Intermolecular and Ionic Forces Comparing IMF Using Percent Evaporation: This lab will explore changing masses during evaporation of organic liquids. The decrease in mass is related to the strength of the intermolecular forces of attraction. As you measure the change in mass during evaporation of three organic liquids, consider the strength of the forces that hold molecules together in each liquid. Will a greater decrease in mass as each substance evaporates indicate stronger or weaker intermolecular forces? Phase Diagrams: For any substance, a phase diagram can be constructed to determine the temperatures (on the x-axis) and pressures (on the y-axis) in which each phase will exist and the curve along which two phases will exist at the same time (in equilibrium). These equilibrium lines, or curves, are where phase changes occur. You will explore an interactive tutorial that explains phase diagrams and how temperature and/or pressure can be changed for a substance to undergo a phase change. Part I: Comparison of evaporation rates In this lab, the degree of evaporative cooling (the temperature drop) of different organic liquids will be measured. Your group will consider how a substance’s intermolecular forces influence the ability of the molecules to evaporate. You will measure the mass of a sample that evaporates after 3 minutes, then take the difference between that mass and the initial mass of the sample to determine the percent by mass of the sample that evaporated. You will explain the results of your data based on the intermolecular forces of the different substances. One group will work at a time in each hood to complete this portion of the lab. While your group is waiting for a hood, you can work on Parts II and III. Part I should take approximately 15-20 minutes to complete. Caution: Organic liquids are highly flammable and volatile. The three organic liquids MUST remain in the hood at all times. The data for this experiment needs to be in your lab notebook. Please read through the procedure at least one time before deciding how to make your data table. Procedure: 1. In this experiment, you will use three organic liquids: ethanol, acetone, and pentane. In the hoods, there are three 10-mL beakers labeled “E” (for ethanol), “A” (for acetone), and “P” (for pentane) for use with each of the liquids. 2. Use a Kimwipe to completely dry the inside of the beaker labeled “E”. Weigh the empty beaker, and record its mass in your lab notebook. 3. In the hood, carefully add only 10 drops of the ethanol liquid into the beaker. Reweigh the beaker, and record its mass in your lab notebook. 4. Take the beaker back to the hood, and set a timer for 3 minutes. 5. When the timer goes off for each liquid, weigh the sample again, and record its mass in your lab notebook. 6. Repeat the procedure (steps 2-5) for the acetone sample. 7. Finally, repeat the procedure (steps 2-5) for the pentane sample. 8. Calculate the mass of the sample at the start for each liquid, then calculate the mass of the sample that evaporated for each liquid by subtracting the “mass of the beaker + sample after 3 minutes” from the “mass of the beaker + sample at the start”. 9. Calculate the percent by mass of the sample that evaporated for each liquid using the formula below: % by mass of sample that evaporated = Mass of sample that evaporated × 100% Mass of sample at the start **Lab Notebook: A table with all the data collected (also including data derived from calculations) should contain the following for each liquid tested (think about the best way of including all these data in one table): • Mass of empty beaker • Mass of beaker and sample before experiment • Mass of beaker and sample after 3 minutes • Mass of sample (only) at beginning of experiment • Mass of sample that evaporated • % of sample calculated **Also be sure to show your calculations within your lab notebook. Part II: Model structure of ice The back bench of the lab has been set up with two model kits to build the structure of solid water (ice). This kit is specifically designed to show the difference between covalent bonds O-H bonds within a water molecule and the hydrogen bridges holding two different H2O molecules together. Groups will rotate through this station. Have your instructor sign your report sheet when your model is complete. 1. Water molecules have already been assembled in this model kit, with the red ball representing oxygen and the smaller white balls representing hydrogen. This is an approximate model showing the overall structure water makes when frozen. However, you will assemble a more accurate model using the plastic molecular models. 2. Use the ruler provided to measure (in cm) the approximate distance from the center of one hydrogen atom to the center of the oxygen atom. Record these data in your lab notebook. 3. Now attach the water molecules to each other using the purple hydrogen bridges provided. You should notice 6-sided, open channels within the structure of ice. (Compare your model to the pre-built structure at this station.). Use as many water molecules as possible. **Lab Notebook: Get your lab instructor’s signature confirming that your model of ice is correct. 4. Use the ruler to measure (in cm) the approximate distance between the center of a hydrogen atom in one molecule and the center of the oxygen atom in a neighboring molecule. Record these data in your lab notebook. 5. When you are done at this station, disconnect the hydrogen bridges (but leave the water molecules intact). Part III: Tutorial of online animations for Intermolecular Forces • • • Scroll down until you see the icon for the green “Intermolecular Forces Tutorial” link. The 13 sections each contain multiple pages. You can click on the “Previous Section” button at the bottom left to return to an earlier section if you would like to review it. Be sure to click on the “Real-World Connections”, “Science Connections”, and “Concept Question” links. Intermolecular and Ionic Forces: Lab Report Name: _______________________ Partner(s): _____________________ Section Number: ________________ PRE-LAB PAGE: Before coming to lab, complete the table and answer the questions on this page. Refer to the figures on page 1 to draw the Lewis structures of the following molecules. Note that H atoms are shown in white, O atoms are shown in red, and C atoms are shown in black/gray on page 1. ethanol (C2H5OH) acetone (C3H6O) (pentane C5H12) Identify the type(s) of intermolecular force(s) between molecules of each substance using LF=London forces, DDF=Dipole-dipole forces, and HB=Hydrogen bridges. Circle the intermolecular force that is strongest in each molecule. ethanol: ______________ acetone:____________ pentane: ____________ Physical Changes: In the “States of Matter” lab, you observed what happened to water when it evaporated and condensed on a watch glass. In the boxes below, draw what happens to water molecules when water evaporates. Begin by drawing 6 molecules in the box on the left. Use to represent H atoms and to represent O atoms. Before evaporation After evaporation Lab Questions You will turn in the pre-lab page, pages 5-8, and your lab book carbon copies Part I: Comparison of evaporation rates 1. Rank the three liquids from in order of increasing percent by mass that evaporated. __________________ < ___________________ < ____________________ lowest percentage highest percentage 2. Would the percent by mass of sample that evaporated be the greatest for the substance with the strongest or the weakest intermolecular forces? Explain. . 3. Rank the three liquids in order of increasing intermolecular forces: _____________________ < ____________________ < ____________________ Weakest intermolecular forces Strongest intermolecular forces 4. If a substance’s boiling point reflects the energy required to vaporize the substance, would the substance with the greatest percent by mass of sample that evaporated have the highest or the lowest boiling point? Explain. 5. Rank the three liquids in order of increasing boiling point: ___________________ < ___________________ < ___________________ lowest boiling point highest boiling point Part II: Model structure of ice 1. Which of the measurements that you recorded in your lab notebook for Part II represents the polar covalent bond within a water molecule and which represents the hydrogen bridge between two water molecules? Explain. 2. Sketch two water molecules (labeling the oxygen and hydrogen atoms) and identify the covalent bonds (as solid lines) holding atoms together and the hydrogen bridge (as a dotted line) attracting one molecule to another. Pay attention to the orientations of the water molecules! Part III: Tutorial of online animations for Intermolecular Forces Most of the questions in this section are taken directly from the tutorial. Each of these questions will be paired with an extension question asking the same content but different examples. 1. Rank the following intermolecular forces in order of increasing strength: ion-ion, dipole-dipole, hydrogen bridging, and London dispersion forces. 2. Which ionic compound would have stronger ion-ion forces? NaF or KBr? ____ Why? 3. Which intermolecular force is only temporary? Rank the following substances from strongest to weakest intermolecular forces for problems 4-6 below. In section 9, the tutorial shows similar types of substances and asks you to rank order them from strongest to weakest based on types of IMF: 4. H2S, H2O, LiF, and Ne: _______ > _______ > _______ > _______ strongest weakest 5. H2O, O2, and CO: _______ > _______ > _______ strongest 6. KF, CaCl2, KBr, AlBr3, SrO: weakest _______ > _______ > _______ > _______ > _______ strongest weakest 7. Would you expect KBr to be solid or liquid at room temperature? Explain why. (Hint: Think about the type and strength of intermolecular force you would find in a sample of KBr.) Part IV: Phase Diagrams Section 3: What does the negative slope in water’s phase diagram tell you about physical properties of the solid versus the liquid? Explain your answer. Section 4: Why does the animation pause at each of the black lines as the water sample is being heated? Section 5: Use the phase diagram below (this is NOT water’s phase diagram) to label the 3 open areas as solid (S), liquid (L), or gas (G); label the three segments as solid-liquid (SL), liquid-gas (LG), or solid-gas (SG) equilibria; label the triple point (TP). What is the normal boiling point (at 1 atm) of this substance? _____________ What is the normal freezing point (at 1 atm) of this substance? _____________ Section 6, Question 2: At 100ºC and 230 atm, water is most stable in the liquid phase as shown in the diagram on the screen. How could the liquid water be converted to steam at this point? Circle your answer. a. Raise the temperate b. Raise the pressure c. Lower the temperature d. Lower the pressure Concept Question: If you heat ice at atmospheric pressure, it first becomes water and then steam. What happens if you heat ice at extremely low pressures? Post-lab Questions: 1. Circle all of the following molecules that can participate in hydrogen bonding: H2O H2S CH4 CO2 HF 2. Use intermolecular forces to explain why bromine is a liquid while chlorine is a gas at room temperature. 3. Rank the following in order of increasing boiling point: KCl, Al2S3, H2O, Br2, HCl, H2. _______ < _______ < _______ < _______ < _______ < _______ lowest b.p. highest b.p. 4. The box on the left is H2O, the box on the right is HCN. Identify what the letters A and B refer to in the structures below – a chemical bond (ionic, polar covalent, or nonpolar covalent) or an intermolecular force. For IMF’s, identify the strongest intermolecular force. (Hint: Write formulas of the compounds and label the atoms in the pictures!) B A A A: A: B: B: B For number 5 below, consider the following six choices: A. ionic bond D. London (dispersion) forces B. polar covalent bond E. dipole-dipole forces C. nonpolar covalent bond F. hydrogen bond 5. Identify the type of bond or intermolecular force between the following: _____ a. I and I in an I2 molecule. _____ b. Na+ and Cl- in solid NaCl. _____ c. HBr and another HBr molecule. _____ d. NH3 and another NH3 molecule. _____ e. H and F in a HF molecule. _____ f. Br2 and another Br2 molecule.