Feel the Heat

Student Exploration: Feel the Heat Vocabulary: calorimeter, conductor, controlled experiment, dissolve, endothermic, exothermic, insulator, solute, solution, solvent, surroundings, system Prior Knowledge Questions (Do these BEFORE using the Gizmo.) 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? They create heat by means of exothermic reaction. The activated carbon helps to evenly disperse the heat produced, which can average 135 °F 2. Where do you think the “cold” and the heat comes from? The cold is the results from the endothermic reaction as the solid matter in cold packs absorbed water to break down, and the hotness comes from formation of new bonds(exothermic) to form new compounds with other chemical. 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 and KCI 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: Molecular view  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? Purple B. Click Play. Watch the animation a few times. What happens to the NaC2H3O2 bonds? They break and bond with water C. What happens to the bonds between water molecules? they break D. What color are the new bonds that form between the solute and solvent? Orange 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? It points at the orange bond and then disappears B. Now replay the animation but focus on the orange arrows. What occurs every time an orange arrow is released? Text C. Fill in the blanks: When bonds are broken, energy is When new bonds form, energy is released absorbed . . 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? When energy is released E. Which bonds do you think are stronger, the bonds that are broken or the bonds that are formed? Explain your reasoning. The bonds that are formed because the products have lower energy than reactants (Activity A continued on next page) Activity A (continued from previous page) 3. Infer: Drag a bottle of KCl from the shelf. Add 20 g of water to 100 mL of water. Click Play and look once again at the molecular view. A. Did the temperature rise or fall? Fall B. Compare the purple and orange arrows. Does more energy go into breaking bonds, Text or is more energy released when new bonds form? C. Which bonds are stronger: the bonds formed or the bonds broken? The bonds that are broken Explain your reasoning. They require more energy 4. Think and discuss: Based on your observations, why do you think the temperature of the NaC2H3O2 solution rose while that of the KCl solution fell? The energy is released from KCI while the energy was absorbed by sodium 5. Think and discuss: If a solute has really strong bonds between its particles, would you expect it to form a solution that gets hold or cold? Explain your reasoning. Text 6. Infer: Some powders produce little to no temperature change when added to water. Find the powder that has the smallest effect on the temperature of the resulting solution. A. What is the identity of this powder? NaBr B. How do the purple and orange arrows compare? same size C. For this powder, how do you think the strength of the bonds that are broken compare to those that are formed? they are equal 7. Predict: Set the water to 200 mL and the powder to 1 g. Test each powder, looking only at the molecular view each time. After carefully observing the arrows, predict the following. Which powder do you think will produce the hottest solution? CaCl2 Which powder do you think will produce the coldest solution? Nh4NO3 Activity B: Test powders Get the Gizmo ready:  Turn off Show molecular view.  Click Reset. Introduction: In order to determine which combination of water and powder will produce the greatest temperature change, it is necessary to perform a controlled experiment. In a controlled experiment only one variable at a time is changed so its effect can be measured. Question: Which combination of powder and water will produce the greatest temperature increase, and which will produce the greatest temperature decrease? 1. Experiment: For the first controlled experiment, decide how much water you want to use. Add some powder. Note the temperature change. Keeping the water amount constant, add a different amount of the same powder and note the final temperature. Keep adjusting only the powder amount until you have discovered how it affects the solution temperature. What relationship did you discover between the amount of powder used and the change in temperature? Text 2. Experiment: For the next controlled experiment, repeat the above procedure, except this time keep the amount of powder constant and vary the amount of water. What relationship did you discover between the amount of water used and the change in temperature? Text 3. Collect data: For each substance, determine which combination of water and powder will give you the greatest temperature increase or decrease. Record data below. Type of powder Volume of water (mL) Mass of powder (g) Initial temp. (ºC) Final temp. (ºC) Temp. change (ºC) NaC2H3O2 50ml 35g CaCl2 50ml 35g 25C 41 16 97 NH4NO3 KCI 50ml 35g 50ml 35g 25C 25C 72 -7 32 11 14 NaBr 50ml 35g NaCI 50ml 35g 25C 26 1 25C 21 4 25C Which powder produced the greatest temperature increase? CaCI2 Which powder produced the greatest temperature decrease? NH4NO3 How do these results compare to the predictions you made at the end of activity A? they are the same Activity C: Create device Get the Gizmo ready:  Select the CREATE DEVICE tab.  Choose LDPE and drag it to the appropriate box. Introduction: In a hot pack, an exothermic reaction occurs, since energy is released from the system to the surroundings. In an endothermic reaction, energy is absorbed by the system, causing the surroundings to get colder. (The hot or cold pack can be defined as the system.) Question: Would it be better to use a material that releases heat quickly or slowly? 1. Experiment: Notice the five types of bag materials. LDPE, HDPE, and Mylar are types of plastic, while latex and neoprene are types of rubber. Find the combination of water and powder that produces the maximum temperature increase. To maintain a controlled experiment, use the same amounts of water and powder in each bag. Record data below: Type of material LDPE HDPE Latex Mylar Neoprene Maximum temp. (ºC) 67 deg 65 deg 65 deg 68 deg 54 deg 2. Analyze: Use the above data table to answer the following. A. Which pack material got the hottest? Myler Why do you think the outside of this pack got hotter than the other ones? Bonds were stronger B. In which pack material did you see the lowest maximum temperature? Neoprene Why do you think the outside of this pack did not get as hot as the others? energy is being absorbed 3. Apply: Turn on Show data to view a graph of the temperature distribution. Take a snapshot of the graph by clicking the camera icon, then paste it into a blank document to turn in. A. Write down your observations about the graph, noting any trends you see. Text B. What material would enable you to warm your hands the fastest? Myler C. What material would keep your hands warm for the longest time? Neoprene (Activity C continued on next page) Activity C (continued from previous page) 4. Analyze: Heat will travel quickly through a conductor, but more slowly through an insulator. A good conductor will heat up quickly and cool down quickly. The best insulators take longer to warm up but then stay warm for a longer period of time. Myler A. Which material is the best conductor? B. Which material is the best insulator? Neoprene C. If the bag was made from a metal (like sturdy aluminum foil), do you think it would lose its heat faster or slower than the plastic bags? Explain your reasoning. Sl0wer because metals are heat conductors 5. Experiment: To construct a cold pack, choose the powder from activity B that produced the greatest temperature decrease. Just like with the hot pack, use the same amounts of water and powder in each pack. Test each of the five types of materials. Record data below: Type of material Minimum temp. (ºC) LDPE 7 deg HDPE Latex Mylar 7deg 7 deg 6 deg Neoprene 12 deg 6. Analyze: Use the above data to answer the following. A. Which material gave you the coldest pack? Myler B. Which material would produce the longest-lasting cold pack? Neoprene 7. Compare: Take a snapshot of the graph by clicking the camera icon, then paste it into a blank document to turn in. Compare the temperature graphs for the cold and hot packs. A. Did the material that got the hottest also get the coldest? yes B. Did the material that stayed warm the longest also stay cold the longest? yes C. What do these observations tell you about the rate and direction of heat flow through Text each specific type of plastic? 8. Interpret: Since hot packs release heat, you might assume that cold packs release cold. Use the definition of endothermic to explain why this view of cold packs is not accurate. Text Extension: Get the Gizmo ready:  Select the CREATE DEVICE tab. Challenge Introduction: Hot and cold packs can be used for a variety of purposes. A pack that is blistering hot or freezing cold might not always be the most practical. For example, a typical hot pack you might buy from the store will only reach a maximum temperature of around 54 ºC. If it gets much hotter it might burn the skin. Some packs are designed to heat or cool quickly, while others last for a longer period of time. In this activity, you will be challenged to design several different packs that each meet certain specifications. Question: Can you make a hot or cold pack that will stay within a certain temperature range for a specified period of time? 1. Think and discuss: Think about all the times a hot pack might be nice to have. List as many practical uses as you can for a hot pack. Heats improve blood flow and warm to help the body 2. Think and discuss: Besides being used to reduce swelling, what are some other possible uses for a cold pack? bruises, bites and other skin injuries 3. Experiment: Use the Gizmo to design a hot or cold pack that meets each of the following requirements. When you solve the challenge, describe the bag material, type of powder, powder mass, and volume of water you used. A. The outside bag temperature reaches 54 ºC (but no higher) within 2 minutes. Mylar, 35g of CaCL2, 120 ml of water B. The temperature does not go above 50 ºC, and does not go below 40 ºC for at least 25 minutes. Neoprene, 35g of CaCL, 65ml of water C. The temperature reaches 8 ºC in less than 2 minutes. Mylar, 35g of NH4NO3, 120 ml of water D. The temperature does not go below 10 ºC, and stays below 20 ºC for at least 15 minutes. Neoprene, 35g of NH4NO3, 65ml of water