Evaporation Rates of Liquids. Sarah Byce

Transcription

1 Evaporation Rates of Liquids Sarah Byce Herrington CHM October 2012

2 Byce 2 Statement of Question How do evaporation rates relate to molecular structure? Based on the information given (Appendix: Table 1) it was predicted the longer the molecular chain the longer it would take to evaporate. Background For this lab, temperatures of liquids were recorded every fifteen seconds until the temperature leveled off. The temperature leveled once the substance had evaporated almost completely. Evaporation occurs when the molecules heat up enough in the liquid phase to become a gas vapor because their attractive forces are being broken. Procedures In this experiment, filter paper was wrapped around the end of a thermometer and attached with a rubber band. Then a small amount, enough to cover the filter paper, of liquid substance was poured into a small test tube. The thermometer was then placed in the test tube. Record the temperature for time zero and then soak for one minute. After one minute, pull the thermometer out and hold over the end of the table. Every fifteen seconds record the temperature. Once the temperature levels off or begins to go up the substance has finished evaporating. Repeat these steps for all the liquids. The liquid, pentane, should be handled carefully; however. Data and Observations *see Appendix: Table 2 Graph *see Appendix: Graph 1 Analysis and Discussion 1. Based on the graph (Appendix: Graph 1) one could conclude, the temperature of the liquids decreases during evaporation. When the temperature levels off or begins to increase, it is because the liquid has evaporated as much as it will. 2. The substance pentane has such a quick drop in temperature because it is evaporating so quickly. It evaporates faster than others because of its structure. Pentane is made of six carbon atoms and twelve hydrogen atoms; because of this the hydrogen atoms will easily bond with air molecules, so their attractive forces between each other are weak.

3 Byce 3 3. Temperatures of the liquids decrease during evaporation because while they are changing from a liquid to a gas they take their kinetic energy with them (Suchocki). This lowers the average kinetic energy of the other molecules in the liquid. Evaporation also cools the surrounding air. Due to the molecules turning into a gas they are moving slower than the molecules in the air which will overall decrease the average kinetic energy of the molecules in the air (Suchocki). 4. Based on the graph (Appendix: Graph 1) the slowest rate of evaporation would be vegetable oil. From there it would be water, 1-propanol, ethanol, and acetone. The liquid with the fastest rate would be pentane. This ranking was determined by the slope of the line formation made by the data points; the steeper the slope, the faster rate of evaporation. 5. If a substance has higher attractive forces it would be more difficult to break the bonds between them. That would mean that it would be harder for a phase change, so the evaporation rate would be slower. On the other hand, if the attractive forces are lower between molecules, the bonds would be easier to break for a phase change and therefore the evaporation rate would be faster. 6. Based on previous information, the liquid with the weakest intermolecular force would be pentane. It would then proceed with, acetone, ethanol, 1-propanol, and water. The strongest intermolecular force would be vegetable oil. This information was based on the graph (Appendix: Graph 1). 7. Molecules with stronger intermolecular forces are mostly based on, the amount and angle of the hydrogen atoms. Based on the information (Appendix: Table 1) the hydrogen atoms that were at a slight angle were the first to evaporate. Then the liquids with hydrogen atoms attached to carbons but with a hydroxyl group were evaporated. Then finally when all hydrogen atoms were bonded with carbons the liquid evaporated. 8. Your body naturally sweats using a process called evaporative cooling. This cools your body because of the kinetic energy of the molecules. In the same way evaporation cools the air and the substance itself, sweating cools the body. When you sweat, water molecules emerge on the skin, and as they evaporate the kinetic energy of the water

4 Byce 4 droplets goes with them. The molecules are also moving slower than the molecules in the air so the air becomes cooler as well. So, for example, when getting out of a warm shower into the cool bathroom air your body feels colder because the water on your skin is evaporating, taking the kinetic energy with it and the evaporated water molecules are decreasing the kinetic energy of the bathroom air, so everything seems cooler. Conclusion In conclusion, it was determined the slower the rate of evaporation the stronger the intermolecular force. Based on this conclusion, the liquid with the slowest rate of evaporation was vegetable oil and the pentane had the fastest rate of evaporation. The goal for this experiment was met when the conclusion, or understanding, was made between the relationship of intermolecular forces and evaporation rates. This lab taught that when the force holding the molecules together is overcome, it takes a lot of kinetic energy and when that energy is released into a vapor it takes heat with it, this in turn cools the liquid down. This concept is important for understanding temperature regulation, such as sweating of the body. The experiment went smoothly, but could be improved upon. It may have been better to get a sample of the class data to compare as well as to get an average for the liquids. It was difficult, sometimes, especially for pentane, to get the temperature reading because it was falling so quickly.

5 Byce 5 Works Cited Suchocki, John. Conceptual Chemistry: Understanding Our World of Atoms and Molecules. 4th ed. New Jersey: Pearson Education, Inc., Print.

6 Byce 6 Appendix: Tables and Graphs Table 1: Liquid Data Table 2: Raw Data Table

7 Graph 1: Temperature vs. Time Byce 7