GCC CHM 130LL: Heat and Energy page 1 of 8 CHM 130LL: Heat and Energy Introduction Energy is defined as the ability to do work, and one form of energy is heat. Heat is defined as thermal energy flowing from an object at a higher temperature to one at a lower temperature. For example, if a chunk of metal at room temperature is placed in a beaker of boiling water, the metal will absorb heat from the water until it is at the same temperature as the boiling water. Scientists also often study the heat associated with different physical and chemical changes. Ideally, the heat changes resulting from physical and chemical phenomena can be harnessed to do work. For example, the burning of gasoline and fossil fuels can be used to run our automobiles and heat our houses. However, in some cases, the heat associated with some processes is transferred to the environment; for example, 90% of the electricity going into traditional incandescent light bulbs generates heat rather than light. This waste of energy is the main reason that incandescent light bulbs are being phased out and replaced by more energy efficient light bulbs such as CFL’s. Most physical and chemical changes are either exothermic or endothermic. Exothermic reactions release energy or heat to increase the temperature of the surroundings; thus, the surroundings are hotter after an exothermic reaction. For example, nitroglycerine exploding is an extremely exothermic reaction. Endothermic reactions absorb energy or heat to decrease the temperature of the surroundings; thus, the surroundings are colder after the endothermic process. For example, cold packs used to relieve swelling joints or muscles often use chemicals that absorb heat when mixed, so the packs feel cold. The amount of heat released when food is burned can even be used to determine the caloric content of food. The number of calories is a measure of the energy a person can get from consuming the food, so the number of calories in two different kinds of food will be compared. In this experiment, you will: - view an animation showing heat transfer between particles at different temperatures - explain various physical changes in terms of heat transfer You will also explore the heat associated with the following physical and chemical changes, then categorize each as exothermic or endothermic: - The recrystallization of a supersaturated solution of sodium acetate - The dissolution (or dissolving) of ammonium chloride in water - The heat of reaction associated with an acid-base reaction - The evaporation of isopropyl alcohol Finally, you will burn a snack food and a nut to determine the number of calories in each, and then you will compare the amount of energy obtained from each type of food.GCC CHM 130LL: Heat and Energy page 2 of 8 Procedure A. Heat transfer 1. Your instructor will demonstrate the HeatTransfer.MOV video (located in the CHM 130LL course folder) on the projection system. 2. Answer the questions regarding heat transfer. B. A supersaturated solution of sodium acetate, NaC2H3O2 1. Check the hotplate cord for any tears exposing wires. If you find any, ask your instructor for another hotplate. Plug in the hot plate. Turn the heat setting to 5. 2. Use your scoopula to add two large scoops of sodium acetate, NaC2H3O2, to just below the 5-mL mark on a 10-mL Erlenmeyer flask. Use your deionized (DI) water bottle to add water to the 5-mL mark on the Erlenmeyer flask. Note: Your mixture should be mostly solid with just a little water for the most dramatic result. There should just be enough water to cover the surface of the sodium acetate. 3. Swirl to mix the solution, then place the Erlenmeyer flask on the hot plate. Heat the resulting slush until the solid has dissolved completely. Do not allow the solution to boil! (If the solution boils, use a paper towel to protect your hand from the hot glass or solution when you transfer the flask in the next step.) 4. Place the piece of colored paper on the lab bench. Once all the solid has dissolved, carefully transfer the Erlenmeyer flask onto the piece of colored paper to cool for at least 20 minutes. (Note: The flask may be a little warm to the touch but not too hot to handle. Be careful not to place the flask directly on the lab bench, which may be cold enough to cause it to recrystallize. Also, do not disturb the flask.) Turn off the hotplate. Work on part C while the solution cools. 5. Make sure the Erlenmeyer flask is completely cool, but the sample has not turned into a solid. If it has turned into a solid, reheat it to dissolve the sample, and allow it to cool again. (Note: The results will not be as dramatic if the solution is still warm, and you may have to start over.) Use your scoopula to get one or two small crystals of sodium acetate, NaC2H3O2, out of the reagent container. Without disturbing the flask, drop the crystals into the center of the solution. You should observe a dramatic change when the crystal is added. Record your observations on your Report Sheet. 6. Place the Erlenmeyer flask in the palm of your hand to feel the temperature change. Record if the temperature increases or decreases when a supersaturated solution of sodium acetate, NaC2H3O2, recrystallizes. Indicate if the recrystallization of sodium acetate, NaC2H3O2, is exothermic or endothermic. Waste disposal: Add water to the NaC2H3O2 solid in the flask and use your stirring rod to make it into a slurry (or slush), then dispose of it in the waste container in the hood.GCC CHM 130LL: Heat and Energy page 3 of 8 C. The dissolution of ammonium chloride, NH4Cl 1. Use your water bottle to add deionized water to a clean test tube until it is 1/3rd full. Feel the test tube to get a sense of the initial temperature of the test tube and water. 2. Use your scoopula to add a scoop of solid ammonium chloride, NH4Cl, to the test tube, and shake the test tube to dissolve the solid. 3. Wrap your hand around the test tube to feel the temperature change. If you do not sense any change in temperature, use a glass stirring rod to dissolve more ammonium chloride, NH4Cl, in the test tube until you feel a change in temperature. Record if the temperature increases or decreases when ammonium chloride, NH4Cl, dissolves. Indicate if the dissolution of ammonium chloride, NH4Cl, is exothermic or endothermic. Waste disposal: Dispose of the solution in the waste container