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Homework 9-solutions (1)

# Homework 9-solutions (1) - ching(rbc537 Homework 9...

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ching (rbc537) – Homework 9 – Brodbelt – (53765) 1 This print-out should have 19 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. Homework 9: Thermochemistry Due November 26, 2008, at 11:00 PM 001 10.0 points A B Reaction progress Energy (kJ) 250 300 350 Graph I A B Reaction progress Energy (kJ) 250 300 350 Graph II Which graph corresponds to an exothermic process? 1. Graph II correct 2. Neither Graph I nor Graph II 3. Graph I 4. Both Graph I and Graph II 5. Cannot be determined with the informa- tion given Explanation: Exothermic processes end with less energy than they began with; the energy was trans- ferred to the surroundings. 002 10.0 points The best definition of heat is 1. temperature. 2. kinetic energy. 3. vibrational energy. 4. the transfer of energy. correct 5. potential energy. Explanation: Heat flows spontaneously from a hotter body to a colder body. 003 10.0 points How much heat is required to change the tem- perature of two cups of water (500 mL) from room temperature (25 C) to boiling? 1. 1.57 kJ 2. 78.5 kJ 3. 157 kJ correct 4. 7.85 kJ 5. 15.7 kJ 6. 0.785 kJ Explanation: ρ water = 1 g/mL, so 500 mL = 500 g q = m c Δ T = (500 g) parenleftbigg 4 . 184 J g · C parenrightbigg (100 C - 25 C) = 1 . 569 × 10 5 J = 156 . 9 kJ 004 10.0 points In the manufacture of nitric acid by the oxi- dation of ammonia, the first product is nitric oxide. The nitric oxide is then oxidized to nitrogen dioxide: 2 NO(g) + O 2 (g) -→ 2 NO 2 (g) Calculate the standard reaction enthalpy for the reaction above (as written) using the following data:

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ching (rbc537) – Homework 9 – Brodbelt – (53765) 2 N 2 (g) + O 2 (g) -→ 2 NO(g) Δ H = 180 . 5 kJ N 2 (g) + 2 O 2 (g) -→ 2 NO 2 (g) Δ H = 66 . 4 kJ 1. - 975 . 0 kJ/mol rxn 2. - 100 . 3 kJ/mol rxn 3. - 114 . 1 kJ/mol rxn correct 4. - 520 . 2 kJ/mol rxn 5. - 690 . 72 kJ/mol rxn 6. - 128 . 2 kJ/mol rxn 7. - 252 . 4 kJ/mol rxn Explanation: Using Hess’ Law and the given standard re- action enthalpies, the first reaction is reversed and added to the second: 2 NO(g) -→ N 2 (g) + O 2 (g) Δ H = - 180 . 5 kJ N 2 (g) + 2 O 2 (g) -→ 2 NO 2 (g) Δ H = 66 . 4 kJ 2 NO(g) + O 2 (g) -→ 2 NO 2 (g) Δ H = - 114 . 1 kJ 005 10.0 points Burning 1 mol of methane in oxygen to form CO 2 (g) and H 2 O(g) produces 803 kJ of en- ergy. How much energy is produced when 3 mol of methane is burned? 1. 2,409 kJ correct 2. 268 kJ 3. 1,606 kJ 4. 803 kJ Explanation: n 1 = 1 mol q 1 = 803 kJ n 2 = 3 mol If we know how much heat is evolved when 1 mole of methane is combusted, then we know how much heat would be evolved if 3 mol of methane were combusted: parenleftbigg 803 kJ 1 mol methane parenrightbigg (3 mol methane) = 2409 kJ 006 10.0 points Calculate the standard reaction enthalpy for the reaction of calcite with hydrochloric acid CaCO 3 (s) + 2 HCl(aq) -→ CaCl 2 (aq) + H 2 O( ) + CO 2 (g) The standard enthalpies of formation are: for CaCl 2 (aq) : - 877 .
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Homework 9-solutions (1) - ching(rbc537 Homework 9...

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