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SCH 4U: Energy Changes and Rates of Reaction Teacher Demo: Hot Ice Topics Exothermic reactions Solubility Timing preparation: 20 minutes + approx. 30 min cooling time demonstration/activity: 10 minutes Specific Expectations SCH4U Introduction Heat transfers in physical and chemical reactions have many practical applications. Exothermic reactions release heat to the surroundings, resulting in an increase in surrounding temperature. Endothermic reactions require heat from the surroundings, resulting in a decrease in surrounding temperature. The most obvious application of exothermic reactions is a combustion reaction, but there are many exothermic reactions that are used in our everyday lives. A sodium acetate solution is found in reusable hand warmers and hot packs. These are safe and non-toxic, and depending on the size of the hot pack, the heat produced can last for over an hour. Boiling the hot packs will dissolve the sodium acetate in the water, and once cooled, they can be used again. The purpose of this demonstration is to illustrate an example of an exothermic reaction that has a practical application. In this demonstration, sodium acetate trihydrate is dissolved in water to produce the supersaturated solution. As it recrystallizes, energy is released in the form of heat. CH3COONa•3H2O(aq) CH3COONa•3H2O(s) + heat Equation 1. Recrystallization of sodium acetate trihydrate, producing heat energy. Materials sodium acetate trihydrate, CH3CO2Na•3H2O heat resistant gloves or tongs 500 mL Pyrex Erlenmeyer flask Parafilm, plastic wrap, or a watch glass larger than the flask opening. electronic balance weighing boat distilled water glass stirring rod hot plate graduated cylinder watch glass safety goggles heat resistant gloves Safety Considerations Wear safety goggles. Sodium acetate is slightly toxic and should not be inhaled or ingested. Use tongs or wear heat resistant gloves while handling hot glassware. Hazardous Materials Identification System Rating (0-minimal 1-slight 2-moderate 3-serious 4-severe) Sodium acetate trihydrate Procedure Preparation 1. Measure 150 g of sodium acetate trihydrate and place in a 500 mL Erlenmeyer flask. 2. Measure 28 mL of distilled water and add it to the sodium acetated trihydrate in the flask. 3. Heat the mixture on a hot plate, stirring frequently, until the solid is dissolved (about 10-15 min). Ensure there are no remaining solid crystals on the side or mouth of the flask. 4. Remove the flask from the hot plate, cover with plastic wrap, Parafilm, or an inverted watch glass. 5. Allow the flask to cool to room temperature. 6. Place a few small crystals of solid sodium acetate trihydrate on a watch glass immediately before performing the demonstration. Predict/Explain 7. Organize your class into groups of 2-4 students per group. Ask each group to predict what will happen when the solution is poured onto the watch glass containing a few solid crystals of the same substance. Each group should also attempt to come up with a rationale for their prediction. 8. Very slowly pour the solution directly onto the solid crystals on the watch glass. Observe 9. Conduct the demonstration and provide time for students to record their observations. Explain 10. Ask the groups to reconvene and revise their explanations, if necessary. Disposal The solid product can be reheated and reused several times. The liquid solution can be neutralized with baking soda (sodium bicarbonate) and discarded. Follow the employer’s protocol for chemical disposed. What happens? As the liquid is slowly poured onto the crystals on the watch glass, it solidifies immediately into a translucent white compound. Depending on the speed at which it is poured, a tall tower will be formed. The solid feels warm, as heat is released. How does it work? When a supersaturated solution of sodium acetate trihydrate comes into contact with a solid crystal of sodium acetate trihydrate, it will recrystallize. This recrystallization is exothermic, so it releases a significant amount of heat. A saturated solution is a solution that contains the maximum amount of solute dissolved in a solvent. A supersaturated solution is a solution that contains more than the maximum amount of solute dissolved in a solvent. When using a solid solute and liquid solvent such as water, heating the solvent will create a supersaturated solution. Increasing the temperature will increase the maximum solubility of many solutes, so when the solution is cooled back down, it will contain more than the maximum amount of dissolved solute. Supersaturated solutions are unstable and will recrystallize with agitation or the addition of even just one crystal. Teaching Suggestions/Hints 1. Students could be asked why a chemical reaction can be suitable for either a hot pack or a cold pack. 2. Anhydrous sodium acetate does not work as well as the trihydrate form. 3. Tap water will not work as well as distilled water. 4. To speed up the cooling process, run room temperature or colder water over the outside of the flask, ensuring no water gets inside the flask. 5. A clean burette could be used to add the solution to the crystal to control the flow. 6. A variation of this demonstration is adding the solid crystals to the flask to show the formation of the solid within the flask. 7. The proportions can easily be scaled up or down depending on how the demonstration will be performed. Next Steps This demonstration can be used to introduce the concept of exothermic reactions. It can also be used to review the concept of solubility from SCH 3U. A natural next step would be for students to perform or even plan an inquiry lab investigating specific heat capacity and enthalpy changes in reactions. Additional Resources 1. A great demonstration example and video from the Royal Society of Chemistry: http://www.rsc.org/Education/EiC/issues/2013July/hot-ice-stalagmite-sodiumacetate-solution.asp 2. Video demonstrations of an endothermic reaction: http://www.dnatube.com/video/4882/Demonstration-on-Endothermic-Reaction 3. Video demonstrations of thermite, an exothermic reaction: http://www.dnatube.com/video/12093/Iron-Video--Periodic-Table-of-Videos and http://www.dnatube.com/video/4892/Exothermic-IronOxide-and-AluminumReaction Specific Expectations SCH4U A1.1 formulate relevant scientific questions about observed relationships, ideas, problems, or issues, make informed predictions, and/or formulate educated hypotheses to focus inquiries or research D3.1 compare the energy changes resulting from physical change (e.g., boiling water), chemical reactions (e.g., bleaching a stain), and nuclear reactions (e.g., fission, fusion), in terms of whether energy is released or absorbed Return to top