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 activat...
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 getscold 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 nitrateinto
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 ownhot 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.
, Get the Gizmo ready:
Activity A:
Click Reset ( ).
Molecular view 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 Watervolume
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 NaC2H3O2bonds?
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 energy that is being absorbed from the
water, while the orange arrows show energy that is being released into the solution.
A. Replay the animation, carefully noting the purple arrows. What does this absorbed energy
do? The absorbed energy breaks apart bonds between the solute particles.
B. Now replay the animation but focus on the orange arrows. What occurs every timean
orange arrow is released? New bonds are formed between solute and solvent.
C. Fill in the blanks: When bonds are broken, energy is absorbed.
When new bonds form, energy is released.
D. Based on the arrow sizes, do you think more energy is absorbed in breaking bonds,or is
more energy released when new bonds form? More energy is released.
E. Which bonds do you think are stronger, the bonds that are broken or the bonds thatare
formed? Explain your reasoning.
The bonds that are formed are stronger. Since more energy is released as a result ofbond
formation than is absorbed to break bonds, the bonds formed must be stronger.
(Activity A continued on next page)
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