Evaporation and Intermolecular Attractions

Transcription

1 Name Partners: Evaporation and Intermolecular Attractions Experiment 1 In this experiment, Temperature Probes are placed in various liquids. Evaporation occurs when the probe is removed from the liquid s container. This evaporation is an endothermic process that results in a temperature decrease. The magnitude of a temperature decrease is, like viscosity and boiling temperature, related to the strength of intermolecular forces of attraction. In this experiment, you will study temperature changes caused by the evaporation of several liquids and relate the temperature changes to the strength of intermolecular forces of attraction. You will use the results to predict, and then measure, the temperature change for several other liquids. You will encounter two types of organic compounds in this experiment alkanes and alcohols. The two alkanes are n-pentane, C 5 H 12, and n-hexane, C 6 H 14. In addition to carbon and hydrogen atoms, alcohols also contain the -OH functional group. Methanol, CH 3 OH, and ethanol, C 2 H 5 OH, are two of the alcohols that we will use in this experiment. You will examine the molecular structure of alkanes and alcohols for the presence and relative strength of two intermolecular forces hydrogen bonding and dispersion forces. Figure 1 MATERIALS Power Macintosh or Windows PC methanol (methyl alcohol) Vernier computer interface ethanol (ethyl alcohol) Logger Pro 1-propanol two Temperature Probes 1-butanol 6 pieces of filter paper (2.5 cm X 2.5 cm) isopropanol 2 small rubber bands n-hexane masking tape Chemistry with Computers 9-1

2 Experiment 9 PRE-LAB QUESTIONS Prior to doing the experiment, complete the Pre-Lab table. The name and formula are given for each compound. Draw a structural formula for a molecule of each compound. Structural formulas are essentially Lewis structures without the lone pairs. For example, to find ethanol s structural formula, Then determine the molar mass of each of the molecules. Dispersion forces exist between any two molecules, and generally increase as the molecular weight of the molecule increases. Next, examine each molecule for the presence of hydrogen bonding. Before hydrogen bonding can occur, a hydrogen atom must be bonded directly to an N, O, or F atom within the molecule. Tell whether or not each molecule has hydrogen-bonding capability. Prelab Data Table Substance Formul a Structural Formulas Molar Mass Hydrogen Bond (Yes or No) Ethanol C 2 H 5 O H propanol C 3 H 7 O H butanol C 4 H 9 O H n-pentane C 5 H Chemistry with Computers

3 Evaporation and Intermolecular Attractions Methanol CH 3 OH n-hexane C 6 H As a substance evaporates, will its temperature increase or decrease? The change in temperature ( t) is the difference between the temperature of the substance in the test tube and the temperature after evaporation has occurred. (1 pt) 20. Will a substance with higher intermolecular forces have a higher t or a lower t than a substance with lower intermolecular forces? Explain your answer. (2 pts) 21. If Substance A and Substance B have the same molecular masses, but A has hydrogen bonding while B does not, which substance would have a higher delta t? Explain your answer.(2 pts) 22. If Substance C and Substance D do not have hydrogen bonding and C has a higher molar mass than D, which substance would have a higher delta t? Explain your answer. (2 pts) PROCEDURE 1. Obtain and wear goggles! CAUTION: The compounds used in this experiment are flammable and poisonous. Avoid inhaling their vapors. Avoid contacting them with your skin or clothing. Be sure there are no open flames in the lab during this experiment. Notify your teacher immediately if an accident occurs. 2. Prepare the computer for data collection. Prepare the computer for data collection by opening the Experiment 9 folder from Chemistry with Computers. Then open the experiment file that matches the probes you are using. On the Graph window, the vertical axis has temperature scaled from 5 to 30 C. The horizontal axis has time scaled from 0 to 250 seconds. 3. Wrap Probe 1 and Probe 2 with square pieces of filter paper secured by small rubber bands as shown in Figure 1. Roll the filter paper around the probe tip in the shape of a cylinder. Hint: 9-3

4 Experiment 9 First slip the rubber band up on the probe, wrap the paper around the probe, and then finally slip the rubber band over the wrapped paper. The paper should be even with the probe end. 4. Stand Probe 1 in the ethanol container and Probe 2 in the 1-propanol container. Make sure the containers do not tip over. 5. Prepare 2 pieces of masking tape, each about 10-cm long, to be used to tape the probes in position during Step After the probes have been in the liquids for at least 30 seconds, begin data collection by clicking Collect. Monitor the temperature for 15 seconds to establish the initial temperature of each liquid. Then simultaneously remove the probes from the liquids and tape them so the probe tips extend 5 cm over the edge of the table top as shown in Figure When both temperatures have reached minimums and have begun to increase, click Stop to end data collection. Click the Statistics button,, then click OK to display a box for both probes. Record the maximum (t max ) and minimum (t min ) values for Temperature 1 (ethanol) and Temperature 2 (1-propanol). 8. For each liquid, subtract the minimum temperature from the maximum temperature to determine t, the temperature change during evaporation. 9. Roll the rubber band up the probe shaft and dispose of the filter paper as directed by your teacher. 10. Based on the t values you obtained for these two substances, plus information in the Pre-Lab exercise, predict the size of the t value for 1-butanol. Compare its hydrogenbonding capability and molecular weight to those of ethanol and 1-propanol. Record your predicted t (#29), then explain how you arrived at this answer in the space provided. Do the same for n-hexane. It is not important that you predict the exact t value; simply estimate a logical value that is higher, lower, or between the previous t values. Data Table 1 Substance t max t min t (t t max t min ) Show one sample calculation: ethanol propanol After you finish Step 10, predict a value for t that is either higher, lower, or between the ethanol and 1-propanol values. Do this before you actually experimentally determine the t. 9-4 Chemistry with Computers

5 Evaporation and Intermolecular Attractions Predicted t Explanation butanol n-pentane * Follow directions in step 12 before answering # Methanol n-hexane 11.Test your prediction in Step 10 by repeating Steps 3-9 using 1-butanol for Probe 1 and n-pentane for Probe 2. Fill the actual values in Data Table 2 (# 37-42). Data Table 2 Substance t max t min t ( t max t min ) 1-butanol n-pentane Methanol n-hexane Based on the t values you have obtained for all four substances, plus information in the Pre-Lab exercise, predict the t values for methanol and n-hexane. Compare the hydrogenbonding capability and molecular weight of methanol and n-hexane to those of the previous four liquids. In Data Table 1 (#33-36), record your predicted t, then explain how you arrived at this answer in the space provided. 9-5

6 Experiment Test your prediction in Step 12 by repeating Steps 3-9, using methanol with Probe 1 and n-hexane with Probe 2. Complete Data Table 2. # should be filled in now. PROCESSING THE DATA 49. Two of the liquids, n-propanol and 2-propanol, had identical molar masses. Which of the two had stronger intermolecular forces? How can you use the data you collected to determine this? How are the two liquids different in molecular structure? What do their structures have to do with the strength of intermolecular forces? (4 pts) 50. According to your experimental data, which of the alcohols studied has the strongest intermolecular forces of attraction? The weakest intermolecular forces? Is this consistent with your predictions? (3 pts) 51. How did the t values for n-hexane and 1-butanol differ? Which of these liquids have a larger molar mass? Based on your data, which of the two had the stronger intermolecular forces? Explain your data. (3 pts) 52. Plot a graph of t values methanol, ethanol, 1-propanol, and 1-butanol versus their respective molecular weights. Plot molecular weight on the horizontal axis and t on the vertical axis. You may use LoggerPro, Excel, or graph paper for this. 15 pts 53. Are there any inconsistencies between your results and your understanding of intermolecular forces? How can you account for them? (3 pts) 54. Were all your pairs of liquids at identical starting temperatures? If they were not, would this have affected results? Explain your answer. (2 pts) 9-6 Chemistry with Computers

7 Evaporation and Intermolecular Attractions 55. Although there are no percent error calculations in this lab, an error analysis is still necessary. There are two types of errors to address in the chemistry laboratory: a. Systematic Errors Errors due to the design and execution of the experiment. They can be identified through a careful analysis of the experiment and associated experiments, and measures can be taken to correct them. Systematic errors occur with the same magnitude and sign every time the experiment is performed, and affect the accuracy of the results, but not the precision. If an experiment has small systematic errors, it is accurate. b. Random Errors Errors due to indeterminate causes throughout the experiment, such as unpredictable mechanical and electrical fluctuations affecting the operation of the instrument or experimental apparatus or even human errors arising from psychological and physiological limitations. They occur with a different sign and magnitude each time Reflect upon and discuss the possible sources of random error in your measurements that contribute to the observed random error. Sources of random error will differ depending on the specific experimental techniques used. Some examples might include reading a burette, the error tolerance for laboratory balances, etc. Sources of random error do not include calculating errors (a systematic error that can be corrected), mistakes in making solutions (also a systematic error), or your lab partner (who might be saying the same thing about you!). (8 pts) 9-7