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Bond Angles Lab - Teacher Support Information Home About Tools Curriculum Accounts Resources Teacher Forum Search Bond Angles Lab Teacher Support Information Table of Contents: 1. 2. 3. 4. 5. 6. Goals Common Topics in High School Chemistry Downloadable Lesson Plans and Handouts Advanced Background National Standards Section Additional Information Goals: Students will perform the Bond Angles Student Laboratory and learn about the geometry of a water molecule. Using energy data obtained from varying the bond angle in a water molecule, students will determine the best geometrical shape for water. Top Common Topics in High School Chemistry: First year chemistry curriculum concepts: Periodic Table, VSEPR, Bonding, Stability Second year chemistry curriculum concepts: Periodic Table, VSEPR, Bonding, Stability, Hybrid Orbitals Top Downloadable Lesson Plans and Handouts: ● ● ● ● Bond Angles Student Lab Coordinates Water Bond Angle Data Table Water Bond Angle Results Top Advanced Background: Typical data for the bond angle of water based on X-ray crystallography is 104.45 degrees. This is less than the predicted http://chemviz.ncsa.uiuc.edu/content/labtest.php (1 of 4) [3/9/2004 7:13:22 PM] Bond Angles Lab - Teacher Support Information value of 109.5 degrees for a CA2E2 molecule (1 central atom, two attached atoms, and two electron pairs). Water, having two nonbonded pairs of electrons and two bonded electrons has a bent shape, although the nonbonded electron pairs exclude more space than bonded pairs. Therefore, based on "negative-negative" or electrostatic repulsion, the nonbonded electrons force each other further apart, resulting in a smaller bond angle for water than expected. Top National Standards Section: All science curricular topics are mentioned in the National Standards for teaching Science. What are these "National Standards"? "The intent of the Standards can be expressed in a single phrase: Science standards for all students. The phrase embodies both excellence and equity. The Standards apply to all students, regardless of age, gender, cultural or ethnic background, disabilities, aspirations, or interest and motivation in science. Different students will achieve understanding in different ways, and different students will achieve different degrees of depth and breadth of understanding depending on interest, ability, and context. But all students can develop the knowledge and skills described in the Standards, even as some students go well beyond these levels." For this lab the Standards that apply are from the PHYSICAL SCIENCE, CONTENT STANDARD B section. As a result of their activities in grades 9-12, all students should develop an understanding of: ● ● ● ● ● ● Structure of atoms Structure and properties of matter Chemical reactions Motions and forces Conservation of energy and the increase in disorder Interactions of energy and matter State Standards also are available for you to review. Top Additional Information: Coordinates: 180 degrees 175 degrees 170 degrees 165 degrees 160 degrees 155 degrees 150 degrees 145 degrees 140 degrees 135 degrees 130 degrees 125 degrees 120 degrees 115 degrees 110 degrees 105 degrees 100 degrees 95 degrees 90 degrees Example Runs: Trial Angle (Example Run) Energy (hartrees) 1 180 degrees -75.9486 2 175 degrees -75.9493 3 170 degrees -75.9514 4 165 degrees -75.9548 http://chemviz.ncsa.uiuc.edu/content/labtest.php (2 of 4) [3/9/2004 7:13:22 PM] Bond Angles Lab - Teacher Support Information 5 160 degrees -75.9594 6 155 degrees -75.9648 7 150 degrees -75.9707 8 145 degrees -75.9771 9 140 degrees -75.9834 10 135 degrees -75.9896 11 130 degrees -75.9954 12 125 degrees -76.0004 13 120 degrees -76.0047 14 115 degrees -76.0078 15 110 degrees -76.0098 16 105 degrees -76.0105 17 100 degrees -76.0097 18 95 degrees -76.0073 19 90 degrees -76.0033 Answer Key: 1. Why would the best angle correspond to the lowest energy? Explain the differences between high and low energies. The lower the energy, the more stable the molecule. High energies are unstable. 2. Why is it important to know the length of the bond? The length of the bond is important when using sines and cosines in determining the x,y coordinates of a particular location. 3. What angle corresponds to the lowest possible energy for the water molecule? In this particular experiment, students should report 105 degrees as the lowest possible energy. 4. Describe the process you would use to determine a more precise value for the angle. http://chemviz.ncsa.uiuc.edu/content/labtest.php (3 of 4) [3/9/2004 7:13:22 PM] Bond Angles Lab - Teacher Support Information Students might try using smaller angles and begin at or around 105°. For instance, running seven trials (102, 103, 104, 105, 106, 107, and 108 degrees), students could follow the same procedure to determine a more precise value for the bond angle of water. Information: In the example runs, the yz slice and the xz slice are switched. Tell the students to ignore this problem if it occurs during class. Students should see the correlation between energy and the stability of a bond. The lower the energy, the more stable the bond. The bond angle experiment is merely a method at arriving at that concept. When running Waltz on a Macintosh Powerbook G4 (approx. 500 MHz), each trial took 30 seconds to input, 3:00 minutes to compute, and another 30 seconds to obtain the energy data. At four minutes per trial, this experiment would take over 70 minutes to complete. If you do not have two periods in which to perform this experiment, consider splitting up the trials between group members, i.e. Member One completes trials 1 - 10, Member Two completes trials 11 - 19. In this way, the data can be reassembled amongst group members once the experiment is completed. 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