May the Force Be with You

Transcription

1 hemistry May the Force Be with You Exploring Evaporation and Intermolecular Forces MATERIALS AND RESOURES EA GROUP aprons data collection device filter paper, any size goggles paper towels 2 sensors, temperature, stainless steel scissors tape, masking 1-butanol 1-propanol ethanol, 95% hexanes methanol n-pentane rubber band, orthodontic TEAER 24 glass vials ABOUT TIS LESSON This lesson is designed to give students the opportunity to make the connections between the structure/imf and the rate of evaporation. The Pre-Lab Exercises guide student thinking. OBJETIVES Students will: ompare the change in temperature of two substances simultaneously Use the results to establish the relationship between the type of IMF and the rate of evaporation LEVEL hemistry T E A E R P A G E S i

2 hemistry May the Force Be with You NEXT GENERATION SIENE STANDARDS ASSESSMENTS!! The following types of formative assessments are embedded in this lesson: Assessment of prior knowledge ASKING QUESTIONS DEFINING PROBLEMS ANALYZING AND INTERPRETING DATA ENGAGING IN ARGUMENT FROM EVIDENE Discussion of Pre-Lab Exercises Visual observations during the laboratory experience Assessment of onclusion Questions PATTERNS STRUTURE AND FUNTION ENERGY AND MATTER The following additional assessments are located on our website: hemistry Assessment: Bonding and States of Matter 2007 hemistry Posttest, Free Response Question 2 LS1: STRUTURES AND PROESSES PS3: ENERGY 2011 hemistry Posttest, Free Response Question 2 Exploring Applications of IMF T E A E R P A G E S ii

3 hemistry May the Force Be with You OMMON ORE STATE STANDARDS ONNETIONS TO AP* (LITERAY) RST Follow precisely a multistep procedure when carrying out experiments, taking measurements, or performing technical tasks, attending to special cases or exceptions defined in the text. (LITERAY) RST Translate quantitative or technical information expressed in words in a text into visual form (e.g., a table or chart) and translate information expressed visually or mathematically (e.g., in an equation) into words. (LITERAY) W.1 Write arguments to support claims in an analysis of substantive topics or texts, using valid reasoning and relevant and sufficient evidence. (LITERAY) W.3 Write narratives to develop real or imagined experiences or events using effective technique, wellchosen details, and well-structured event sequences. (MAT) S-ID.4 Use data from a sample survey to estimate a population mean or proportion; develop a margin of error through the use of simulation models for random sampling. AP EMISTRY 2 B.1 London dispersion forces are attractive forces present between all atoms and molecules. London forces are often the strongest net intermolecular force between large molecules. B.2 Dipole forces result from the attraction among the positive ends and negative ends of polar molecules. ydrogen bonding is a strong type of dipole-dipole force. B.3 Intermolecular forces play a key role in determining the properties of substances, including biological structures and interactions. *Advanced Placement and AP are registered trademarks of the ollege Entrance Examination Board. The ollege Board was not involved in the production of this product. T E A E R P A G E S iii

4 hemistry May the Force Be with You TEAING SUGGESTIONS This activity is designed to let students discover the relationship between the type of IMF and the evaporation rate of different substances. In the Pre-Lab Exercises, students are instructed to draw the structural formula of the listed molecules and determine the types of IMF present. Struggling students may benefit from a brief review of how to draw Lewis structures. It may be helpful to elicit student help to draw the first molecule, calculate the total number of electrons, and identify the intermolecular forces together as a class before sending students home with this assignment. Before students begin the laboratory activity, check their Pre-Lab Exercises and redirect those students with incorrect structures, electrons, or IMF. While students are in the laboratory, move between lab groups and ask for brief explanations of their predictions for changes in temperature. After collecting the data, students relate the magnitude of the t to the type of IMF present in the molecule. This activity usually produces very clear relationships that help solidify the effect IMF has on the behavior of liquids. The inclusion of the vapor pressure graph is intended to extend student awareness of the relationship between vapor pressure, boiling point, and intermolecular forces for a few common substances. This activity is best performed with two stainless steel temperature probes running simultaneously. Stainless steel temperature probes are recommended as the pentane can penetrate probes made of other material. The stainless steel probes yield better results when wrapped with a sock made of filter paper or some other slightly absorbent material (piping/ cord casing, for example). This allows for a more uniform evaporation of the liquid. Snug-fitting rubber bands can be used to secure the sock to the probe. Orthodontic rubber bands are available from Flinn Scientific, or you may cut small sections from a smalldiameter rubber hose or surgical tubing. This method uses very small amounts of liquids. Sets of liquids can be dispensed in small, stoppered test tubes or vials with tight-fitting lids. Adjust the level of liquid in the vial so that it will be above the top edge of the filter paper. Because several of these liquids are highly volatile, keep the room well ventilated. ap the test tubes or vials when not in use. The experiment should not be performed near any open flame. T E A E R P A G E S iv

5 hemistry May the Force Be with You PRE-LAB EXERISES 1. omplete Table 1 with the structural formula, number of electrons, and intermolecular forces for each compound. The following information may be useful throughout the investigation: Dispersion forces exist between any two molecules, and generally increase as the molecular weight of the molecule increases because the more electrons in the molecule, the more polarizable the electron cloud. For hydrogen bonding to occur, a hydrogen atom must be bonded directly to a nitrogen (N), oxygen (O), or fluorine (F) atom within the molecule. See Table Describe what is occurring at the molecular level when a liquid evaporates. Support your description by drawing a particulate-level diagram illustrating evaporation. When a liquid evaporates, the molecules gain energy from collisions. When enough energy is gained, the molecules are able to overcome the IMF and escape into the gas phase. Student illustrations may vary but the idea is that particles gain energy and leave the surface of the liquid (Figure A). 3. The volume of water in an open container decreases over time, whereas the volume of water in a closed container remains constant. A student explains this phenomenon by citing that the evaporation of the water stops when the vapor pressure is reached in the closed container. Write a statement informing the student of the error in this argument. When water evaporates from an open container, the water molecules escape into the surroundings and are not likely to return to the container; therefore, the volume of water decreases. In a closed container, the volume of water remains constant not because evaporation stops but rather because a dynamic equilibrium is established between the molecules in the vapor phase and the liquid phase. Although nothing is visible to the eye, molecules are continuously escaping from the vapor phase and returning to the liquid phase. A N S W E R K E Y Figure A. Evaporation of a liquid v

6 hemistry May the Force Be with You PRE-LAB EXERISES (ONTINUED) Table 1. Various Organic ompounds Substance Formula Structural Formula Ethanol 2 5 O O Total Number of Electrons Type(s) of IMF 26 LDF and hydrogen bonding 1-Propanol 3 7 O O 34 LDF and hydrogen bonding 1-Butanol 4 9 O n-pentane 5 12 O 42 LDF and hydrogen bonding 42 LDF only A N S W E R K E Y Methanol 3 O O 18 LDF and hydrogen bonding n-exane LDF only vi

7 hemistry May the Force Be with You DATA AND OBSERVATIONS Substance Ethanol 1-Propanol 1-Butanol n-pentane Methanol n-exane t 1 ( ) t 2 ( ) Table 2. Temperature hange During Evaporation t (t 1 t 2 ) ( ) Explanation alculated Predicted Varies < 4.9 Butanol, propanol, and ethanol all exhibit hydrogen bonding and LDF but 1-butanol has more electrons than the others. Its LDF are stronger and it will evaporate less with a smaller t Varies > 8.3 Pentane only has LDF. As a result, it should evaporate more and with a greater t Varies > 8.3 Like the other alcohols, methanol exhibits hydrogen bonding and has LDF. owever, it has the smallest number of electrons, and therefore should evaporate more quickly and with greater t Varies < 16.1 exane should be similar to pentane with only a slightly smaller t because of its large number of electrons and greater LDF. A N S W E R K E Y vii

8 hemistry May the Force Be with You ANALYSIS 1. What relationship exists between the rate of evaporation of the liquid and the change in temperature? The more quickly a liquid evaporates, the greater the change in temperature. 2. Justify the direction of the temperature change in this investigation by describing the movement of energy between the system and surroundings that will occur during the evaporation process. The temperature decreases as each liquid evaporates. Evaporation is an endothermic process; energy must be absorbed by the system from the surroundings. For evaporation to occur, the liquid molecules must gain enough energy to overcome the intermolecular attractive forces and escape into the vapor phase. 3. Which of the alcohols studied has the strongest intermolecular forces of attraction? Support your claim using evidence from the data. Butanol has the strongest IMF and methanol has the weakest. All the alcohols have hydrogen bonding capability, so the difference in their t is the result of their differing LDF. Butanol has the greatest number of electrons, and therefore has the most polarizable electron cloud, resulting in the strongest LDF of the group. With the least number of electrons, methanol has the weakest LDF of the group, and thus evaporates most readily. This comparison shows us that it is not simply the presence or absence of hydrogen bonding that determines the evaporation of the molecules. The ability to evaporate is affected by considering all the IMF and their strengths within the molecule. 4. Which of the alkanes studied has the strongest intermolecular forces? Support your claim using evidence from the data. exane has stronger IMF than pentane. With more electrons in hexane (50 versus 42), the electron cloud is more polarizable, and therefore forms stronger LDF between hexane molecules. The stronger IMF result in a decreased tendency to evaporate, and thus give hexane a smaller t than pentane. 5. Plot a graph of t values of the four alcohols versus their respective number of electrons on the grid provided. Plot the number of electrons on the horizontal axis and t on the vertical axis. Label your graph appropriately. Write a caption for the graph summarizing your results. See Figure B. aption: The greater the number of electrons, the smaller the change in temperature hange in Temperature ( ) x Figure B. hange in temperature vs. number of electrons x Number of Electrons x x A N S W E R K E Y viii

9 hemistry May the Force Be with You ONLUSION QUESTIONS 1. Two of the liquids, n-pentane and 1-butanol, have the same number of electrons but significantly different t values. Justify the difference in t values of these substances based on their intermolecular forces. Because pentane and butanol have the same number of electrons, we expect their LDF to be approximately equal. The difference in evaporation rates arises from the hydrogen bonding that occurs between butanol molecules. Pentane molecules do not exhibit hydrogen bonding. The hydrogen bond between neighboring butanol molecules requires more energy to overcome and separate the molecules than the LDF in pentane. As a result, not as many molecules are able to escape into the vapor phase, and the t is not as large. 2. Given the following pairs of substances, identify which substance has the greater boiling point. Justify your answer using appropriate terminology regarding intermolecular forces. a. Methane, 4, or carbon tetrachloride, l 4 arbon tetrachloride has a higher boiling point (76.72 ) than methane ( ). Methane and carbon tetrachloride are both nonpolar molecules that have LDF as their only IMF. Methane has 10 electrons and carbon tetrachloride has 74 electrons. With more electrons, carbon tetrachloride has a more polarizable electron cloud, and therefore develops stronger LDF. With stronger LDF, the substance has a higher boiling point as more energy is required to overcome these forces. b. Ammonia, N 3, or phosphine, P 3 Ammonia has a higher boiling point ( ) than phosphine ( 87.7 ). Ammonia has the ability to form hydrogen bonds and LDF between neighboring molecules, and phosphine can form dipoledipole forces and LDF. With hydrogen bonds, the IMF in ammonia are stronger, and therefore the boiling point is higher. c. Dimethyl ether or ethanol These molecules have an identical number of electrons and identical molecular formulae. Some might predict that their boiling points are equal. owever, ethanol (78.4 ) has a higher boiling point than dimethyl ether ( 24 ). In ethanol, there is a hydrogen atom bonded to the oxygen atom, therefore this molecule is capable of forming hydrogen bonds between neighboring molecules. Even though the dimethyl ether molecule has an oxygen atom, the hydrogen atoms in that molecule are all bonded to carbon and not able to form a hydrogen bond with a neighboring ether molecule. A N S W E R K E Y ix

10 hemistry May the Force Be with You ONLUSION QUESTIONS (ONTINUED) 3. Draw a structural formula for each compound represented in the graph: chloroform, l 3, ethanol, 3 2 O, and acetic acid, 3 OO. l O 3 OO l l 4. What does the horizontal line represent? Why is this value significant? The horizontal line is drawn at 760 torr, which is standard (sea level) atmospheric pressure. This value is significant when finding normal temperatures for phase changes. l O O O 5. Identify the relationship between vapor pressure and boiling point. Arrange the substances shown in the graph from lowest boiling point to highest boiling point. When the vapor pressure above the liquid is equal to atmospheric pressure, the substance boils. The greater the vapor pressure, the lower the boiling point. The substances should boil in this order: chloroform (61.2 ), ethanol (78.4 ), and acetic acid (118 ). 6. Which substance exhibits the strongest intermolecular forces? Support your claim with data from the graph, your structural drawings, and your knowledge of intermolecular forces. Acetic acid exhibits the strongest intermolecular forces. From the graph, it is shown to have the lowest evaporation rate, indicating stronger IMF. From the structure drawn, acetic acid is a polar molecule with LDF and hydrogen bonding. hloroform is polar, exhibiting LDF and dipoledipole attractions. Ethanol is polar and does exhibit LDF and hydrogen bonding, however with fewer electrons than acetic acid, the LDF forces are not as great. A N S W E R K E Y x

11 hemistry May the Force Be hemistry with You May the Force Be with You Exploring Evaporation and Intermolecular Forces MATERIALS aprons data collection device filter paper, any size goggles paper towels 2 sensors, temperature, stainless steel scissors tape, masking 1-butanol 1-propanol ethanol, 95% hexanes methanol n-pentane rubber band, orthodontic The temperature of a substance gives a measure of the average kinetic energy of all the molecules in a sample. For a liquid, some molecules may have enough energy to overcome intermolecular attractions and enter into the vapor phase. If the liquid is in a closed system, equilibrium between the gas and liquid phases is established when the gas reaches a pressure specific to that substance at that temperature. This pressure is referred to as the vapor pressure of the liquid. The vaporization of a liquid below its boiling point is referred to as evaporation. In this experiment, temperature probes are placed in various liquids. Evaporation, an endothermic process, occurs when the probe is removed from the liquid s container (Figure 1). Like viscosity and vapor pressure, the rate of evaporation is related to the strength of intermolecular forces (IMF) of attraction. ere 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 pentane, 5 12, and hexane, Alcohols are distinguished from alkanes by the presence of the O functional group attached somewhere on the carbon chain. Methanol, 3 O, and ethanol, 2 5 O, are two of the alcohols that you will investigate in this experiment. You will examine the molecular structure of alkanes and alcohols to determine the presence and relative strength of two intermolecular forces, hydrogen bonding and dispersion forces. 1

12 hemistry May the Force Be with You Figure 1. Temperature probes PURPOSE In this experiment, you will study temperature changes caused by the evaporation of several liquids, and relate these temperature changes to the strength of intermolecular forces of attraction. SAFETY ALERT!» Goggles are required throughout this entire lab.» The compounds used in this experiment are FLAMMABLE and POISONOUS. Avoid inhaling their vapors. Avoid contact 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

13 hemistry May the Force Be with You PROEDURE 1. Set up your device for data collection: a. onnect the temperature probes to the interface. b. Open the data collection program. c. onfigure the data collection system to collect readings every second for a total of 4 minutes (240 seconds). 2. Wrap each temperature probe 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. int: First, slip the rubber band on the probe, then wrap the paper around the probe, and finally slip the rubber band over the paper. The paper should be even with the probe end. 3. Place Probe 1 into the ethanol container and Probe 2 into the 1-propanol container. Make sure the containers do not tip over. 4. Prepare two pieces of masking tape, each about 10 cm long, to tape the probes in position during Step After the probes have been in the liquids for at least 30 seconds, begin data collection. 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 up so the probe tips extend 5 cm over the edge of the table top, as shown in Figure Data collection will stop after 4 minutes unless data collection has been manually stopped before time has elapsed. Examine the graph of temperature vs. time. Based on your data, determine the maximum temperature, t 1, and minimum temperature, t 2, for both probes. Record t 1 and t 2 for each probe. For each liquid, subtract the minimum temperature from the maximum temperature to determine t, the temperature change during evaporation. 3

14 hemistry May the Force Be with You PROEDURE (ONTINUED) 7. Based on the t values you obtained for these two substances plus information from the Pre-Lab Exercises, predict the size of the t value for 1-butanol. ompare its hydrogen-bonding capability and number of electrons to those of ethanol and 1-propanol. Record your predicted t, and then explain how you arrived at this answer in the space provided. Do the same for n-pentane. It is not important that you predict the exact t value; simply estimate a logical value that is greater, lesser, or between the previous t values. 8. Test your prediction from Step 7 by repeating Step 4 through Step 6 using 1-butanol with Probe 1 and n-pentane with Probe Based on the t values you have obtained for all four substances plus information from the Pre-Lab Exercises, predict the t values for methanol and n-hexane. ompare the hydrogen-bonding capability and number of electrons of methanol and n-hexane to those of the previous four liquids. Record your predicted t, and then explain how you arrived at this answer in the space provided. 10. Test your prediction from Step 9 by repeating Step 4 through Step 6 using methanol with Probe 1 and n-hexane with Probe 2. 4

15 hemistry May the Force Be with You PRE-LAB EXERISES 1. omplete Table 1 with the structural formula, number of electrons, and intermolecular forces for each compound. The following information may be useful throughout the investigation: Dispersion forces exist between any two molecules, and generally increase as the molecular weight of the molecule increases because the more electrons in the molecule, the more polarizable the electron cloud. For hydrogen bonding to occur, a hydrogen atom must be bonded directly to a nitrogen (N), oxygen (O), or fluorine (F) atom within the molecule. 2. Describe what is occurring at the molecular level when a liquid evaporates. Support your description by drawing a particulate-level diagram illustrating evaporation. 3. The volume of water in an open container decreases over time, whereas the volume of water in a closed container remains constant. A student explains this phenomenon by citing that the evaporation of the water stops when the vapor pressure is reached in the closed container. Write a statement informing the student of the error in this argument. 5

16 hemistry May the Force Be with You PRE-LAB EXERISES (ONTINUED) Table 1. Various Organic ompounds Substance Formula Structural Formula Ethanol 2 5 O Total Number of Electrons Type(s) of IMF 1-Propanol 3 7 O 1-Butanol 4 9 O n-pentane 5 12 Methanol 3 O n-exane

17 hemistry May the Force Be with You DATA AND OBSERVATIONS Substance Ethanol 1-Propanol 1-Butanol n-pentane Methanol n-exane t 1 ( ) t 2 ( ) Table 2. Temperature hange During Evaporation t (t 1 t 2 ) ( ) alculated Predicted Explanation 7

18 hemistry May the Force Be with You ANALYSIS 1. What relationship exists between the rate of evaporation of the liquid and the change in temperature? 2. Justify the direction of the temperature change in this investigation by describing the movement of energy between the system and surroundings that will occur during the evaporation process. 3. Which of the alcohols studied has the strongest intermolecular forces of attraction? Support your claim using evidence from the data. 4. Which of the alkanes studied has the strongest intermolecular forces? Support your claim using evidence from the data. 8

19 hemistry May the Force Be with You ANALYSIS (ONTINUED) 5. Plot a graph of t values of the four alcohols versus their respective number of electrons on the grid provided. Plot the number of electrons on the horizontal axis and t on the vertical axis. Label your graph appropriately. Write a caption for the graph summarizing your results. 9

20 hemistry May the Force Be with You ONLUSION QUESTIONS 1. Two of the liquids, n-pentane and 1-butanol, have the same number of electrons but significantly different t values. Justify the difference in t values of these substances based on their intermolecular forces. 2. Given the following pairs of substances, identify which substance has the greater boiling point. Justify your answer using appropriate terminology regarding intermolecular forces. a. Methane, 4, or carbon tetrachloride, l 4 b. Ammonia, N 3, or phosphine, P 3 c. Dimethyl ether (left) or ethanol (right) O O 10

21 hemistry May the Force Be with You ONLUSION QUESTIONS (ONTINUED) Questions 3 6 refer to the graph shown in Figure 2. Figure 2. Vapor pressure of various substances vs. temperature 3. Draw a structural formula for each compound represented in the graph: chloroform, l 3, ethanol, 3 2 O, and acetic acid, 3 OO. 11

22 hemistry May the Force Be with You ONLUSION QUESTIONS (ONTINUED) 4. What does the horizontal line represent? Why is this value significant? 5. Identify the relationship between vapor pressure and boiling point. Arrange the substances shown in the graph from lowest boiling point to highest boiling point. 6. Which substance exhibits the strongest intermolecular forces? Support your claim with data from the graph, your structural drawings, and your knowledge of intermolecular forces. GOING FURTER Research one of the alkanes or alcohols used in this experiment. Write a brief report on the history, the production, the uses of the substance, and at least two other interesting facts. Be sure to include references. 12