Lab #8: Virtual Geometry

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Name: _______________________________________________
MPHS Chemistry
Lab #8: Virtual Geometry
Objective: To explain how a molecule’s geometry can be predicted from its Lewis structure using VSEPR
theory.
Introduction: The figure below shows some common molecular shapes and their names. We refer to the threedimensional shape made by the bonded atoms in a molecule as its molecular geometry.
Linear
Bent
Tetrahedral
Trigonal Planar
Trigonal Pyramidal
Pre-Lab Questions:
1. If a molecule consists of a central atom bonded to 2 peripheral (outside) atoms, what are its possible
geometries?
2. If a molecule consists of a central atom bonded to 3 peripheral atoms, what are its possible geometries?
An electron domain is a region of space around the central atom occupied by a lone pair or a chemical bond to
another atom (the bond may be single, double, or triple).
For example, in the Lewis structure below, the central atom A is surrounded by 4 electron domains.
lone pairs
x1 =
1 ED
single bonds
x2 =
2 EDs
double bonds
x1 =
1 ED
triple bonds
x0 =
0 EDs
TOTAL =
3. Determine how many electron domains surround each B atom.
4 ED's
Procedure:
Part I. Exploring Electronic Geometries
1. Open the "Molecular Shapes" simulation at http://phet.colorado.edu/en/simulation/molecule-shapes.
2. Choose the MODEL tab and select the bond angle and electronic geometry boxes.
A bond angle is any angle made between three points - an outside atom, the central atom, and another outside
atom.
3. Experiment by clicking and dragging the electron domains (single bonds, multiple bonds, lone pairs) around
the central atom. Try to bring them closer together. What happens when you do this? What is the
equilibrium or final resting position of the electron domains? What does this have to do with the VSEPR
theory?
Electronic geometry is the 3-D shape made by the electron domains around a central atom.
4. Experiment by using different numbers of electron domains around the central atom. For each total number
of electron domains, record the electronic geometry and the bond angle(s) below.
Number of electron
domains
Electronic Geometry
Bond angle(s)
1
2
3
4
5.
How is the definition of electronic geometry different from molecular geometry?
Part II. Exploring Molecular Geometries
1. Click on the tab for REAL MOLECULES and select the REAL radio button.
2. The attached chart below shows a number of actual molecules. (Some of the Lewis structures violate the
octet rule. Ignore this for now and focus on the geometry.)
3. For each molecule, sketch the shape. Record the electronic geometry, molecular geometry, bond angle(s),
and the number of electrons domains that are bonding and non-bonding.
4. Under what conditions do electronic geometries differ from molecular geometries?
2
Analysis: Once you have completed the table, look for patterns in the data.
1. Decide whether the following statements are TRUE or FALSE based on your observations in this lab.
a)
b)
c)
d)
e)
f)
Electron domains exert an attractive force on each other.
Electron domains try to orient themselves as far apart from each other as possible.
The bond angle is the angle made between 3 terminal atoms.
If a central atom is surrounded by four atoms, the molecular geometry is tetrahedral.
If a molecule consists of only 2 atoms, it has a linear molecular geometry.
The molecular geometry is identical to the electronic geometry if lone pairs are present on the central
atom.
2. Construct a rule that allows you to predict from the Lewis structure when a molecule is
a) linear vs. bent
b) trigonal planar vs. trigonal pyramidal
Extension:
1. Using the simulation, determine the bond angles in CH4, NH3, and H2O for the Real vs. Model (click the radio
button). Fill in the chart below.
Bond angle
Model
Real
CH4
NH3
H2O
2. Scientists have determined that lone pairs (non-bonding electron domains) are physically larger and repel
more than bonding domains (single, double, or triple bonds). Discuss how your bond angles from above
provide evidence of this.
3
Assessment: The following questions will be assessed using Criterion C: Knowledge and Understanding of
Science.
Due: _________________________________________
1. Draw a Lewis structure for each of the molecules or ions below. Use what you have learned in this activity
to predict the molecular geometry of the following molecules or ions:
a) CH2O
b) PH3
c) SiF4
d) O3
e) SCl2
f) HCN
2. Match each molecule in question #1 to a molecule that has the same geometry from your table in Part II.
3. Provide your best estimate of the bond angle for each molecule in question #1.
4. The molecule phosphorus pentachloride, PCl5, is an exception to the octet rule. The central phosphorus atom
is able to make 5 bonds to chlorine atoms. The Lewis structure is shown below. The molecular geometry of
PCl5 has been determined and is shown below. This geometry is known as trigonal bipyramidal.
a) Applying what you have learned in this lab, label the Cl-P-Cl bond angles in the molecular geometry
and determine their value in degrees. Note: There are 3 different angles.
b) Use the VSEPR theory to explain why the geometry shown below is not a very likely molecular geometry
for PCl5.
4
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