Hot Pack Cold Pack Exothermic & Endothermic Processes
Lesson Plan
Guiding Question: How do hot packs and cold packs change temperature?
Overview: Explore endothermic and exothermic processes by making a cold pack or hot pack.
Learning Goals:
● Compare and contrast endothermic and exothermic processes. ○ Exothermic Process = −ΔH The system loses energy (heat) to the surroundings. G raph (↓) ○ Endothermic Process = +ΔH The system absorbs energy (heat) from the surroundings. graph (↑) ● Recognize that enthalpy of solution depends on the combined enthalpy for two competing processes: the hydration enthalpy a nd lattice enthalpy. ● Observe the hydration enthalpy of ions from salts in solution. ● Calculate the enthalpy of solution using Hess’s Law.
Key Concepts: E xothermic and Endothermic Processes, Dissociation, Enthalpy, Heat Flow, Heat of Solution
Link to Simulation: https://interactives.ck12.org/simulations/chemistry/exothermic-and-endothermic/app/index.html
Background Video
If you play sports, or are generally active, you’ve probably gotten injured at some point. Maybe you’ve sprained a muscle or were hit by a ball and had to apply heat or ice to help with the pain and recovery. It isn’t always easy to keep large amounts of ice around or a hot pack warm on a rainy day. Instant hot and cold packs remain at the surrounding temperature until you need them to become really hot or really cold. The packet is filled with a solid compound, usually a salt, and a small inner pouch of water. To activate it, the pack is squeezed, which breaks open the inner water pouch, and the pack starts to change temperature! Yikes! Looks like someone was injured! Can you figure out how to make a hot pack or cold pack for them?
Teacher Guide | Page 1
Simulation Overview
Chemical Graph of Energy vs. Time This selector allows you to choose what ionic compound This graph shows how energy is released or absorbed as will be used in the hot/cold pack. Different salts have a result of dissolving the ionic compound. After the trial is different hydration/solution enthalpies which determine if complete, the arrow indicating the total amount of heat dissolving them in water will heat or cool the solution. dissipated or sequestered will appear.
Ion Labels Enthalpy Diagram This toggle will add or remove charge symbols to the This shows an enthalpy diagram for the ionic compound ionic compound being dissolved in the animation. being dissolved after the trial is completed. This shows students how small changes in the Lattice enthalpy and Hydration Enthalpy of each chemical determines if it will heat or cool the solution in the pack.
Teacher Guide | Page 2
Molecule Explorer
Sodium Chloride - NaCl Sodium chloride, commonly known as table salt, is good for more than just making food taste good! When sodium chloride dissolves in water, each molecule of sodium chloride becomes a sodium ion (Na+) and a chloride ion (Cl- ) Lattice Enthalpy = -790 kJ/mol Hydration Enthalpy = +794 kJ/mol Solution Enthalpy = +4 kJ/mol
Lithium Chloride - LiCl Lithium chloride is an ionic compound that dissolves easily in water. When it is in a humid environment, it attracts water from the air. Lattice Enthalpy = -864 kJ/mol Hydration Enthalpy = +827 kJ/mol Solution Enthalpy = -37 kJ/mol
Sodium Hydroxide - NaOH
Unlike the other chemicals in this simulation which are salts, sodium hydroxide is a strong base. This means that, like salts, sodium hydroxide breaks apart into its ions (Na+ and OH-) in water. What makes it a base and not a salt is the OH- ion. Bases by definition are substances that dissolve in water to produce OH- ions. Like other strong bases, sodium hydroxide is caustic, meaning that in high enough concentrations it will burn skin and should be handled with extreme care. A solution of sodium hydroxide is used to make soda and pretzels. Lattice Enthalpy = -892 kJ/mol Hydration Enthalpy = +847 kJ/mol Solution Enthalpy = -45 kJ/mol
Potassium Chloride - KCl
Potassium chloride is an ionic compound often used as a salt substitute for people who have too much sodium in their diets. However, sprinkling potassium chloride on your potatoes might not lead to the desired results. Most people find potassium chloride to have a bitter, not salty, taste. Lattice Enthalpy = -720 kJ/mol Hydration Enthalpy = +737 kJ/mol Solution Enthalpy = +17 kJ/mol
Water - H2O
Water dissolves so many other substances we sometimes call it "the universal solvent." Here water is the solvent and salt is the solute. You may have noticed H2O is written above the arrow in the chemical equation. This is because salt dissolving in water is a physical change; it does not chemically alter the substances involved.
Teacher Guide | Page 3
Real-World Connections
What is a calorie? One measure of heat flow is the calorie (cal) - the quantity of heat required to raise the temperature of 1 gram of
water by 1°C. The term Calorie (with a capital 'C') is Why can’t we just make water? commonly used when referring to the amount of energy released by the food you eat. Calories contained within It's not that easy! In order to make water, two hydrogen food are actually kilocalories (kcal). To say that the snack atoms need to bond to one oxygen atom in an exothermic 'contains' 100 Calories means that 100,000 cal of energy reaction. The problem is that individual hydrogen and is released when that snack is processed by your body. oxygen atoms are very flammable and can result in a dangerous explosion when trying to provide the energy needed to cause them to combine.
How do air activated hand warmers work?
Hand warmers help to keep people warm in cold climates through the oxidation of iron when exposed to air - which is an exothermic process.
Teacher Guide | Page 4
Name: ______Date: ______
Hot Pack Cold Pack Exothermic & Endothermic Processes
Exploration Questions
Challenge Me Questions 1-7: As you explore the simulation, answer the following questions.
1. Describe the transfer of energy in hot packs. Where does the energy come from?
2. Describe the different terms: lattice enthalpy and hydration enthalpy.
3. Which process in question 2 is exothermic? Which process is endothermic? Explain your reasoning.
4. Why does the amount of heat released or absorbed in these processes change over time?
5. Why do hot packs and cold packs eventually return to room temperature?
6. Describe the difference(s) between exothermic and endothermic processes.
7. Why is dissolving salts sometimes an endothermic process and sometimes an exothermic process? Use the terms lattice energy and hydration energy as part of your explanation.
Exploration Worksheet | Page 1
Name: ______Date: ______
Check My Understanding
Questions 8-15: Circle the choice that best answers each question.
8. In order for the process to be endothermic, the 12. In exothermic reactions, the system _____ energy required to break the lattice has to be energy, while the surroundings _____ in energy ______than the energy released as the ions go a. absorbs, increases into solution. b. releases, decreases a. Greater c. releases, increases b. Less d. absorbs, decreases
9. What material would make the most efficient hot pack? 13. When energy is released, what happens to the energy of the system (the pack)? a. sodium chloride a. The energy increases b. lithium chloride b. The energy decreases c. sodium hydroxide c. The energy stays the same d. potassium chloride
14. When energy is released by the pack, what 10. What material would make the most efficient happens to the energy of the surroundings cold pack? (outside the pack)? a. sodium chloride a. The energy increases b. lithium chloride b. The energy decreases c. sodium hydroxide c. The energy stays the same d. potassium chloride
15. What happens to the energy in the bonds during 11. In endothermic reactions, the system _____ a “hot pack” reaction? energy, while the surroundings _____ in energy a. The energy in the bonds is released a. absorbs, increases b. The energy in the bonds is absorbed b. releases, decreases c. The energy in the bonds stays the c. releases, increases same d. absorbs, decreases
Exploration Worksheet | Page 2