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EQA
Learning By Modeling
LEARNING BY MODELING
Spartan 1.1 Practice working with Spartan. Select New from the Toolbar or File menu to access the
library of atom fragments and structural units.
(a) Select the
(b)
(c)
(d)
(e)
button from among the atom types, then click in the window to place a
C
tetrahedral carbon on the desktop. Click at the end of a bond to add a second tetrahedral
carbon to the first. The model is now CH3CH3. Repeat. The model is now CH3CH2CH3.
Select View from the Toolbar or Build menu to display the model in various formats.
Select Add Fragment from the Toolbar or Build menu and convert CH3CH2CH3 to
(CH3)2CHCH2CH3 by adding tetrahedral carbons as appropriate.
Select Delete from the Toolbar or Build menu and convert (CH3)2CHCH2CH3 to
CH3CH3 by deleting carbons. (Click on an atom to delete it.)
Select OCl from the atom library and click on the end of a bond to convert CH3CH3 to
CH3CH2Cl.
Select OOO and double click on the chlorine atom; O replaces Cl. View a model of
CH3CH2OH in various formats. Configure the Labels on Model menu to show atom
symbols.
(f) Scale, rotate, and translate the model.
(g) Select Close from the Edit menu.
Spartan 1.2 Carry out the indicated operations on a model of CH3CH3.
(a) From the list of Groups, select Carboxylic acid, and connect it to the CH3CH3 model by
clicking on a bond. You now have a model of CH3CH2CO2H. Clear the desktop.
(b) From the list of Rings, select Cyclopentane, and connect it to a CH3CH3 model. Copy
and Paste the resulting compound as a Tube model into a word-processing document.
Spartan 1.3 Use the
C
,
C
, and
C
atom fragments to construct molecular models of
ethane (C2H6), ethylene (C2H4), and acetylene (C2H2).
(a) How do ethane, ethylene, and acetylene compare with respect to their geometries and
bond angles (Geometry menu) predicted on the basis of VSEPR?
(b) What is the trend in COH and COC bond distances (Geometry menu) in going from
ethane to ethylene to acetylene?
Spartan 1.4
(a) Make a model of CH3OCH3 and measure the COOOC bond distances and bond angle.
Select Minimize on the Toolbar or Build menu. What are the minimized bond distances
and bond angle?
(b) Click on the OSO button, then double click on the oxygen atom of CH3OCH3 to transform it to CH3SCH3. What are the COSOC bond distances and bond angles before and
after minimization?
(c) Based on your experience with CH3OCH3 and CH3SCH3, how do you think the bond
distances and angles of (CH3)3N will compare with those of (CH3)3P? Test your prediction by making minimized models.
Spartan 1.5 How many different models can you make that have the formula (a) CH2Cl2;
(b) Cl2CPCH2; and (c) ClCHPCHCl?
Spartan 1.6 Spartan can make models to illustrate inorganic chemistry too. In the gas phase, elemental phosphorus exists as P4 molecules. P4 is a regular tetrahedron. You can make it as follows:
atom fragments
±£
a
c
POPOP
A
P
b
make bonds
and Minimize
uuuuy
a and c
a and b
b and c
POOOP
H E
P
A
P
OO
O
P
O
OO
Connect four
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CHAPTER ONE
EQA
Structure Determines Properties
Many of the problems in later chapters illustrate the modeling of properties by quantum mechanical
calculations. You won’t do the calculations, but will call up their results from Spartan’s Database. There
are two ways to do this. One is to Retrieve a molecule from the File menu on the basis of its name.
Retrieving a molecule gives a model and its calculated properties (accessed via the Display menu). The
other method is to make a model in the usual way. If the model is in the Database, its name will appear
at the bottom of the window. Clicking on the word Retrieve beside the name replaces your model with
its counterpart (including calculated properties) from the Database.
Spartan 1.7 One compound in each of the following pairs has a dipole moment and the other does
not. Choose the one that has a dipole moment and specify its direction. Compare your answer with
that provided for the corresponding model in the Database. Dipole is an option under Properties in
the Display menu. The direction of the dipole moment can be displayed by clicking in the box next
to Dipole.
(a)
BF3 and
(b)
F3CCH3 or
F
NF3
To access boron, go to the “expert”
atom fragment library.
(c)
F3CCF3
F
CPC
H
F
or
H
H
CPC
H
F
Spartan 1.8 Which compound is more consistent with the electrostatic potential map shown? Is it H2O,
CO2, or SO2? Why? Does the molecule have a dipole moment? Check your answers with the molecular models in the Database.
Spartan 1.9
(a) The species BH4, CH4, and NH4 all have an octet of electrons around a central atom
and a tetrahedral geometry. Their electrostatic potential maps are shown. Match the map
with the correct species. The color scale is the same for the three maps. Check your
answer with the models in the Database.
(b) Turn off the potential map and select Properties from the Display menu. Click on each
atom to determine its charge. What is the distribution of charges among all of the atoms
in each species and what is the net charge in each?
(c) The extent of the electron density surface as shown in the maps is a measure of the size
of the various species. Which species is largest? Which is smallest? Why?
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EQA
Learning By Modeling
Spartan 1.10
(a) Write Lewis formulas for three species in which carbon is bonded to three hydrogens.
These species should have formal charges of 0, 1, and 1, respectively, for carbon.
(b) Match your Lewis formulas with the electrostatic potential maps shown. The color scale is
the same for the three maps. Check your answer with the models in Database.
(c) Using the same procedure as in the preceding problem, view the charge on each atom and
add them to determine the net charge on each species.
(d) Apply VSEPR to predict the geometry of each species based on their Lewis formulas and
rank the three structures in order of decreasing HOCOH bond angle. Do your geometries
and order of bond angles match those of the models?
O
Spartan 1.11 There are three different kinds of hydrogen environments, labeled a, b, and
c, in glycolic acid. Rank hydrogens a, b, and c in order of decreasing acidity. Refer to
the glycolic acid model in the Database. How does your predicted order of acidity cor- HOCH2COH
respond to the degree of positive charge calculated for each hydrogen?
a
b
c
Spartan 1.12 Amino acids represent a key class of compounds in biochemistry and can exist in two
isomeric forms.
O
B
H2NCH2COOOH
O
I
B
L
H3NCH2COO
E
D
This interconversion obeys the customary rule for acid–base reactions in that it lies to the side of the
weaker acid. Compare the charge on the hydrogen of the OH group in one structure with a hydrogen
of the H3N group of the other. Assume the more acidic proton has the larger positive charge to determine which isomer predominates.
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