Unit 6 Topic: Molecular Geometry VSEPR Date: February 25, 2011 NSES: STS A-E; ASE A-E; SCS 9-12 B Grade level: 10 th and 11 th SOL: CH.2 The student will investigate and understand that the placement of elements on the Subject: General Chemistry periodic table is a function of their atomic structure. The periodic table is a tool used for the investigations of f) trends including electronegativity CH.3 The student will investigate and understand how conservation of energy and matter is expressed in chemical formulas and balanced equations. Key concepts include c) writing chemical formulas (molecular, structural) d) bonding types (ionic and covalent) Daily Question: How does VSEPR (valence shell electron pair repulsion) dictate the shape of a molecule? How can you determine if a solute is soluble in polar or non-polar solvents? Procedures for Learning Experience Guiding Questions Materials Needed Housekeeping: Instructor should check every students HW 1.6 (on electronegativity) grading the assignment for completion (0-5 points). Once, homework is checked the instructor should address any misconceptions observed and answer any questions regarding the homework. Instructor should have students turn in the electronegativity worksheet completed in class after the unit 5 test last class period (25 point classwork grade). Note: students turn papers into the basket on left corner of teachers desk. What is electronegativity? What is the most/least electronegative atom on the periodic table? Check off Sheet on clipboard Evaluation (Assessment) Informal assessment of student work by observation. Approximate Time Needed 10 Engagement: Instructor should initiate class discussion by asking the guided questions listed in order to get students thinking about the concepts they will see in the exploration. How does electronegativity predict bond types (covalent or ionic)? How does electronegativity impact how molecules interact with each other? None. Informal assessment through discussion. 5
Exploration: Instructor should distribute the Polarity Exploration handout and read the introduction to the students. The instructor should then have students hypothesize what will occur in both Petri dishes (one with whole milk, one with skim milk), recording their thoughts on the handout. The instructor should project the demonstration so students can see what is occurring in the milk mixture in the Petri dish from a birds-eye perspective. Instructor should choose a student to perform the procedure. MAKE SURE STUDENT IS WEARING GOGGLES. See specific procedure below. What is the difference between skim milk and whole milk? Why did the food coloring begin to swirl? Did the food coloring swirl in both Petri dishes? Why or why not? How does dish soap clean your dishes? What component of dish soap attracted the food coloring? Petri dish, whole milk, skim milk, dish detergent, food coloring, goggles, document camera, Polarity Exploration handout Collect handout after students have finished the exploration. Grade the exploration on the thoroughness and content presented in the conclusion (15 points). 20 Have students record their observations. Instructor should engage class in a discussion to have students determine the science behind what they observed. What can we conclude from this exploration? Have students turn in their demonstration handout in the basket at the front of the room and have students pick up the guided notes next to the basket. Explanation: Lecture covering: electronegativity, polarity, solvation ( like dissolves like ), and molecular shapes(valence shell electron pair repulsion (VSEPR)). How can electronegativity be used to predict polarity? What determines if a solute is soluble in a solvent? Guided notes, powerpoint, remote clicker, toothpicks, marshmallows, gumdrops, Ziploc bag Informal assessment of student participation. Instructor should introduce electronegativity, polarity, and solvation through student conclusions from the exploration. What must you know about the molecule in order to determine polarity? 40 See attached PowerPoint slides and notes section below for lecture details. Instructor should Why do the water molecules orient themselves differently
encourage students to utilize the guided notes given. For molecular shape, students should be able to name the structure, identify bond angles, and determine polarity for linear, tetrahedral, pyramidal, bent, and trigonal planar shapes. Students must have the shapes and angles memorized! Before introducing each shape instructor should have students draw the Lewis dot diagram for the compound in their guided notes. Then the instructor should distribute a Ziploc bag filled with the appropriate molecule constructing tools and have students create a 3D shape for the 2D Lewis dot diagram that they drew. Regular marshmallows = unshared electrons Mini marshmallows = shared electrons Toothpicks = bonds Gumdrops = atom around sodium and chlorine ions? What is largest bond angle obtained by methane? When a lone pair of electrons replaces an atom, what happens to the bond angle? What pattern do you observe as the number of lone pairs on the central atom increases? Why was a mini-marshmallow used to show a shared pair of electrons and a regular marshmallow used to show an unshared pair? How can VSEPR model help to predict bond angles? Conclude lecture with the terminology that guided their molecular shape creations (VSEPR). Extension: Instructor should distribute the Practice with VSEPR, Polarity, and Solvation worksheet to give students reinforced practice with these new concepts. Instructor should go through the first problem (BF 3 ) with the students and then allow the students to first work individually and then with their bench partner. How did drawing a Lewis structure help you to determine the geometry of each of your substances? Could an ionic compound be soluble in both water and alcohol? Why? Handout Collect student work for class work completion grade and to assess student comprehension of material. 15 Instructor should put the answer key at the front of the classroom so students can check their work by working at their own pace. If students do not finish the practice problems they should complete them at
home. The practice problems will be collected next class period. Notes: 1. Homework to be checked: a. Read Periodic Trends in Electronegativity pages 168-169. Answer questions: page 169 #20-21, page 175 #69. b. Name the most and least electronegative atoms on the periodic table. 2. Polarity Demonstration Procedure: a. Instructor and student helper should both be wearing goggles. Note that milk, food coloring, and soap are harmless chemicals but they should not be ingested due to potential residue from other reactions on the laboratory glassware. b. Instructor should turn on and project the document camera. c. Instructor should ask student to place the Petri dish under the camera and fill the dish with skim milk until it is half full. d. The student should add one drop of food coloring at 12, 3, 6, and 9 o clock to the milk. Note the demonstration is most effective when a different color is used for each of the drops added in the four locations. e. Student should add one drop of dish soap (Palmolive works best) to the center of the dish. f. All students should record their observations. g. Instructor should choose another student to perform the procedure again but using whole milk instead of skim milk. 3. Detailed Lecture Plan: a. Slide 1: i. Why does the trend for electronegativity increase up and across the periodic table? ii. If lithium and fluorine were bonded together, where would you find the majority of the electrons distributed in the compound? Why? b. Slide 2: i. Visually answers the second question from slide 1. ii. Why does Cl 2 display no ionic character, but HI displays about 7% ionic character? iii. Why does LiF display more ionic character than LiCl? c. Slide 3: i. Thinking about electron distribution, how would this apply to ionic compounds dissolved in a solvent like water? ii. What does dissolving really mean? 1. Note this question is very important because research shows that students struggle with this concept. See CTS: mixtures and solutions for more information. iii. Show students hyperlinked simulation of NaCl dissolving in water. http://www.chem.iastate.edu/group/greenbowe/sections/projectfolder/flashfiles/thermochem/solutionsalt.html iv. Why do the water molecules orient themselves differently around sodium and chlorine ions? d. Slide 4: i. Go through the definitions of polar and nonpolar molecules, building from what the students observed with the behavior of water with the salt ions.
ii. For determining polarity discuss symmetry and how symmetrical molecules are nonpolar and asymmetrical molecules are polar. Demonstrate this idea using a yardstick and the images displayed on the screen. Encourage students to use their pencils when looking at their Lewis dot diagrams to determine polarity. e. Slide 5: i. Differentiate between a polar molecule and a polar bond. Repeat this several times. Module how electronegativity values help to confirm the polarity of a molecule, the more polar a molecule the more ionic character it displays. Reference back to first slide with electronegativity values if needed. f. Slide 6: i. The guided notes display the images portrayed on this slide. Use the think-pair-share method when having students predict the polarity of the molecules displayed. ii. Go through each answer explaining and demonstrating why its either a polar or nonpolar molecule. g. Slide 7: i. Before putting up this slide have students draw the Lewis dot diagrams for the molecules on page 2 of the guided notes. Since students are still struggling with drawing their diagrams have them compare their answers to their neighbors. Instructor should circle classroom checking student work. ii. Introduce the activity: Creating 3-D Molecular Shapes. 1. Students are not allowed to consume their materials until the activity is completed. If consumption occurs instructor should confiscate student materials. 2. Instructor should demonstrate the basic construction of a molecule using the materials. 3. Instructor should distribute a materials bag to each student. The bag includes: 5 gum drops, 8 toothpicks, two large marshmallows, and six mini marshmallows. 4. Students are prompted to construct a methane molecule, the first molecule they drew a diagram for. 5. Note: students who have motor deficit skills should be given a modeling kit to facilitate construction of shapes. h. Slide 8-12: i. Review the five molecular shapes in the following order: tetrahedral (methane), trigonal pyramidal (ammonia), trigonal planar (boron trifluoride), bent (water), and linear (carbon dioxide). ii. A 3D simulated molecule (hyperlinked in PowerPoint) should be shown to students so they can verify their constructed molecular shape as well as determine the bond angles for the molecules. http://3dchem.com/ iii. Prompting questions for students creating 3D molecules: 1. Is that the furthest apart the atoms can go? Can you come up with another way to separate them? Why did you spread them out that way? 2. How many shared pairs of electrons does the central atom have? How many unshared pairs of electrons? What are the bond angles? 3. When a lone pair of electrons replaces an atom, what happens to the bond angle? What pattern do you observe as the number of lone pairs on the central atom increases? i. Slide 13:
i. Introduce students to the terminology used for when atoms within a molecule arrange themselves to minimize the repulsion of the electrons. When they have been predicting the molecular shapes, they have inadvertently been following the VSEPR model. ii. How does the VSEPR model help us to predict molecular shape and bond angles? iii. What role does electronegativity plan within the VSEPR model? 4. Homework due next class: a. HW 2.6 i. Read VSEPR Theory on pages 259-261. Answer numbers 54-55 ii. Draw the Dot diagrams and predict the shape and bond angle for the following molecules: SiF 4, PCl 3, H 2 S, NH 3, O 2 iii. Read Electronegativity and Polarity on pages 263-267. Answer numbers 64-70 iv. Predict which atom in the following bonds would carry a partial negative charge: H-N, S-S, O-P, C-F, B-O, H-Cl v. Predict whether the following molecules are polar: SiF 4, PCl 3, H 2 S, NH 3, O 2 b. HW 6.6 i. Read & summarize page 455-456 Solvation and Read Solubility of polar molecules on page 266 ii. Determine which molecules on page 266 #60-63 will dissolve in water c. Instructor should distribute the EXTRA CREDIT webquest that reinforces the content taught in this lesson. The extra credit is due the day of the unit 6 test.