Group Quiz 2 Blue KEY

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School

University of Wisconsin, Madison *

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Course

103

Subject

Chemistry

Date

Dec 6, 2023

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Chem 103 Group Quiz 1_version #1 Date last revised 10/12/2021 Page 1 of 2 Group Quiz #2 v1 TA should evaluate this document (circle one): YES NO 10 points All group member’s names: __________________________________________________ ___________________________________________________________________________ Directions: Upon entry to your discussion classroom, you will be placed into a randomized group of 3-4 students. You will have 20 minutes to complete the quiz. • For the first 10 minutes, you will work with your group members to answer the quiz questions. • For the following 10 minutes, your group will be able to interact with other groups to compare how you’ve answered the quiz questions. • At the end of the 20-minute quiz time, quizzes will be collected. All members in a group will receive the same quiz score. All quiz materials must be turned in, but only one document per group will be evaluated. All other usual quiz rules are still in place: Show all work; circle your final answer. Materials allowed are limited to approved calculators, writing instruments, and provided quiz resources. No electronic devices or other materials may be used. TAs will proctor the quiz time, but they are not allowed to answer questions to help you solve the quiz problems. 1. U.S. military MREs (meals, ready-to-eat) are heated by the oxidation of magnesium metal by liquid water to produce magnesium hydroxide and hydrogen gas. A. Write a balanced chemical equation for this reaction. [3 points] Mg(s) + 2H 2 O(l) → Mg(OH) 2 (s) + H 2 (g) + Energy B. Is this process endothermic or exothermic (remember that these are used in heating food)? [1 point] Exothermic C. Add the term “energy” to the proper side of your equation in part A to match your determination of endo- or exothermic in part B. [1 point] See above /5 points 1 point correct species 1 point correct phases 1 point properly balanced Energy term graded in part C

Page 2 of 2 2. Two choices for a fuel in some appliances are propane (C 3 H 8 ) or butane (C 4 H 10 ). Combustion reactions are shown below. C 3 H 8 (g) + 5O 2 (g) → 3CO 2 (g) + 4H 2 O(l) + 1190.0 kJ 2C 4 H 10 (g) + 13O 2 (g) → 8CO 2 (g) + 10H 2 O(l) + 5755.0 kJ A. Determine the energy released per gram of propane combusted. [1.5 points] 1190.0 kJ 1 mole C 3 H 8 ∗ 1 mole C 3 H 8 44.11 g = 26.98 kJ/g B. Determine the energy released per gram of butane combusted. [1.5 points] 5755.0 kJ 2 mole C 4 H 10 ∗ 1 mole C 4 H 10 58.14 g = 49.49 kJ/g C. If propane costs $0.0080 per gram and butane costs $0.013 per gram, which gas is more cost effective (which one will provide more energy per dollar)? [2 points] Propane: 26.98 𝑘𝐽 1 𝑔 ∗ 1 𝑔 $0.0080 = 3400 kJ/dollar Butane: 49.49 𝑘𝐽 1 𝑔 ∗ 1 𝑔 $0.013 = 3800 kJ/dollar Butane is more cost effective. OR Propane: $0.0080 1 𝑔 ∗ 1 𝑔 26.98 𝑘𝐽 = $0.00030/kJ = $3.0 x 10 -4 /kJ Butane: $0.013 1 𝑔 ∗ 1 𝑔 49.49 𝑘𝐽 = $0.00026/kJ = $2.6 x 10 -4 /kJ Butane is more cost effective /5 points 1 pt stoichiometry/setup 0.5 pts correct MW – 0.5 pts. if no units 1 pt stoichiometry/setup 0.5 pts correct MW – 0.5 pts. if no units 0.5 pts correct kJ/$ for propane 0.5 pts correct kJ/$ for butane 1 pt correct conclusion 0.5 pts correct $/kJ for propane 0.5 pts correct $/kJ for butane 1 pt correct conclusion Part C should be graded based on answers to parts A and B. Conclusion should match answers to parts A and B (e.g., if work in A and B is incorrect, but would lead to conclusion that ethanol is more cost effective, then students must answer ethanol in part C to receive credit.

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