 665 K 3. 349 K 4. 335 K correct Explanation: Δ H 0 f , CHCl 3 ( l ) = - 134 . 5 kJ / mol S 0 CH 3 Cl 3 ( l ) = 202 J / mol · K Δ H 0 f , CHCl 3 (g) = - 103 . 1 kJ / mol S 0 CHCl 3 (g) = 295 . 6 J / mol · K Δ H vap = s n Δ H f , prod - s n Δ H f , rct = ( - 103 . 1 kJ / mol) - ( - 134 . 5 kJ / mol) = 31 . 4 kJ / mol Δ S vap = s n Δ S f , prod - s n Δ S f , rct = (295 . 6 J / mol · K) - (202 J / mol · K) = 93 . 6 J / mol · K = 0 . 0936 kJ / mol · K Δ G = Δ H - T Δ S At the boiling point Δ G = 0, so solve for T of the boiling point: T = Δ H Δ S = 31 . 4 kJ / mol 0 . 0936 kJ / mol · K = 335 . 47 K 009 (part 1 of 1) 12 points
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Unformatted text preview: Consider the reaction CH 4 (g) + X 2 (g) → CH 3 X(g) + HX(g) , where X is a halogen-like atom (Group 7A). Calculate Δ H for this reaction if bond en-ergy tables give the following values: C H : 416 kJ/mol X X : 235 kJ/mol H X : 322 kJ/mol C X : 258 kJ/mol Correct answer: 71 kJ / mol. Explanation: C H H H H + X X----→ C X H H H + H X Δ H rxn = s BE rct-s BE prod = b 4 (C H) + (X X) B-b 3 (C H) + (C X) + (H X) B = b (C H) + (X X) B-b (C X) + (H X) B = b (416 kJ / mol) + (235 kJ / mol) B-b (258 kJ / mol) + (322 kJ / mol) B = 71 kJ / mol 010 (part 1 of 1) 10 points Which of the following would probably have a positive Δ S value?...
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## This note was uploaded on 01/23/2011 for the course CH 301 taught by Professor Fakhreddine/lyon during the Spring '07 term at University of Texas.

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