# Laboratory 14. Vapor Pressure and the Heat of Vaporization. Objectives. Introduction

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1 Laboratory 14 Vapor Pressure and the Heat of Vaporization Objectives Use experimental techniques to record temperature and volume data for known and unknown compounds in which the liquid and gas are in equilibrium, and determine the boiling point of the liquid. Utilize the Clausius Clapeyron equation to calculate the heat of vaporization. Introduction The boiling of a liquid is a common phenomenon familiar to all of us. In both the kitchen and the laboratory we have all observed the boiling of water. We have also observed what happens when a puddle of water evaporates on a sunny day. In these examples the water molecules become gaseous, leave the system (the water) and enter the air (the surroundings). Would the boiling of water, or any other liquid for that matter, be affected if the gaseous molecules remained part of the system? What happens if the liquid and the gas above it are a closed system, i.e., a system in which energy can be exchanged with the surroundings but matter cannot? In this laboratory experiment you bring together the concepts of intermolecular forces, boiling point, and vapor pressure and develop a model to be able to answer questions like these. 1

2 Laboratory 14 Discussion Molecules can escape from the surface of a liquid into the gas phase by evaporation. In order to escape the liquid the molecule must overcome the intermolecular forces that attract it to other molecules in the liquid phase. If a liquid is heated the motion of the molecules overcome the attractive forces holding it in the liquid phase. The amount of energy required to vaporize a mole of liquid is the heat of vaporization vap. The units for heat of vaporization are generally kj mol 1 : H 2 O(l) \$ H 2 O(g vap = 40.7 kj mol 1 Question 14.1: a. The heat of vaporization is always a positive number because energy is required to overcome the intermolecular forces that are holding the molecules in the liquid phase. b. The heat of vaporization may be positive or negative depending on the particular intermolecular forces present in the liquid phase. c. A strong intermolecular force corresponds to a small heat of vaporization. What happens after the particles enter the gas phase? If the vessel is open to the atmosphere they may disperse and move away from the liquid. However, if they happen to strike the surface of the liquid they can be recaptured and return to the liquid phase. Obviously, as the concentration of gas-phase molecules increases the probability that they will strike the surface and condense, i.e., go from the gas phase to the liquid phase, also increases. We therefore have two processes taking place simultaneously; evaporation of the liquid and the condensation of the gas. When two processes occur simultaneously at equal rates, they are said to be in dynamic equilibrium. Even though it may appear nothing is happening, a great deal is happening as some of the molecules continuously pass from liquid to gas state, and from gas to liquid state. Suppose the evaporation of the liquid and condensation of the gas takes place in a closed system. When the molecules are in the gaseous phase they will exert a pressure as they strike the walls of the container (see Figure 14.1). The vapor pressure of a liquid is the pressure exerted by its vapor when the liquid and vapor are in dynamic equilibrium. 2

3 Vapor Pressure and the Heat of Vaporization P gas = equilibrium vapor pressure Hayden-McNeil, LLC Liquid ethanol (a) Liquid before any evaporation (b) At equilibrium, molecules enter and leave liquid at the same rate. Figure 14.1 Illustration of the equilibrium vapor pressure over a liquid. In (a) we imagine that no molecules of the liquid are in the gas phase initially; there is zero vapor pressure in the vessel. In (b), after equilibrium is reached, the rate at which molecules leave the surface equals the rate at which gas molecules return to the liquid phase. These equal rates produce a stable vapor pressure that does not change as long as the temperature remains constant. What do you predict will happen if there are initially no gas molecules above the liquid (Figure 14.1a)? If there is no external pressure being exerted on the liquid s surface, it will be easier for the gas molecules to escape into the vapor. In other words, the evaporation of a liquid is affected by the external pressure acting on the liquid surface. When the vapor pressure equals the external pressure acting on the liquid, it is boiling. The boiling point of a liquid at 1 atm (760 torr) pressure is called its normal boiling point. Vapor pressure (torr) Diethyl ether 34.6 C 78.3 C 100 C Normal boiling point Ethyl alcohol (ethanol) Water 200 Ethylene glycol Temperature ( C) Figure 14.2 Vapor pressure for four common liquids as a function of temperature. 3

4 Laboratory 14 Question 14.2: What do the liquids shown in Figure 14.2 have in common? Select all true statements: a. They all have the same vapor pressure at 100 C. b. They all boil at the same temperature. c. They all boil at the same vapor pressure. d. They all have the same ΔH vap. e. As the temperature increases, the vapor pressure increases. There is a quantitative relationship between vapor pressure, temperature, and heat of vaporization known as the Clausius Clapeyron equation, THvap 1 ln P = + C R T vap is the heat of vaporization, R is the gas constant, J mol K, and C is a constant. The units of R makes clear that vap will have units of J mol. Notice that the equation has the form of the equation of a straight line. If the vapor pressure vap vap R. vap and one of the points on the graph will be the boiling point, i.e., the temperature at which the vapor pressure equals atmospheric pressure. Question 14.3: Which statement correctly states the relationship between intermolecular force, vapor pressure, and normal boiling point? a. Strong intermolecular forces High vapor pressure High boiling point b. Strong intermolecular forces Low vapor pressure High boiling point c. Weak intermolecular forces High vapor pressure High boiling point d. Weak intermolecular forces Low vapor pressure Low boiling point Analyzing Your Data The procedure has three parts: a. Determine temperature volume data for a known compound. 4 b. Determine temperature volume data for an unknown compound. c. Determine the boiling point of the unknown compound.

5 40 Vapor Pressure and the Heat of Vaporization The temperature volume data is obtained by injecting a small amount of the compound to be studied into a 30-mL syringe. The syringe is immersed in a water bath and the water is slowly heated. The experimental setup is shown in Figure Volume readings are taken pressure is then calculated from volume measurements using Dalton s law and the combined gas law. The known compound is one in a set of three related compounds, and data for the other two known compounds in the set will be obtained from students designated by your lab instructor. After the syringe is cleaned and dried, the procedure is repeated with the unknown sample. Syringe 600ml 5% Thermometer Rubber band 600-mL beaker Hayden-McNeil, LLC Figure 14.3 Determining temperature volume data. In the third part of the experiment the boiling point of the unknown sample will be determined. The setup for the boiling point determination is shown in Figure The key to a successful boiling point determination is to keep everything on small scale: A 100-mL beaker is used for the water bath; a 4-mL test tube contains 1 ml of the sample. A capillary tube is broken to a length of approximately 3 cm and placed, open end down, in the capillary tube as vapor from the unknown liquid forces air from the capillary tube. After a few seconds, the capillary tube will contain only vapor from the unknown. The burner is removed and the liquid slowly starts to cool. When the vapor pressure falls below atmospheric pressure, liquid will start to enter the capillary tube. This temperature, at which vapor pressure equals atmospheric pressure, is the boiling point. 5

6 Laboratory 14 Hayden-McNeil, LLC Thermometer Thermometer 4-mL test tube 4-mL test tube ml 5% 100-mL beaker 25 Rubber band Capillary tube (closed end up, open end down) Sample (~1 ml) (a) Capillary tube in test tube (b) Test tube in beaker Figure 14.4 Boiling point determination. As noted above, ΔH vap the vapor pressure of the liquid at temperature T in Kelvins. The vapor pressures must be calculated from recorded volumes. The syringe contains both air and vapor from the liquid sample. As the temperature is increased, two things happen: conditions of pressure and volume change for the air and more of the liquid vaporizes. At each temperature, the liquid is assumed to be in equilibrium with the vapor. The pressure inside the syringe is equal to the barometric pressure, P B, and is the sum of the pressure of the sample, P S, and the pressure of the air, P A (Dalton s law): P B = P S + P A (1) The pressure of the sample is what we need to know; it is the vapor pressure of the liquid: P S = P B P A (2) An expression for the pressure of air at each temperature is obtained from the combined gas law: PV T R P V = (3) T B R A 6

7 Vapor Pressure and the Heat of Vaporization The left side of the equation represents the initial conditions: the volume of air (V R ) in the syringe at room temperature (T R ) and the barometric pressure (P B ). The right side of the equation represents a second set of conditions: pressure and volume at some higher temperature. Solve the equation for pressure of air, P A : Equation (4) can now be substituted into equation (2): VR T PA = PB (4) V T R VR T PS= PB PB (5) V T R VR T PS= PBf1 p (6) V T In your analysis, calculate the pressure of the sample at each set of conditions. Graph ln P S the same graph. Prepare a second graph for your unknown compound. Include the boiling point for each compound on the graph. The vapor pressure for the boiling points taken from the literature is mm Hg. For the experimentally determined boiling point, the vapor pressure is the barometric pressure. Each student must independently generate their graphs. It is suggested you use a graphing program (e.g., Excel) for your calculations and graphs. If you make your worksheets look like those in the lab manual, you can submit them in place of the Report Sheets in the manual (without having to copy all the data to the Report Sheet). However, you must turn in the Report Sheet (even if blank). If data is copied to the Report Sheet, you still need to turn in the worksheets with your graphs. In the graphing program, add a linear line to each data set. Set the option to print the equation for the line and the R 2 vap. vap you should show a sample calculation for the slope of one of the lines, even if you use the slope generated by the program. vap (kj mol 21 ). vap = (slope) R where R = J mol 1 K 1. R vap for the three known compounds and for your vap for the three knowns. Your discussion should in- vap values for the knowns corresponds to the attractive forces present in the compounds (i.e., did you get the correct trend based on the attractive forces? What are those forces?). Also, discuss whether the values are reasonable (based on what vap and boiling point for the unknown are reasonable and make sense. 7

8 Laboratory 14 Materials Required Equipment 30-mL syringe 2-mL syringe with needle two serum stoppers thermometer 600-mL beaker 100-mL beaker 4-mL test tube capillary tube rubber bands Bunsen burner, wire gauze thin-walled rubber tubing ring stand, ring utility clamp Chemicals One of the following compounds: Set 1 Alcohols methanol (1 ml) ethanol (1 ml) 1-propanol (1 ml) unknown sample acetone (15 ml) Common Equipment none Set 2 Alkanes n-hexane (1 ml) n-heptane (1 ml) n-octane (1 ml) Cautions unknowns can be irritating. Fume hoods should be on. Procedure Your TA will assign you to a group for Part A each student does a different known in the assigned set. Your TA will also distribute an unknown. A. Determine temperature volume data for the known compound. 1. Clean and dry a 30-mL syringe. Remove the glass plunger from the syringe and rinse it with a 5-mL portion of acetone. Lubricate the plunger with graphite by rubbing the entire surface with a pencil. Temporarily place the plunger in a 400-mL beaker. Rinse the barrel with acetone using two 5-mL portions. Dry the barrel by drawing air through it with the aspirator Attach a serum stopper to the syringe. The small end of the serum stopper goes around folded back around the outside. See Figure The preferred method is to fold the not be covered (see Figure 14.5).

9 Vapor Pressure and the Heat of Vaporization folded back serum stopper syringe stopper attached Hayden-McNeil, LLC Figure 14.5 Attaching the serum stopper to the syringe. 3. Insert a loose hypodermic needle into the center of the serum stopper. This allows air to escape as you replace the plunger. Push the plunger down to about the 5-mL mark, and remove the needle. The plunger should turn easily in the barrel but should not spin freely. Test the plunger now. 4. Use two rubber bands to attach a thermometer to the syringe with the bulb near the 1-mL mark. Make sure you don t attach the rubber bands to the plunger, or it won t move. 5. Position a ring on the ring stand at a height such that a burner can be placed beneath it. Above the ring, place a utility clamp and use it to hold the syringe, clamping it just below the top of the barrel. Do not clamp too tightly or the movement of the plunger will be restricted. Record the temperature and volume of air in the syringe. 6. Clean a small syringe and rinse it with a portion of the known compound. Use the small syringe and draw up about 0.4 ml of the compound. Invert the syringe so the air bubbles rise toward the needle. Press the plunger to force out the air and liquid (into a small Kimwipe) until the plunger is on the 0.20 ml mark. Insert the syringe into the 30-mL syringe through the serum stopper and carefully inject the 0.20 ml of the compound. Make sure to not inject any more air with the sample. Remove the small syringe. Some of the compounds can vaporize enough at room temperature so the plunger may rise. 7. Position a 600-mL beaker containing room temperature water on the ring. Lower the clamp so that the syringe is immersed at least to the 26-mL mark. Your setup should look like Figure Gently heat the water, stirring with a stirring rod to assure uniform temperature. Heating must be slow enough so that the sample is at the same temperature as the bath and has established its equilibrium. When the volume reaches approximately 8 ml, rotate the plunger. (For 1-propanol and n-octane, take readings at approximately 7 ml.) It is stirring the water and when the volume stabilizes, record the temperature and volume. 9. Continue heating and stirring; record temperatures and volumes at approximately 12, 16, 20, and 24 ml. (For 1-propanol, take readings at approximately 9, 11, 13, and 15 ml. For n-octane, take readings at approximately 8, 9, 10, and 11 ml.) 10. Measure the barometric pressure using the barometer in the balance room. 9

11 Vapor Pressure and the Heat of Vaporization What do your graphs show with respect to the vap values for the unknowns? What trends are evident and do they make sense in terms of the attractive forces present in the molecules? What are these attractive forces? Are the vap values reasonable based on what you've learned in lecture and the textbook? Do the normal boiling points you looked up show the same trends as the vap and follow the correct trends based on the attractive forces? Is the value for vap for the unknown reasonable? Is the boiling point reasonable? What errors are present? As always, make sure to include at least two inherent errors and how they can affect the results and be corrected. 11

12 12 Laboratory 14

13 Vapor Pressure and the Heat of Vaporization Laboratory 14 Report Sheet Name: Date: TA Name: Known compound Temp. Vol. data obtained from T R V R P B T ( C) 1/T (1/K) Volume (ml) P S (mm Hg) ln P S Boiling Point vap Known compound Temp. Vol. data obtained from T R V R P B T ( C) 1/T (1/K) Volume (ml) P S (mm Hg) ln P S Boiling Point vap 13

14 Laboratory 14 Report Sheet My Known Compound T R V R P B T ( C) 1/T (1/K) Volume (ml) P S (mm Hg) ln P S Boiling Point vap My Unknown Compound T R V R P B T ( C) 1/T (1/K) Volume (ml) P S (mm Hg) ln P S Boiling Point vap For one of your compounds, show sample calculations for 1/T, P S vap. Attach both graphs. 14

15 Vapor Pressure and the Heat of Vaporization Laboratory 14 Answer Clinic Question 14.1: Answer a. The heat of vaporization is always a positive number because energy is required to overcome the intermolecular forces that are holding the molecules in the liquid phase. b. The heat of vaporization may be positive or negative depending on the particular intermolecular forces present in the liquid phase. c. A strong intermolecular force corresponds to a small heat of vaporization. The heat of vaporization is always a positive number because energy is required to overcome the intermolecular forces that are holding the molecules in the liquid phase. Question 14.2: What do these liquids have in common? Select all true statements. Answer They all boil at the same vapor pressure, and as the temperature increases the vapor pressure increases. Question 14.3: Which statement correctly states the relationship between intermolecular force, vapor pressure, and normal boiling point? Answer Strong intermolecular forces, low vapor pressure, high boiling point. 15

16 Laboratory 14 Answer Clinic 16

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