Reptile Rescue

Introduction & Motivation

The goal of engineering is to design an object or process to solve a problem.  Engineers need to define the features that will make the object or process successful and those that may interfere with the success. In this activity, you will use engineering to design a device that uses a chemical reaction to reach a certain temperature range.  This will require testing, measuring, and refining the quantities of substances and modifying the materials for an optimal design, as well as testing a model/prototype and modifying, improving, and optimizing the prototype based on testing.

Split the students into two groups.  One group of students will work with the hot packs and exothermic reactions, one group will work with cold packs and endothermic reactions. Using the story as motivation, students are presented with an engineering design challenge.

To the exothermic group, propose this situation: You must rescue reptile eggs from a new construction site. Build one portable device which can keep a reptile egg physically safe and in the desired temperature range (28 – 32 °C) while it is moved from the construction site to a nearby reptile conservation center. 

To the endothermic group propose this situation: You must rescue turtles hibernating in mud from a new construction site. Build one portable device to safely transport a hibernating turtle within the desired temperature range (13 – 18 °C) while it is moved from the construction site to a nearby aquarium.

After observing different heat packs/cold packs, students will discuss the criteria and constraints related to designing a hot/cold pack as the basis for their device.  Students investigate calcium chloride as an exothermic dissolver and ammonium nitrate as an endothermic dissolver.  Students then move on to calcium chloride or ammonium nitrate and baking soda as the exothermic/endothermic chemical reaction which will serve as the heat/cold source for their device.

Students adjust the amount of the reactants (water, calcium chloride/ammonium nitrate and baking soda) to achieve the right temperature range and then test a prototype created in a zip-closing plastic bag. Students use their findings and ideas about insulation and heat transfer to draw up an optimized design that: 

1) Keeps an egg at the ideal temperature range (28 – 32 °C) or a turtle in the temperature range for hibernation (13 – 18 °C)
2) Protects the egg from impact during transport

Exothermic Reaction: Calcium chloride and water, or calcium chloride, baking soda, and water:

CaCl₂ (s) + H₂O (l) → Ca²⁺ (aq) + 2Cl^– (aq)

NaHCO₃ (s) + CaCl₂ (s) + H₂O (l) → CaCO₃ (s) + CO₂ (g) + 2NaCl (aq) + HCl (aq) + H⁺ (aq)

Endothermic Reaction: Ammonium nitrate and water:
NH₄NO₃ (l) + H₂O (l) → NH₄⁺ (aq) + NO₃^–(aq)

Chemical Safety:

1. Calcium chloride
2. Ammonium nitrate

Materials

• Digital thermometers
• Calcium chloride (CaCl₂)
• Ammonium nitrate (NH₄NO₃)
• Baking soda
• Water
• 2 small clear plastic cups
• Measuring spoons (⅛ tsp., ¼ tsp. and ½ tsp.)
• Plastic wrap
• Ziploc bags of various sizes
• Small cuvettes, film canisters, small containers (get a large variety)
• Aluminum foil
• Nitrile gloves
• Commercial hand warmers 
• Instant chemical ice packs

Procedure

Take time to explain endothermic/exothermic reactions and what they mean. In an endothermic reaction, energy must enter the system for the reaction to proceed. In an exothermic reaction, the system releases energy as a result of the reaction. Ask students all the ways they think energy can be transferred into/out of a system (sound, light, heat, etc.)

Emphasize that we can harness that energy in useful ways. In this case, chemical energy comes from breaking and forming bonds between atoms and is made useful by heating our precious cargo.

1. Have students try out the exothermic reaction or endothermic reaction with different amounts of calcium chloride or ammonium nitrate, sometimes adding baking soda.  Make sure they record the amounts of each added and the temperature changes of each reaction.
   
2. Show the students both a commercial hand warmer and commercial chemical ice pack. Let students experiment with them, perhaps cut them open to see how they work.  Make sure they are wearing gloves!
   
3. Have students conduct their chemical reaction in a sealed bag as a prototype heat pack/ice pack.
   • Let the students realize that the addition of baking soda produces a gas in the case of the exothermic reaction
   • Remind students that their challenge is to make a heat pack to warm and safely transport snake eggs or a cold pack that will allow a turtle to be transported while hibernating.  Explain that they’re conducting the chemical reaction in a sealed bag to see if the temperature and amount of gas produced will do the job. 
   • Ask the students if they think a fully inflated bag or a partially inflated bag will protect the reptile more.
     
4. The most common way to create warmers/ice packs is to seal a packet of water along with a powdered form of the reagents inside of a bag. When the water packet is ruptured, it mixes with the reagents and reacts. (e.g. use small capsule to contain water where the top can be unscrewed or otherwise removed inside the pack.) Describe to students this approach but emphasize that they are free to pick another approach!
   
5. Have the students design their pack and then test it several times without the ammonium nitrate (NH₄NO₃) or calcium chloride (CaCl₂).
   
6. Once they have come up with a preliminary design, have the students add about 50 mL of water, and 20 grams of either CaCl₂ or NH₄NO₃. Make sure students measure the temperature change of the pack’s surface. This measurement should be at the surface because this is where the pack is touched during use!
   
7. Put an endothermic and exothermic group together.  Have them test each other’s packs and give feedback about usability/what they think/etc.  Have them try to explain to the other group what is going on in their pack and why its getting either hot or cold.

Discussion

1. After the students have made their packs, ask them how they could make them hotter/colder.
   • e.g. Using more reactants! (Generally discuss the idea of enthalpy per mole vs. total enthalpy)
   • e.g. Using another material for the container such as aluminum foil (Discuss heat transfer – bring sample of wood, plastic and metal. They are all the same temperature, but the metal feels the coldest because it transfers heat the fastest.)
     
2. Ask the students to come up will ideas on how to keep their packs hotter for longer.  If they had different/more materials, how would they use them to improve their packs?