Exam (elaborations) PHY 111 (PHY 111)

Feel the Heat Answer Key Vocabulary: calorimeter, conductor, controlled experiment, dissolve, endothermic, exothermic, insulator, solute, solution, solvent, surroundings, system Prior Knowledge Questions (Do these BEFORE using the Gizmo.) [Note: The purpose of these questions is to activate prior knowledge and get students thinking. Students are not expected to know the answers to the Prior Knowledge Questions.] Eduardo hurts his knee during a basketball game. The trainer applies a cold pack, which gets cold after being squeezed. The next day, Eduardo’s friend Beth is going sledding. Beth buys some hand warmers and puts them into her gloves. All morning, her hands stay toasty warm. 1. How do you think these devices work? Answers will vary. [Cold packs typically work by dissolving a salt such as ammonium nitrate into water. Commercial hot packs work by crystalizing sodium acetate or by oxidizing iron.] 2. Where do you think the “cold” and the heat comes from? Answers will vary. Gizmo Warm-up As you saw with the instant cold pack and the hand warmers, some reactions absorb energy while others release it. In the Feel the Heat Gizmo, you will explore these energy changes while making your own hot and cold packs. To begin, select the TEST POWDERS tab. Drag a bottle of NaC2H3O2 (sodium acetate) from the shelf. Using the sliders, decide how much water and powder to add. Note the beginning temperature, and then press Play ( ) to see what happens. 1. Did the temperature go up or down? Up 2. Experiment with different powders until you find one that produces the opposite effect. What powder did you use? NH4NO3, KCl, and NaCl will produce a temperature decrease. When a powder dissolves in water a solution is formed. The powder is the solute and the water is the solvent. Oftentimes energy changes accompany the formation of a solution. When added to water, some powders cause the resulting solution to get hot, while others make it cold. Some powders don’t produce a temperature change at all. When energy changes do occur, they can be put to good use. Activity A: Molecular view Get the Gizmo ready: • Click Reset ( ). • Drag a bottle of NaC2H3O2 from the shelf. Introduction: The reactions in this activity are performed within a calorimeter, an insulated device that keeps heat from escaping, enabling you to accurately record temperature changes. Question: Why does the temperature change when a powder is dissolved in water? 1. Observe: Turn on Show molecular view, and notice the water molecules. Set the Water volume to 100 mL and the Powder mass to 20 g, and then click Play. Click Pause ( ) after adding the powder. You should now see show some sodium acetate in the water. A. What color represents the bonds between the particles of NaC2H3O2? Orange B. Click Play. Watch the animation a few times. What happens to the NaC2H3O2 bonds? They are broken. C. What happens to the bonds between water molecules? They are broken. D. What color are the new bonds that form between the solute and solvent? Blue 2. Investigate: In the animation, the purple arrows represent....... .....................................................Continued....................