Phase Diagrams Revised: 1/27/16 PHASE DIAGRAMS. Adapted from Bill Ponder, Collin College & MIT OpenCourseWare INTRODUCTION

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1 PHASE DIAGRAMS Adapted from Bill Ponder, Collin College & MIT OpenCourseWare INTRODUCTION A phase diagram is a graphical representation of the physical states of a substance as they relate to temperature and pressure (Figure 1). A typical phase diagram has pressure on the y-axis and temperature on the x-axis. Supercritical Fluid Pr ess ur e Solid Critical Point Liquid Gas Triple Point Temperature Figure 1: Example of a general phase diagram. The labels on the graph represent the physical state or phase of the substance at equilibrium under all possible pressure/temperature conditions. The three common physical states are solid, liquid, and gas. These phases are defined by their physical properties, such as the organization of the atoms or molecules with respect to one another (Figure 2). In solids, particles are closely organized in a predictable order, causing them to hold a shape unless force is applied. Particles in 1

2 a liquid are still closely packed, but are not organized, allowing them to conform to the shape of the container in which held. A gas has no definite shape or volume, but occupies the entire container in which it is confined. SOLID LIQUID GAS Figure 2: Physical states The lines on the phase diagram represent combinations of pressures and temperatures at which two phases coexist in equilibrium. By moving across the lines or curves on the phase diagram, a phase change occurs. The orange line divides the solid and gas phases, corresponding to sublimation (solid to gas) and deposition (gas to solid). The green line dividing the solid and liquid phases represents melting (solid to liquid) and freezing (liquid to solid). The blue line divides the liquid and gas phases, representing vaporization (liquid to gas) and condensation (gas to liquid). Phase changes occurring when temperature is increased (moving left to right on the phase diagram) require energy. No methods exist to predict where these boundaries are for any given compound, so they must be determined experimentally. In addition, two important points exist on the diagram: the triple point and the critical point. The triple point represents the pressure and temperature at which all three phases exist at equilibrium. The critical point terminates the liquid/gas phase line and relates to the critical pressure (typically a very high pressure), the pressure above which a supercritical fluid forms (typically a very high pressure). Supercritical fluid has the physical properties of a gas, but behaves like liquids due to its high density, making it useful in certain applications. For example, supercritical carbon dioxide is used in the production of decaffeinated coffee. Over the next two weeks, experimental data will be obtained for the construction of a portion of the phase diagram of tert-butanol. 2

3 The phase diagram in Figure 1 is for a pure compound. When a second compound is introduced to the system forming a homogeneous solution however, the phase diagram drastically changes. For example, the addition of a solute to a pure solvent (making a solution) can disrupt important interactions between solvent molecules, changing the temperature at which the solvent would typically freeze or boil. If we take a closer look at the phase change between liquid and solid, we can identify a property known as freezing point depression. Freezing point depression is a colligative property; that is, it depends of the ratio of solute and solvent particles and not on the nature of the substance. The equation showing this relationship is: ΔT = K f m Where ΔT is the freezing point depression, K f is the freezing point depression constant specific for a given solvent, and m is the molality of the solution. Molality (m) is used because it is independent of the volume changes that can occur with variations in temperature, unlike molarity (M). molality (m) = moles of solute / kg of solvent In Part D of the experiment, the freezing point depression of tert-butanol solutions will be observed, and the K f of tert-butanol will be determined using the data obtained. Lastly, phase changes can be used to purify chemical compounds. One such example is the use of sublimation to purify solids. In the final part of the experiment, sublimation will be used to purify ferrocene (for more information on ferrocene, see the ferrocene experiment introduction). OH HO OH Fe Tert-butanol Ethylene glycol Figure 3: Chemicals used Ferrocene 3

4 Before starting the experiment, the TA will ask you to do a quick demonstration or talk-through one of the following: 1) How to use parafilm 2) Show the apparatus set up for Part A (Two students can tag-team to show the set up) 3) What is the purpose of boiling chips 4) Show the apparatus set up for Part E SAFETY Safety goggles, aprons, and gloves must be worn at all times in the laboratory. Tert-butanol is extremely flammable and harmful by inhalation, ingestion, and in contact with skin. Any container holding tert-butanol should be capped when not in use to prevent evaporation, as it is harmful when inhaled. Tert-butanol, ethylene glycol, and ferrocene must be placed in appropriate waste bottles and can NEVER be poured down the drain. PROCEDURES Work in pairs. Answer all italicized questions in your observations. Week 1: Part A: 1. Wrap a small amount of parafilm around the temperature probe approximately one-half inch from the top (Figure 4). Insert the probe into the two-hole stopper until the parafilmed portion is seated within the stopper. What is the purpose of the parafilm? Next, construct the experimental apparatus as shown below using a 50 ml filter flask and 125 ml filter flask (Figure 5). 4

5 Phase Diagrams Figure 4: Temp. probe Revised: 1/27/16 Figure 5: Apparatus 2. Plug the pressure sensor into Ch. 1 and temperature probe into Ch. 2 of the LabQuest2. Change the units of pressure and temperature to atm and C, respectively, by clicking on each window and selecting change units (Figure 6). Figure 6: LabQuest2 3. Test the apparatus for leaks by turning the vacuum pump on and completely closing the needle valve (Figure 7). The pressure should reach ~0.01 atm. If a leak is suspected, use parafilm to seal any joint. If there are no leaks, slowly open the needle valve and turn off the vacuum pump. 5

6 Phase Diagrams Revised: 1/27/16 Figure 7: Needle valve 4. Make sure that the 50 ml filter flask is clean and dry. Add approximately 20 ml of tertbutanol to the 50 ml filter flask along with several boiling chips. 5. Stopper the 50 ml filter flask, ensuring the tip of the temperature probe is immersed in the tert-butanol (Figure 8). Draw a molecular illustration representing the chemical phases present in the flask. (Use spheres to represent tert-butanol molecules, as in Figure 2) Boiling chips Figure 8: Temperature probe and Boiling chips in tert-butanol 6. Confirm that your needle valve is in the open position and turn on the pump. Note the temperature and pressure reported on the LabQuest2. The pressure inside the 50 ml filter flask containing the tert-butanol can be adjusted by closing the needle valve (Figure 7). Confirm this by slowly turning the knob of the valve and noting the decrease in pressure. 7. Completely close the valve and closely examine the tert-butanol contained in the flask. Also, closely monitor both temperature and pressure readings. Eventually the tert-butanol will both freeze and boil simultaneously. What is this called? Record the temperature and pressure. Draw a molecular illustration representing the chemical phases present in the flask. 6

7 8. Slowly open the needle valve. Once the pressure within the system has equalized with atmospheric pressure, turn off the vacuum pump. 9. Melt the tert-butanol completely by suspending the 50 ml filter flask in a warm water bath. 10. Repeat the experiment two or three more times, recording the temperature and pressure of each trial. What is the average of the trials? Part B: 1. Heat a 1 L beaker of water on a hot plate to near boiling. Use the same apparatus from Part A. 2. Set up a water bath by filling a 250 ml beaker equipped with a stir bar with tap water. Suspend into the bath the 50 ml filter flask containing the tert-butanol. While stirring, slowly warm the tert-butanol to approximately 40 C by adding hot water from the 1 L beaker to the water bath. Make sure the needle valve is fully open. Use an alcohol thermometer to determine the temperature of the water bath. 3. Turn on the pump and slowly decrease the pressure within the system by partially closing the needle valve. Continue decreasing the pressure until the tert-butanol begins to boil. Draw a molecular illustration representing the chemical phases present in the flask before and after the pressure has been reduced. 4. Record the pressure and temperature and then completely open the needle valve to raise the pressure within the system. The boiling should stop immediately. 5. Adjust the water bath temperature to ~50 C and repeat the experiment, recording the temperature and pressure when the tert-butanol begins to boil. 6. Continue to collect data for temperatures of ~60 C, ~70 C, and ~80 C, presenting your data as a table in your ELN. What is the literature value for the boiling point of tert-butanol? What is the pressure at which the literature value is measured at? What part of a phase diagram does this data represent? 7. When you have finished collecting all of the data, dispose of the tert-butanol in the proper waste container and disassemble the apparatus. Rinse all glassware with acetone into the waste container, let air dry in the hood, and return to their place of origin. 7

8 Week 2: Begin this week by setting up Part E. Part C: 1. Set up a water bath by filling a 250 ml beaker equipped with a stir bar with tap water. Cool the bath to approximately 15 C using ice. 2. Using the same apparatus from Part A (including the 20 ml of tert-butanol in the clean and dry 50 ml filter flask), remove the rubber stopper on the 50 ml filter flask and add a small stir bar to the container. Replace the rubber stopper. Warm the tert-butanol to approximately 30 C using a warm water bath. Use hot tap water for the warm water bath. 3. Now, suspend the 50 ml filter flask containing the tert-butanol in the 15 C water bath, and initiate stirring. 4. Closely observe the tert-butanol. When the tert-butanol becomes turbid (cloudy), record the temperature and pressure from the LabQuest2. What is the literature value for the freezing point of tert-butanol at 1 atm? Draw a molecular illustration representing the chemical phases present in the flask as the tert-butanol freezes. 5. Remove the 50 ml filter flask containing the tert-butanol from the cold water bath and warm the tert-butanol to approximately 30 C using a warm water bath. 6. Turn on the vacuum pump and adjust the pressure using the needle valve to approximately 0.75 atm. 7. Return the 50 ml filter flask containing the tert-butanol to the 15 C water bath. 8. When the tert-butanol becomes turbid (cloudy), record the temperature and pressure. 9. Adjust the pressure to ~0.5 atm and repeat the experiment, recording the temperature and pressure when the tert-butanol begins to become cloudy. 10. Continue to collect data for pressures of ~0.25 atm, and ~0.1 atm, presenting your data as a table in your ELN. What part of a phase diagram does this data represent? 11. When you have finished collecting all of the data, dispose of the tert-butanol in the proper waste container. Rinse the 50 ml filter flask with acetone into the waste container and dry the flask before moving on. Part D: Ethylene glycol solutions 1. Repeat Part C of the experiment using 1 m and 0.5 m solutions of ethylene glycol in tertbutanol. Which liquid is the solute and which is the solvent? Your TA will assign you one of 8

9 the solutions and an approximate pressure at which to run the experiment. Share your results with your lab section. Some solutions may not freeze in the 15 C cold bath. Add a few pieces of ice to the bath to slowly reduce the temperature if this is the case. How much (in ml) ethylene glycol is in 20 ml of a 1 m solution in tert-butanol? Provide the procedure for making the 1 m solution, including glassware used. 2. When you have finished collecting all of the data, dispose of the ethylene glycol/tert-butanol solution in the proper waste container and disassemble the apparatus. Rinse all glassware with acetone into the waste container, let air dry in the hood, and return to their place of origin. Part E: Sublimation of Ferrocene 1. Weigh out approximately 200 mg of crude ferrocene and record the actual mass. 2. Distribute the crude ferrocene in the center of a culture dish to a thickness of no more than 5 mm. 3. Place the cover on the culture dish and place the apparatus on a hotplate. Turn the hotplate on LOW and place a 400 ml beaker filled with ice water on the top of the culture dish (Figure 9). Figure 9: Sublimation apparatus 4. Allow the sample to heat up slowly. As sublimation occurs, crystals will form on the cover of the culture dish. Do NOT heat the sample over 100 C. A small beaker filled with water can be placed on the hotplate next to the apparatus to assure it is not too hot. How will you know if the hotplate has become too hot? Why is it important not to over heat the ferrocene? Draw 9

10 a molecular illustration representing the chemical phases present in the petri dish during heating. Represent ferrocene molecules as squares. 5. Once a layer of crystals have formed on the cover, turn off the hotplate and allow the apparatus to cool. Carefully remove the ice bath from the top of the culture dish by sliding it off the top. Lifting the ice bath straight up may disturb the crystals too much. 6. Carefully remove the cover of the culture dish and scrape off the solids using a metal scoop. Mass the pure solid. Obtain Raman IR spectra of both the crude ferrocene from the bottle and purified ferrocene samples. Place the remaining pure solid in a labeled vial, and submit it to your TA. What does the pure ferrocene and remaining crude ferrocene look like? 7. Rinse the remaining ferrocene into the proper waste container using acetone. Dry the culture dish with paper towels. Calculations and Discussion Part A, B, and C 1. Using the data obtained, construct a phase diagram in Excel. Be sure to include the average temperature and pressure from Part A in both data sets, so that your two lines intersect at a common point. Also include the literature value for the boiling point of tert-butanol as a data point. 2. Using your general knowledge of phase diagrams, sketch in the missing phase change and label all the phases. Part D 1. Using the shared data from your class, construct a graph of the 3 freezing point data sets: pure tert-butanol, 1 m ethylene glycol, 0.5 m ethylene glycol. 2. What does your graph tell you about the relationship between freezing points at various pressures? 3. Why does the solution of ethylene glycol in tert-butanol freeze at a different temperature than pure tert-butanol? Include in your answer a discussion of molecular interactions. 4. Using the data obtained at atmospheric pressure, calculate K f for tert-butanol. 5. The literature value for K f is 9.1. Calculate the percent error. 10

11 Part E 1. Calculate your percent yield of purified ferrocene. 2. Compare the crude and pure ferrocene Raman spectra. Has the spectrum changed from sublimation? What does this tell you about the purity of the crude ferrocene used? Abstract writing An abstract is a brief summary of a research study, included in nearly all scientific writing situations. It describes the objectives of the study, the methods used, the major results, and their interpretation and implications. Typical abstracts are usually no more than 5 to 6 sentences long and include a single figure to help demonstrate the results. Write your own abstract for the entire experiment. (Hints: Focus on Parts A-C in your abstract and include a figure of the phase diagram) Spartan Extra Credit (up to 10 points) Perform one or more Spartan calculations that model any of the physical changes that occur during this lab. On a new ELN page in this experiment s folder provide the procedure used (Spartan directions), Spartan files, data, and a discussion of the data s meaning and how it relates to the experiment done in lab. 11