MCTP Maryland Collaborative for Teacher Preparation Laboratory Experiment: Chemical Reactions and Energy Thomas C. O'Haver Department of Chemistry and Biochemistry University of Maryland College Park, MD 20742 (301) 4051831 to2@umail.umd.edu NSF Cooperative Agreement No. DUE 9255745 Student handout ---------------------------------------------------------------------- Chemistry 121/122 Name________________________________ Laboratory 3, Oct. 17, 1994 Partner ____________________________ Chemical Reactions and Energy You must wear your goggles for this experiment. Chemical reactions often involve the release or the absorption of energy in the form of heat or light. You are already familiar with combustion reactions, such as burning a candle, which involves the release of both heat and light. You have also learned about the conversion of oxygen to ozone in the upper atmosphere, which is caused by the absorption of ultraviolet photons (ultraviolet light) from the sun. In this experiment, we will investigate several reactions and observe 1. Hydrochloric acid plus magnesium a. Measure out 30 mL of hydrochloric acid (HCl) solution into a styrofoam cup. Place the cup in a 250 mL beaker to keep it from tipping over, place your temperature probe in the solution, and record the temperature of the solution. Temperature: __________________ b. Obtain a 0.1 gram piece of magnesium metal from the instructor and add it to the HCl solution in the cup. Gently stir to ensure good mixing. Closely observe the system and record your observations. Watch the temperature until it stops changing and record the temperature. Maximum temperature:__________________ c. List three ways that you could tell a chemical reaction was taking place. d. Is this reaction exothermic or endothermic? (See pages 97 and 98 of the textbook). 2. Citric acid plus baking soda (sodium bicarbonate) a. Measure out 30 mL of citric acid solution into a styrofoam cup. Place the cup in a 250 mL beaker to keep it from tipping over, place your temperature probe in the solution, and record the temperature of the solution. Initial temperature: __________________ b. Weigh out 10 grams of baking soda (sodium bicarbonate) onto a piece of weighing paper. Add the baking soda to the citric acid solution and gently stir to ensure good mixing. Closely observe the system and record your observations. Watch the temperature until it stops changing and record the temperature. Maximum temperature: __________________ c. List three ways that you could tell a chemical reaction was taking place. d. Is this reaction exothermic or endothermic? 3. Lightsticks a. Obtain a lightstick from the instructor and weigh it carefully to the nearest 0.01 gram. Initial mass of lightstick______________________ Is a lightstick like a solid plastic glow in the dark toy that you have to activate by exposing it to a strong light? Try it. Hold the lightstick in strong light and see if it glows in the dark. b. Follow the instructions that come with light stick to get it started. What do you observe? c. Why do you suppose the instructions tell you to bend the lightstick and then to shake it? d. Lay your temperature probe along the length of the lightstick, positioning the tip of the probe near the center of the lightstick. Use a piece of Parafilm (sticky transparent film) to wrap around the lightstick and the temperature probe to hold the two together. Monitor the temperature closely. Do you see any evidence of temperature change while the lightstick operates? e. Since light is a form of energy, where do you suppose the light energy is coming from? f. Obtain some ice and pack the lightstick and its temperature probe in ice. Observe the light intensity (brightness) and the temperature. Does cooling the lightstick have any effect? g. Place the lightstick and its temperature probe in warm (80C) water for 5 minutes. Dont use water hotter than 80C or the plastic case might melt. Does warming the light have any effect on the light? h. Allow the lightstick to return to room temperature, remove the temperature probe, dry the lightstick carefully and weigh it. Do you observe any change in mass compared to its initial mass? Did you expect this? Explain. i. Do you think there is a chemical reaction occurring inside the lightstick? What evidence do you have for this? j. The conversion of oxygen to ozone in the upper atmosphere is caused by the absorption of photons (light) and might be written schematically Reactants + Light --> Products where the reactants in this case are oxygen molecules and the products are ozone molecules. Assuming that there is a reaction of some sort going on inside the lightstick, how could you modify this general schematic to represent the lightstick reaction? k. Why do you think that heating and cooling the lightstick changed its brightness? (Hint: think of other more familiar chemical reactions, such as in cooking or storing food, that are speeded up or slowed down by temperature changes?) At the end of the period, one of you in each working pair of may take the lightstick with you. Observe it periodically during the rest of the day. At our next class meeting, I'll ask you to report back to the rest of the class: How long it take for the light to disappear? Why did it stop?