Evaporation and Intermolecular Forces

Transcription

1 Evaporation and Intermolecular Forces 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 rate of the 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. Two types of organic compounds will be used in this experiment alkanes and alcohols. 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. Before coming to lab, you should be able to draw Lewis structures for all six compounds to be used and you should know the compounds' molecular weights. You should also read the section in your textbook to clarify the differences between hydrogen bonding and dispersion forces. Figure 1 1

2 PROCEDURE 1 Calibration and Setup of Logger Pro: Prepare the computer for data collection. Go to Ch 1 Choose Sensors Temperature Direct Connect Temperature Probe. Calibrate the temperature probe with room-temperature water and ice water. Go to Experiment then Data Collection and set mode to time based and set the length to 400 seconds. Data Collection: Wrap the end of the probe with filter paper secured by wire as shown in Figure 1. Roll the filter paper around the probe tip in the shape of a cylinder. The paper should be even with the probe end. Stand the temperature probe in the ethanol container. After the probe has been in the liquid for at least 45 seconds, begin data collection by clicking on the Start button. To establish the initial temperature of the liquid, monitor the temperature of the probe while it is still immersed. Once the temperature has been stable for at least 20 seconds, remove the probe from the liquid and tape it so that the probe tip extends 5 cm from the edge of the table top, as shown in Figure 1. When the temperature has reached a minimum and has begun to increase, stop data collection. Record the initial and minimum temperatures for ethanol. Autoscale and print out your graph. Repeat the procedure using propanol, butanol, and pentane with new filter paper. Based on the T values you obtained for all four substances, predict the T values for methanol and hexane. Compare the hydrogen-bonding capabilities and molecular weights of methanol and hexane to those of the previous four liquids. Record your predicted T values, and explain how you arrived at this answer, in your notebook. Show your instructor your predictions, and then test your predictions. 2

3 PROCEDURE 2 Place a small watch glass on the pan of an analytical balance. Put a piece of filter paper in the center of the watch glass, and drop methanol onto the paper until it is saturated. Tare the balance and record the mass lost to evaporation every 15 seconds for three minutes. Repeat the procedure with ethanol. Open Graphical Analysis by going to Start Programs Division Software Vernier Software Graphical Analysis. Determine what your independent variable (X) and dependent variable (Y) are and enter the mass lost and time data in the appropriate columns. Create a new calculated column by going to Data New Calculated Column. You must first figure out how to convert from mass of alcohol lost in grams to mole of alcohol lost. Use the Equation option of Graphical analysis to convert your raw data on mass lost from grams to mole. Plot moles of compound lost versus time. Use the regression function (Linear Fit) to find the slope of the relatively linear region of the loss curve. Print out your graph with the regression results showing. Questions: 1. Draw the Lewis structure and VSEPR 3D shape for all the compounds used, and list all intermolecular forces for each compound. 2. Two of the liquids, n-pentane and 1-butanol, had nearly the same molecular weights, but significantly different T values. Explain the differences in T values of these substances, based on their intermolecular forces. 3. Which of the alkanes studied has the stronger intermolecular forces of attraction? Explain using the results of Procedure Plot a graph of T values of the four alcohols versus their respective molecular weights. Plot molecular weight on the horizontal axis and T value on the vertical axis. Explain the appearance of this plot, using the information you collected in Procedure 2 of this experiment. 5. Describe the relationship between T, the rate of evaporation, and the strength of intermolecular forces. 3

4 Salting Effects 1 In this portion of the lab you are going to explore the strength of intermolecular forces by studying the solubility of food coloring in different solutions. In order for a solute to be soluble in a solvent the strength of the intermolecular forces formed between the solute and solvent molecules must be stronger than in pure solvent. Addition of an ionic compound to a water/alcohol solution, containing food coloring will be used to demonstrate the different strengths of intermolecular forces. Procedure: Add 15 ml of water and one drop of green food coloring to a 50 ml test tube, cap with a rubber stopper, and mix. Add 15 ml of rubbing alcohol (70% isopropyl alcohol) to the test tube, cap with a rubber stopper, and mix. Add 7 g of ammonium sulfate that has been ground to a fine powder with a mortar and pestle, cap, and shake vigorously for 10 s. Once the solution in the test tube has separated into two layers carefully remove 1 ml of each layer and place on separate watch glasses. Test the watch glass sample using a flame to determine which layer is the alcohol and which is water. Add another 15 ml of water and record your observations. 1. Write a short paragraph, outlining what intermolecular forces are present in the solution during the experiment, explaining why the solution behaves as observed. 4

5 1. Person, E. C.; Golden, D. R.; Royce, B. R., Salting Effects as an Illustration of their Relative Strength of Intermolecular Forces. Journal of Chemical Education 2010, 87 (12),