Download Bond Angles Lab - Teacher Support Information

Bond Angles Lab - Teacher Support Information
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Bond Angles Lab
Teacher Support Information
Table of Contents:
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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.
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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
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Downloadable Lesson Plans and Handouts:
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Bond Angles Student Lab
Coordinates
Water Bond Angle Data Table
Water Bond Angle Results
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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
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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.
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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:
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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.
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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
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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.
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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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