Summary

In this demonstration, students will observe temperature changes in chemical hot and cold packs and discuss processes of endothermic and exothermic changes. They will also see that common household products can be used to make a hot and cold pack.

Grade Level

Middle and high school

AP Chemistry Curriculum Framework

This demonstration supports the following unit, topic, and learning objective:

• Unit 6: Thermodynamics
  • Topic 6.1: Endothermic and Exothermic Processes
    • ENE-2.A:Explain the relationship between experimental observations and energy changes associated with a chemical or physical transformation.

The extension lesson additionally supports the following unit, topic, and learning objective:

• Unit 6: Thermodynamics
  • Topic 6.4: Heat Capacity and Calorimetry
    • ENE-2.D: Calculate the heat q absorbed or released by a system undergoing heating/ cooling based on the amount of the substance, the heat capacity, and the change in temperature.

NGSS Alignment

This demonstration will help prepare your students to meet the performance expectations in the following standards:

• HS-PS3-4. Plan and conduct an investigation to provide evidence that the transfer of thermal energy when two components of different temperature are combined within a closed system results in a more uniform energy distribution among the components in the system (second law of thermodynamics).
• Science and Engineering Practices: Plan and conduct an investigation individually and collaboratively to produce data to serve as the basis for evidence, and in the design: decide on types, how much, and accuracy of data needed to produce reliable measurements and consider limitations on the precision of the data (e.g., number of trials, cost, risk, time), and refine the design accordingly.

• Crosscutting Concepts:
  • Systems and System Models: When investigating or describing a system, the boundaries and initial conditions of the system need to be defined and their inputs and outputs analyzed and described using models.

Objectives

By the end of this demonstration, students should be able to

• better understand what happens in terms of energy when a substance dissolves.
• define endothermic and exothermic processes.
• classify a change as either endothermic or exothermic.

Chemistry Topics

This demonstration supports students’ understanding of

• Energy/Thermodynamics
• Exothermic/endothermic processes
• Energy flow between a chemical system and the surroundings

Time

Teacher Preparation: 15 minutes
Lesson: 20 minutes

Materials

• Chemical Hot Pack (2)
• Chemical Cold Pack (2)
• Scissors
• Ice Melt with calcium chloride as the main ingredient (25g)
• Salt Substitute with potassium chloride as the main ingredient (50g)
• Snack size Ziploc® bag (2)
• Quart size Ziploc® bag (2)
• Water (tap water is fine)
• Electronic balance
• Scoopula
• Graduated cylinder (100ml)
• Temperature probe or thermometer (optional)
• Insulated cups (4)

Safety

• Always wear safety goggles when handling chemicals in the lab.
• Students should wear proper safety gear during chemistry demonstrations. Safety goggles and lab apron are required.
• Students should wash their hands thoroughly before leaving the lab if they have handled any materials.
• Follow the teacher’s instructions for cleanup of materials and disposal of chemicals.
• Potassium Chloride SDS
• Calcium Chloride SDS

Teacher Notes

• You will only need two hot packs and two cold packs for this demonstration. Budget permitting, you may want to get more so that the students can easily pass them around.
• The hot packs and cold packs can contain a variety of substances. Choose one that has a packet of water and a solid chemical that is activated by squeezing the pack to break the water pouch so that it mixes with the water.
• Teaching Points
  • Hot Pack: When the hot pack is activated the dissolving process produces heat, making it an exothermic process. When students touch the activated packet ask them how the energy is flowing (from pack to hand, which is why hand feels warm).
  • Cold Pack: When the cold pack is activated the dissolving process absorbs heat, making it an endothermic process. When students touch the activated packet ask them how the energy is flowing (from hand to pack, which is why hand feels cool).

• Before beginning the lesson, carefully cut open one of the hot and one of the cold packs. These will be passed around for students to observe the contents during the lesson. Carefully place them in a small plastic container with a lid so that you can store them for future lessons.
• You only need to make one homemade hot pack and cold pack. Budget permitting, you may want to get enough materials for each student, or pair of students, to make a hot pack and a cold pack.
• For a more advance lesson you can collect data while assembling the chemical hot and cold pack and use it to calculate the enthalpy of solution for each of the chemicals used.

Lesson

1. Define or review the terms exothermic and endothermic process with the students.
2. Activate the hot pack and pass it around for students to touch and ask them to define how the energy is flowing in terms of the pack and their hands. Ask students which type of process (exothermic or endothermic) is happening in the pouch.
3. Show the students the opened hot pack so that they can observe the contents.
4. Activate the cold pack and pass it around for students to touch and ask them to define how the energy is flowing in terms of the pack and their hands. Ask students which type of process (exothermic or endothermic) is happening in the pouch.
5. Show the students the opened cold pack so that they can observe the contents.

1. Show the students how they can make a chemical hot pack at home:
   a. Put 200ml of tap water into a snack size Ziploc® bag. Seal the bag, pressing out a much of the air as possible.
   b. Mass approximately 25g of Ice Melt and add it to the quart size Ziploc® bag.
   c. Put the sealed bag of water into the quart Ziploc® bag with the Ice Melt.
   d.Carefully seal the quart bag.
   e.To activate the bag press down on the inner bag of water until it pops open. Shake the contents and feel the bag.

1. Show the students how they can make a chemical cold pack at home:
   a. Put 200ml of tap water into a snack size Ziploc® bag. Seal the bag, pressing out a much of the air as possible.
   b. Mass approximately 50g of Salt Substitute and add it to the quart size Ziploc® bag.
   c. Put the sealed bag of water into the quart Ziploc® bag with the Salt Substitute.
   d. Carefully seal the quart bag.
   e. To activate the bag press down on the inner bag of water until it pops open. Shake the contents and feel the bag.
2. Disposal: Dispose of the resulting solutions in accordance with the Safety Data Sheet (SDS).

Extension Lesson

1. Collect data while making the hot and/or cold packs to allow students to do calculations.
2. Pour the water from the bag into two, stacked insulated cups. Record mass of water.
3. Use a temperature probe or thermometer to record the initial temperature of the water.
4. Record mass of the solid chemical and pour it into the insulated cup of water, stirring with the temperature probe.
5. Record the highest or lowest temperature reached.
6. Assume that the heat capacity of the solution is 4.18 J/g °C
7. For mass to mole conversions, assume that the Ice Melt is completely calcium chloride and the Salt Substitute is completely potassium chloride.
8. Calculate the heat absorbed by the resulting solution and the enthalpy of solution for the solid using the following equation:
   q = mCΔT
   
   q = heat (J) m = total mass (g) of water and solid
   C = heat capacity (J/g °C) ΔT = temperature change (°C)
   
   
9. Divide the value of q for the dissolving process by the moles of solid involved to get the enthalpy of solution. Note: q_solution and ΔH_dissolving have opposite signs.
   a. For an exothermic dissolving process the value of heat (q) for the solution will be positive and the enthalpy of solution for the chemical will be negative.
   b. For an endothermic dissolving process the value of heat (q) for the solution will be negative and the enthalpy of solution for the chemical will be positive.
10. Common misconception: For physical changes, a temperature increase indicates an exothermic change and a temperature decrease indicates an endothermic change. Many students try to infer the same for chemical changes. Focus students on the flow of energy between the reaction system and the surroundings for chemical changes.
    a. When a reaction is exothermic it produces energy it releases it to the surroundings and the surroundings will feel heat from the addition of energy.
    b. When a reaction is endothermic it absorbs energy from the surrounds and the surroundings will feel cold from the loss of energy.