Determine the polarity of several molecules and their solubility.

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1 Exploring Polarity & Solubility Using WebMO The purpose of this activity is to compare the polarity of solute and solvent molecules to determine if their polarity affects how well they mix. Solute and solvent molecules that readily mix are said to form a solution. A common measure of how well they mix is referred to as solubility. Solubility measures the amount of solute that can mix in a given amount of solute. You will be using the Internet to access a program called WebMO. Using the program will allow you to view the geometry of a molecule and determine the polarity. During the activity students will import the following molecules and determine the polarity direction and strength of each molecule: C 6 H 14 hexane CH 3 CH 2 CH 3 -- propane H 2 O water CH 3 CH 2 -OH ethanol H 3 C-O-CH 3 dimethyl ether CO 2 -- carbon dioxide HF hydrogen fluoride Polarity is a measure of how symmetrically electrons are distributed relative to the proton distribution in the atoms of a molecule. Molecules in which the electrons are not symmetrically distributed are said to possess a dipole moment. The dipole moment is expressed in the unit Debyes and can be represented by an arrow. The head of the arrow points in the direction of greater electron concentration. The tail of the arrow points to the region of the molecule with a lower electron concentration. You will examine the molecules to determine the direction of the dipole arrow (polarity direction) and identify the strength of the dipole moment (calculated value in Debyes). To begin the activity, access the Internet and Go to Wait for the site to redirect to the login page. Sign on with the username and password provided by your teacher. Exercise 1 Determine the polarity of several molecules and their solubility. The molecules for this activity have been assembled and saved as Jmol files (*.mol) for import. You will need to import the files one at a time and process them as indicated below in order to observe the molecular geometries and dipole moments. Importing the Molecules: Your instructor has saved the Jmol files in a directory to which you have access. You will be able to access the files using the browse control during import. Preparing to import a molecule: This part assumes you have logged onto WebMO and are at the Job Manager" screen. Click the New Job menu option at the top left of the WebMO viewing screen and select the "Create New Job" option on the drop down menu list shown below University of Illinois Board of Trustees

2 1 A Build Molecule" window will open as shown below. Click on the "Import Molecule" button at the bottom right of the window. This will open the "Import Molecule" window shown below. Click on the "Browse" button at the center right to locate the file for import. Double click on the file name to upload it to the import molecule window. With the file selected, click on the "Import Molecule" button at the bottom left and the file will be imported into the "Build Molecule" window as shown below.

3 2 The next step is to process the molecule through the calculation engine of the software. To begin this process, click on the blue arrow at the bottom right of the Build Molecule screen. This will open the "Choose Computational Engine" screen shown below. A list of five selectable engines can be seen at the center left of the window. Select the engine titled "Mopac" by clicking on the circle to the left of the engine name. Once the engine has been selected, click on the blue arrow at the bottom right of the window. This will open the "Configure Mopac Job Options" window shown below. Modify the job name by appending your initials or other identifier after the chemical formula (C2H6O in the example). Use the pull down tab in the "Calculations menu to view the drop down list. Select "Geometry Optimization" as shown in the example. Finally, click on the blue

4 arrow at the bottom right of the screen to submit the job. This will return you to the main Job Manager" screen as show below. 3 You will see your job listed as a line entry (Number, Name, Description, Date, Status, Time, and Actions). When the computer is finished with its computations, the job status will be shown as "complete" (as shown in the example above). You are now ready to access the job, view the molecule, and determine the direction and magnitude of the dipole moment. Click on the job name to open the window shown below. The molecule viewing window is located at the top of the screen. If you scroll down, you will find the "Calculated Quantities" window. Locate the dipole moment listing in the calculated quantities. Record the value given in the table provided within the lab handout. If you click on the magnifying glass icon, the molecule in the viewing window will display a blue arrow showing the location and direction of the dipole moment. You can rotate the molecule using your mouse to view the dipole moment from different vantage points. Repeat the process until you have observed all of the molecules. Gathering solubility and experimental dipole moments for your molecules: Information on the solubility of the molecules included in this activity can be found at a variety of websites. To search for information of the solubilities in water or hexane, one might use a search query such as: ethanol solubility in water

5 In a similar manner, one might search for experimentally measured dipole moments using a query such as: ethanol dipole moment To completely fill in the chart below, you will need to do a search on the Internet for the experimental dipole moments and solubilities of each of the molecules in water and hexane. Record the dipole moments obtained from WebMO (calculated) and the experimental dipole moments from the Internet search. Use parentheses to set off the experimental value. The polarity of a molecule increases as its dipole moment increases. 4 Formula Name Dipole (Debye) C 6 H 14 H 2 O CH 3 OCH 3 CH 3 CH 2 OH HF CH 3 CH 2 CH 3 CO 2 Polarity Solubility Predictions H 2 O C 6 H 14 xxxxxxxxxx xxxxxxxxxxx A completely nonpolar molecule is expected to have a dipole moment at zero. The dipole moment is a measure of the force of attraction or repulsion per unit of mass on the affected particle. The positive end of a dipole attracts the negative end of other dipoles; it repels the positive end of other dipoles. Hence, polar molecules tend to exhibit mutual attraction for one another. They interact only very weakly with nonpolar molecules. From this tendency we get the simple rule Like dissolves like. Using this rule, predict which of the molecules in this activity are soluble in H 2 O and/or C 6 H 14. Compare your predictions with the solubilities found on the Internet. Dipole Moment Order Formula Solubility (include units) H 2 O C 6 H 14 Least To Greatest

6 Questions: 1) What relationship do you see between the polarity of a substance and its solubility in water? (In the first six molecules.) What reasons can you give for this relationship. (Hint: think about what is happening in a polar molecule vs. a nonpolar molecule.) 2) Rank the substances in order of their polarity from least polar to most polar. Does the solubility of these molecules in water show a pattern? If you observe a general pattern, or relationship, between polarity and water solubility, describe the pattern. 3) Does the solubility and polarity of CO 2 fit the pattern? If not, what possible explanation can you offer? 4) Assuming you are able to locate the solubilities of the molecules in hexane, does the solubility of the molecules in hexane show a pattern? If you observe a general pattern, or relationship, between polarity and hexane solubility, describe the pattern. 5) In what ways has this activity helped you understand the relationship between solubility and polarity? 5

7 Teacher Notes: Determining Solubility: Keeping in mind that Like dissolves like, which of the following are soluble in H 2 O and/or CCl 4. Formula Name Dipole (Debye) Polarity Soluble in H 2 O Soluble in C 6 H 12 C 6 H 12 hexane nonpolar xxxxxxxxxx 20ºC H 2 O water polar xxxxxxxxxxx (1.85) CH 3 OCH 3 dimethyl ether polar 20ºC (1.30) CH 3 CH 2 OH ethanol polar miscible (1.69) HF Hydrogen fluoride polar miscible (1.91) CH 3 CH 2 CH 3 propane nonpolar 20ºC (0.08) CO 2 Carbon dioxide nonpolar ºC 6