7 How much heat can a phase change produce?

This lesson is designed to allow students to relate the heat energy involved in a phase change to useful applications. In the process students will perform calculations using q= mCΔT as well as q = ΔH_solidm. This lesson also asks students to compare and contrast the phase change chemistry in a science fiction passage to an actual home heating application.

Learning outcomes

Students will:

• describe the energy in a change of state as endothermic or exothermic.
• write a thermochemical equation to represent the endothermic or exothermic change of state.
• explain how a house can heat and cool itself.
• read and interpret the phase change diagram for water.
• use calorimetry to determine the heat released by a reusable hand warmer.
• relate and apply chemical principles to their own lives.

Teacher planning

Time required

Three 85-minute class periods

Materials needed

• materials for endothermic demonstration:
  • 32 g of Ba(OH)₂ 8H₂O g
  • 250 ml Erlenmeyer flask
  • 11 g of NH₄Cl weighed on a piece of weigh paper
  • a piece of wood to place the flask on
• hand warmers or feet warmers (brand names include “Hot Hands” and “Toasty Toes”) for the exothermic demonstration (1 per class) Author’s note: These can be purchased at a drug store or online. They are inexpensive, but not reusable so I use them as a demonstration. The directions are on the packet. The reaction is a simple oxidation of iron.
• PowerPoint presentation: “Heat in Changes of State”
• sodium acetate reusable hand warmers (one for each group). Author’s note: These can be purchased from amazon.com or other online sources. Simply search for “reusable hand warmer.” Purchase after winter for the best prices. They will cost about $3.00 a piece.
• Cat’s Cradle by Kurt Vonnegut (one copy, or access to a copy, for each student)
• student worksheet: “Endothermic and Exothermic Reactions and Thermochemical Equations” (1 per student)
• Science Friday video clip
• computer with internet access and projector to show video clip
• student activity sheet: “How Much Heat In A Reusable Hand Warmer?” (1 per student) — includes answer key and rubric with teacher notes
• calorimeters (1 for each lab group)
• student worksheet: What Do the House and Cat’s Cradle Have in Common?” (1 set of questions per student) Note: The set of questions appears three times on this page to reduce printing.

Pre-activities

• Before the lesson, prepare the endothermic demonstration:
  • Put 32 g of Ba(OH)₂ 8H₂O g in a 250 ml Erlenmeyer flask.
  • Have 11 g of NH₄Cl weighed on a piece of weigh paper.
  • Have a piece of wood available to place the flask on.
• Cut apart the questions on the worksheet “What do the House That Heats Itself and Cat’s Cradle Have in Common? so you can hand out one set of questions to each student.
• Students should have an understanding of types of reactions and dimensional analysis.
• Students should distinguish between heat and temperature.
• Students should have an understanding of enthalpy and be able to solve for q using the formula q= mCΔT

Activities

Day one

1. Review endothermic and exothermic reactions using a demonstration of each as follows:
   1. Add 11 g of ammonium chloride to an Erlenmeyer flask containing 32 g of barium hydroxide. Swirl to mix and have students touch the flask and comment on what they see and feel.
   2. As the flask gets cold, set it on a wet piece of wood. The reaction is so endothermic that the water will free and the flask will be stuck to the wood.
   3. To demonstrate an exothermic reaction activate a (non-reusable) hand or foot warmer and pass it around so students can feel the heat released.
2. Present slides 1-12 from “Heat in Changes of State” PowerPoint presentation. These slides also cover thermochemical equations.
3. Practice some problems from the “Endothermic and Exothermic Reactions and Thermochemical Equations” worksheet in class. This can be completed for homework.
4. Present slides 13-20 and discuss heat during a phase change.
5. Have students read chapters 19-23 from Kurt Vonnegut’s Cat’s Cradle prior to the next class. Provide a little background information to let students know that the story picks up with an interview of Dr. Breed who worked with Dr. Hoenikker. The narrator, John, is writing a book about the bombing of Hiroshima and he wants to include information about Dr. Hoenikker, a Nobel prize winning physicist and one of the fathers of the atomic bomb. Dr. Hoenikker is no longer alive, so John is talking with Hoenikker’s coworker, Dr. Breed. Chapters 19-23 discuss a fictional substance called ice nine. You can check the SparkNotes website for a summary of the book.

Day two

1. Present the video clip from the Science Friday website. This clip shows how we can utilize the fact that heat is absorbed and released during phase changes.
2. Present slides 21-25 from the PowerPoint to explain calorimetry as a technique used to measure heat released as a result of a chemical or physical change. Demonstrate the proper use of the equipment throughout the discussion.
3. Hand out and introduce the “How Much Heat In A Reusable Hand Warmer?” activity by giving each lab group one reusable hand warmer. Show them how to activate the hand warmer by bending the metal disc. Allow them to observe the reaction so they will understand that heat is generated as the sodium acetate changes state. (Note: For purchasing information for these hand warmers, see the materials list above.
4. Have each lab group explain their observations on a white board and in turn share these ideas with the class.
5. Have students work in their lab groups to answer all pre-lab questions and write a procedure so they are ready to perform and complete the lab during the next day’s class.

Day three

1. Have students perform the “How Much Heat In A Reusable Hand Warmer?” lab. The specific heat of sodium acetate (C= 2.5 J/g °C) and the heat of fusion for sodium acetate (Hf= 264 - 289 J/g) are included on the student sheet. Background information for lab:
   • The specific heat of sodium acetate is 3.0 J/g°C.
   • Heat of fusion and heat of solidification are the same amount of heat. Heat of fusion is a positive value because it requires the input of heat to melt a substance while heat of solidification is a negative value because heat is released when a substance solidifies. The known value for heat of fusion of sodium acetate is 264–289 kJ/kg which is the same as 264–289 J/g.
   • The heat of solidification values obtained in the lab tend to be low which allows for a discussion of where there heat may have gone or other possible errors. Remember that we did not include the plastic casing.
   • Note: In order for students to obtain decent results in this activity, it is necessary to guide them to understand that the heat released when the hand warmers are activated is used to increase the temperature of the water in the calorimeter as well as to increase the temperature of the hand warmer itself.
2. Have students complete the “What Do the House That Heats Itself and Cat’s Cradle Have in Common?” worksheet for homework as a way to once again relate change of state to heat.

Assessment

• Assess the “How Much Heat In A Reusable Hand Warmer?” activity using the rubric and answer key on the worksheet.
• Assess by students’ completion of the “What Do the House That Heats Itself and Cat’s Cradle Have in Common?” worksheet.
• You may wish to have students complete a unit test including concepts from this lesson.

Modifications

This lesson was designed and used with honors chemistry students. Academic chemistry students will require instruction on how to go about doing the calculations so they recognize that the heat released during the solidification of the sodium acetate is transferred to the water thereby increasing the temperature of the water and it is also increasing the temperature of the sodium acetate. We ignore the plastic casing of the hand warmer, although it could also be included. The final equation looks like this: -ΔH_solm (sodium acetate) = mCΔT (water) + mCΔT (sodium acetate).

Critical vocabulary

• Calorie
• calorie
• joule
• enzyme
• energy
• thermochemistry
• calorimetry
• calorimeter
• heat of fusion
• heat of solidification
• endothermic
• exothermic

• Next: Alternative energy: Where's the chemistry?

National Standards

In addition to meeting objectives of the North Carolina Standard Course of Study, this lesson plan addresses the following national standards.

National Science Content Standard A

All students should develop an understanding of:

• Abilities necessary to do scientific inquiry
• Understandings about scientific inquiry

National Science Content Standard B

All students should develop an understanding of:

• Structure of atoms
• Structure and properties of matter
• Chemical reactions
• Motions and forces
• Conservation of energy and increase in disorder

• Next: Alternative energy: Where's the chemistry?
• Previous: Units of heat: How much energy is that anyway?

Helpful hint

The reusable hand warmers can be reactivated by putting them in boiling water. Watch then carefully, and when they are liquid again allow them to remain in the water until they cool to room temperature for best results.

Learn more

Related topics

• Search LEARN NC for more resources on chemistry, demonstrations, endothermic reactions, exothermic reactions, hands-on, and science.