Phase Diagrams Revised: 1/27/16 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 Critical Point Pr Liquid ess Solid ur e 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 Phase Diagrams Revised: 1/27/16 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 Phase Diagrams Revised: 1/27/16 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 = Kf · m Where ΔT is the freezing point depression, Kf 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 Kf 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 OH Fe HO Tert-butanol Ethylene glycol Ferrocene Figure 3: Chemicals used 3 Phase Diagrams Revised: 1/27/16 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 Phase Diagrams Revised: 1/27/16 Figure 4: Temp. probe 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 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 tert- butanol 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 Phase Diagrams Revised: 1/27/16 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.