# L32-answers-2 - Which requires more energy A"cooling graph...

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1 Which requires more energy? A. Warming 15.0 g of water from 25 o C to 37 o C. B. Warming 60.0 g of aluminum from 25 o C to 37 o C For H 2 O (l): Compound Al (s) Specific Heat Capacity q = m x c x T = (15.0 g) (4.18 J g 1o C 1 ) ( 12 o C) 0.90 J g 1o C 1 H 2 O (s) 2.06 J g 1o C 1 H 2 O (l) 4.18 J g 1o C 1 For Al (s): q = m x c x T = (60.0 g) (0.90 J g 1o C 1 ) ( 12 o C) A “cooling graph” for substance Y at constant pressure: 1. Which has the smallest specific heat capacity: Y(s), Y(l), or Y(g)? 2. Which is larger, the heat of fusion or the heat of vaporization? A. Y(s), H vap B. Y(l), H vap C. Y(g), H vap D. Y(l), H fusion E. Y(s), H fusion Y(g) is cooling Y(g) Y(l) Y(l) is cooling Y(l) Y(s) Y(s) is cooling Bond Enthalpies Is the following reaction endothermic or exothermic ? H 2 (g) + Cl 2 (g) 2 HCl (g) Bond Bond Enthalpy (kJ/mol) H Cl H H Cl Cl 431 436 242 A. Endothermic B. Exothermic C. Not enough information Breaking 1 mol of H-H bonds: + 436 kJ Breaking 1 mol of Cl-Cl bonds: + 242 kJ Forming 2 mol of H-Cl bonds: 2( 431 kJ) 184 kJ S, H, and G: 2 H 2 (g) + O 2 (g) 2 H 2 O (g) Predictions: S < 0 (3 moles of gas goes to 2 moles of gas) H < 0 (recall demos, fuel cells, etc!) G o Temp H o S o 0 0 At what temperature does this reaction change from product-favored to reactant- favored? G o = 0 = H o T S o T = H o / S o So we need to have actual values for H o and S o . Calculate these from thermodynamic tables (Appendix J in your text). . H o = 2 H o f (H 2 O(g)) 2 H o f (H 2 (g)) −∆ H o f (O 2 (g)) = 483.64 kJ S o = 2 S o (H 2 O(g)) 2 S o (H 2 (g)) S o (O 2 (g)) = 88.86 J/K T at which G o

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L32-answers-2 - Which requires more energy A"cooling graph...

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