Designing a Hand Warmer (Kathryn Smith)

Author(s)

Kathryn Smith, Hart High School, AP/Honors Chemistry

NGSS Engineering Standards

HS-PS1-4: Students who demonstrate understanding can develop a model to illustrate that the release or absorption of energy from a chemical reaction system depends upon the changes in total bond energy.

Science and Engineering Practices: Planning and carrying out investigations, Analyzing and interpreting data, Engaging in argument from evidence, Obtaining, evaluating, and communicating information (also, working within constraints)

Materials needed

• Thermometers

• Styrofoam Cups with Tops (or Calorimeters)

• Balance

• 100 mL Graduated Cylinder

• Scoops

• Weighing Boats (or Dixie Cups)

  • Sodium Chloride

  • Calcium Chloride

  • Sodium Carbonate

  • Sodium Acetate

  • Lithium Chloride

  • Ammonium Nitrate

  • Magnesium Sulfate

Procedure

1. Demonstrate to the kids how dissolving an ionic compound in water can change the temperature (LiCl or NH₄NO₃ make surprising observations) but don't tell them what salt it is.

2. Challenge the kids to design an experiment to collect all the necessary data to design the most cost effective hand warmer that can fit in the average sized pocket.

   1. Students will come up with a variety of experimental methods. Check their procedures before letting them in lab. It is crucial that they collect enough data to use the q=mCDT formula to calculate the heat change (mass of water and salt, initial and final temperature). Suggest a mass of 7-10 g of salt and 75-100 mL of water.

3. Have students share data with each other. To do this lab in a single class period, assign a different salt to each lab group.

4. Have the students calculate the enthalpy of dissolution in kJ/mol. If you let them, an advanced group may decide to do this on their own. Usually you have to explain to kids that you can just compare the heat amount because of the different masses for each salt. Finding the kJ/mol of salt will allow students to see which is the best heat producer.

5. Have students look up the costs of the salts. Flinn Scientific is a great site for this. Most of the time, I tell my kids to find the cost per mol of material.

6. In the lab report, students should make a claim to the salt of choice for the hand warmer, the amount of salt and water necessary, and the cost per unit.

   1. Possible answer choices are calcium chloride or lithium chloride. Calcium chloride is more cost effective than lithium chloride, but lithium chloride is more heat effective. Depending on how large they want the hand warmer to be, either could be the correct answer.

Questions

• How can we measure the heat change of a chemical reaction in the lab? What sort of materials will we need to use?

  • We can measure a heat change by applying the first law of thermodynamics, the law of conservation of energy. If we create a relatively isolated system by using insulating materials, such as styrofoam, any heat movement can be measured as a result of temperature change (thermometer).

• How much heat is produced when 5 g sodium acetate is added to 45 mL of water (C = 4.18 J/(g^oC) ) when a temperature change of 4.8 is observed?

  • 1003.2 J

• What is the enthalpy of dissolution (in kJ/mol) of LiCl if 5 g of LiCl are added to 45 mL of water (C = 4.18 J/(g^oC) ) displaying a temperature change from 20.0 ^oC to 38.0 ^oC.

  • -34.3 kJ/mol

Photos

Movies

References

AP Chemistry Guided-Inquiry Experiments: Applying the Science Practices