EVERYDAY CHEMISTRY THE CHEMISTRY OF FIRST AID INTRODUCTION If you’ve ever sprained your ankle, bumped your head or jammed a finger, you might know that putting ice on a soft tissue injury helps reduce the pain and swelling. However, many injuries of this kind occur in places where there’s no convenient supply of fresh ice, like while hiking and during sports events. Chemists, using their knowledge of the properties of chemical compounds, were able to design a portable “cold pack” that can be carried around and stored at room temperature. It contains water and a small pouch of a salt named ammonium chloride. One of the properties of ammonium chloride is that energy is required to make it dissolve in water, so it pulls thermal energy from its surroundings, which lowers the temperature of the water. In this experiment, you will examine the results of this process yourself using a temperature probe – an electronic device that functions like a thermometer. You will be able to observe how the temperature changes over time when the salt is added to water, and plot a graph of the relationship between temperature and time. MATERIALS Data collection equipment o Computer o LoggerPro software o LabPro interface o Temperature probe Chemicals o Ammonium chloride (NH4Cl) o Distilled water Lab equipment o 50 mL graduated cylinder o 250 mL glass beaker o Foam cup o Digital scale SAFETY Ammonium chloride requires special handling. Obtain an MSDS for this chemical and review the safety precautions before beginning the procedure. PROCEDURE 1. Connect the temperature probe to the LabPro, then connect the LabPro to the computer and start Logger Pro. Check that Logger Pro correctly finds the temperature probe. Make sure that all cords are placed safely around your work area. 2. Place the foam cup inside the 250 mL beaker. (The beaker will help keep the cup from tipping over while the temperature probe is resting inside.) 3. Using the graduated cylinder, measure 50 mL of distilled water and transfer it to the foam cup. 4. Using the digital scale, carefully weigh out 10 g of ammonium chloride. 5. Place the tip of the temperature probe in the water. Wait for the temperature displayed on the screen to stabilize (this should take about 15 to 20 seconds), then click ⊳ Collect. 6. Wait about 10 seconds, to record the initial temperature of the water, then carefully add the ammonium chloride to the foam cup. Gently stir the mixture with the probe. 7. Observe the behavior of the contents of the cup, as well as the data being collected on screen. Record your observations in your lab report. 8. When the temperature of the solution begins to increase again, click █ Stop. 9. Remove the temperature probe from the foam cup and rinse it off with distilled water. Dispose of the contents of the foam cup following the directions of your instructor. ANALYSIS 1. Examine the plot of temperature vs. time generated by Logger Pro. On the graph, find and mark the following points: a. The time when you added the ammonium sulfate, and the corresponding temperature b. The time when the mixture reached its lowest temperature, and that temperature 2. Record the these temperatures and times in the data table. 3. Save a screenshot of the graph. Make sure the axes are scaled appropriately. DATA Starting temperature °C Starting time seconds Minimum temperature °C Time of min. temp. seconds Temperature change °C Time needed to reach lowest temperature seconds DISCUSSION QUESTIONS 1. In your own words, describe what you observed when you added the ammonium chloride to the water. Were there any visible changes? How did the temperature change? Use detailed words 2. Before you started the experiment, your group was asked to obtain and read the MSDS for ammonium chloride. a. Summarize the safety requirements necessary for handling ammonium chloride. b. What precautions did your group decide to take while using it? 3. Why do you think the water was placed in a foam cup, rather than a glass beaker? (What properties does Styrofoam have that would be of interest for this experiment?) 4. The coach of the soccer team comes to your chemistry class one day and asks your group to design a new cold pack for his team to use. a. What properties do you think a good cold pack should have? b. Which variables would you consider testing? c. How would you test the effect of one of these variables?