Experiment 5
Structure and Physical Properties of Compounds
What laboratory techniques and skills will you practice?
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Building molecular models
Observing solubility/miscibility
Handling flammable chemicals
Pre-lab preparation

Read through and write notes in
the margins of this document
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Complete your flowchart.

Attempt to answer margin
question prompts, as they will
prepare you for the worksheet
completed in the lab.
What chemical concepts will you apply?
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Generate valid Lewis diagrams
Identify bonds of significant polarity, functional groups, and bond angles
Build VSEPR diagrams
Assign electron-pair geometry and molecular shapes to atoms bonded to
two, three, or four other electron groups
Use intermolecular interactions to explain/predict relative miscibility and
boiling points.
Complete and pass (>60%) the
online pre-lab quiz in d2l.
What to bring to lab
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**Lab coat & goggles
Pen, pencil, eraser
What communication and reporting skills will you use?
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Describing observations
Comparing observed data to predicted IMFs present
Background
Understanding the structure of a chemical compound is important in understanding
the chemical and physical properties of that compound and the reactions it can
undergo.
When a chemist is given a molecular formula for a small molecule (or condensed
formula for a larger species) they should be able to determine a "best" Lewis diagram
for the chemical species that the formula represents (e.g. for NH3 see Figure 1a).
Remember that a Lewis diagram only shows the connectivity between atoms. The
connectivity is a chemist’s understanding of what type of bonding forces and bond
dipoles (electron density differences) exist in a chemical species.
The connectivity between atoms in a compound is only half of the picture. It is
important to know how the connectivity is arranged in 3D space (VSEPR diagram).
VSEPR gives an understanding of the "shape" of a molecule, and a rough idea of the
overall electron density distribution within a molecule or its molecular dipole (see
Figures 1b and 1c). These visual representations help one understand how different
species can approach each other, and the location of potential "sites of interaction".
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VSEPR: Attention to Detail
Can you spot the four errors in
the following VSEPR diagram of
propanoyl chloride below?
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Figure 1a. The Lewis diagram
of NH3.
Figure 1b. The VSEPR diagram of NH3, including bond
angles, bond dipoles, and partial charges.
Figure 1c. The overall electron
density distribution for NH3.
This experiment will examine how chemical structure can be related to physical
properties such as miscibility and boiling point.
Miscibility
Miscibility is the ability of liquids to mix and form a homogeneous mixture (looks like
a single solution). For covalent molecules it is observed that like dissolves like, i.e.,
polar solvent dissolves polar species and non-polar solvent dissolves non-polar
species. Polarity of a covalent molecule is based on the presence of molecular dipoles.
The “direction” of molecular dipoles can be approximated by looking at relevant bond
dipoles. Bond dipoles are based on electronegativity differences between atoms (∆χ).
In general, relevant bond dipoles are those that have a ∆χ of at least 0.5. Sometimes
the structure of a covalent molecule can have both polar and non-polar portions in
which case the compound exhibits solubility in both polar and non-polar solvents.
You and a lab partner will examine the miscibility for a variety of compounds and see
how chemical structure affects solubility/miscibility in polar and non-polar media.
Before you start interpreting your data you should make yourself aware of the
structures. By assembling models of the compounds that you are dealing with in
Experiment 5, you and your lab partner will study how to draw 2D diagrams that most
effectively convey the 3D shapes of molecules (VSEPR diagrams). There are often
many ways one can draw a "correct" 3D VSEPR structure, but determining which one
is the "best" drawing is more difficult. You will then use these drawings to examine
molecular polarity and then rationalize the trends you see.
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IMFs and Miscibility
When justifying the miscibility of
“A” with “B”, it’s important to
consider the cumulative,
relative strengths of IMFs in the
following situations:
A/A, A/B, and B/B.
If “A” and “B” are miscible, the
IMFs between A/B are
stronger/more favorable than
the IMFs between A/A or
between B/B.
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Hydrogen Bonding
Consider the pairs of molecules
below, and decide if hydrogen
bonding is possible between
them (hint: 2/4 sets are possible)
Set #1:
Set #2:
Set #3:
Set #4:
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Boiling Point
The boiling point of a liquid relates to the interactions between the molecules of that
liquid. The stronger the interactions between molecules the higher the boiling point.
The interactions are known as intermolecular forces (IMFs). To understand these
interactions one must look at the overall electron density distributions within the
molecules and whether they are permanent (dipole-dipole forces, hydrogen bonding)
or temporary (London dispersion) forces. In particular, it is critical to look at the
overall shape of the molecule (especially the carbon backbone of an organic molecule)
as well as the functional group location along that backbone (Figure 2).
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LDFs & Molecular Shape
Consider the structures and
boiling points for the two
isomers of C5H12 below:
309 K
283 K
Given that LDFs are the only
forces in pure samples of each
isomer, why do their boiling
points differ?
Figure 2. A VSEPR diagram of methylamine (left), shown with an
overall molecular dipole. A generalized representation of the
molecule is shown based on the overall shape (right).
FYI:
When purchasing chemicals from a chemical company it can be confusing on
how to name a compound. Even if it is implied that we should use IUPAC
nomenclature instead of a common name (i.e. propanone instead of acetone), what if
we are trying to purchase from a company whose native language is not English? This
is why chemical companies have gone to purchasing based on CAS registry number.
Though they will not actually be used in this experiment, you recorded the CAS
registry number of a list of chemicals as part of your pre-lab exercise.
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Procedure
Students will work in pairs. Record data on your own worksheets as you work.
Your responses to questions must be in your own words.
Preparation
1. Obtain the following supplementary equipment:
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Marking pen
Model kit
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9 test tubes (18 x 150 mm)
9 test tube stoppers
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2. Using your model kit, each student is to build ONE of the following
compounds (your TA will assign you one): 1-butanol, 2-butanol, 2-methyl-1propanol, methanol, propanol and butanone.
Part 1 – Miscibility
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1. Label your 18x150 mm test tubes with numbers 1 through 9. In test tube #1
place both 1 mL of water and 1 mL of hexanes. Record your observations
in Table 1.
2. In test tubes #2 to #5 add 1 mL of water to each. In test tube #2, add ~15 drops
of methanol. In test tube #3, add ~15 drops of propanol. In test tube #4, add
~15 drops of 1-butanol. In test tube #5 add ~15 drops of butanone. Stopper
the tubes, shake and observe whether or not a homogeneous solution is
formed. Record your observations in Table 2.
3. Repeat step 2 with test tubes #6 to #9 but this time using 1 mL of hexanes
instead of water. Make sure the test tubes you are using are completely dry.
4. Use your data observed and your understanding of intermolecular forces to
answer all the discussion questions on the last page of your worksheet.
Part 2 – Using Structure to Rationalize Relative Boiling Points
Molecular Model Kits
Use all the models built by
the class as a whole to help
analyze and draw VSEPR
structures/ line drawings in
Tables 1-4. These models will
help rationalize boiling points
and miscibility.
Schlieren Lines
The presence of Schlieren
lines is due to a change in
the refractive index of a
solution, and indicates that
something is dissolving.
When combining reagents in
the experimental procedure,
be aware of this
phenomenon.
!
Waste Disposal +
Cleaning
All tube contents are
disposed of in the organic
waste container in the
fumehood. Tubes should
be rinsed with acetone,
and allowed to dry upside
down in a test tube rack
until the end of the lab
period.
Uh-oh! Someone ripped the boiling point data off of some chemical compounds! We
need you and your lab partner to use your understanding of intermolecular forces to
rationalize and match the compounds with their boiling points.
1. Look to the PowerPoint presentation to see two sets of compounds with
unmatched boiling points, which you will analyze in Tables 3 and 4.
2. Based on your understanding of intermolecular forces and how they change
with a change in structure, rationalize the relative boiling points for a given
set of compounds with differing functional groups in Table 3, and then repeat
the process for a given set of compounds with the same functional group in
Table 4.
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Expectations for Drawing Structures in the Worksheet
In your worksheet, you will be asked to draw line/Lewis and VSEPR structures, including identifying the most polar
bonds by placing δ+ and δ- charges on their atoms, listing bond angles, bond dipoles and evaluating if there is a molecular
dipole. Here is a sample structure:
Line Drawing
VSEPR Diagram
Propanal
Bond Angle(s)
Bond
Dipole(s)
Molecular Dipole
Remember single bonds
are rapidly rotating
(compare the initial line
drawing with the one
above), you want to
consider this when
evaluating a molecular
dipole
Figure 3. Example of how to fill in the tables in your worksheet.
**C-H bond polarity is minimal (∆χ = 0.3) and can be omitted from diagrams**
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WORKSHEET LABORATORY REPORT
CHEMISTRY 201
SPRING 2019
Experiment 5: Structure and Physical
Properties of Compounds
Student Name:
Date Experiment Performed:
Home Lab Section:
Home Lab Room:
Home Lab TA Name:
Lab Partner Name:
Student Signature:
Date:
Please staple the worksheet pages (7 pages, including this one) AND the pages for the procedure
given in the lab manual before handing them to your lab TA.
Submit your completed worksheet BEFORE leaving the laboratory room.
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Part 1 – Miscibility Tests
A. Hexanes and water are:
 Miscible
 Immiscible
B.
Knowledge of structure explains IMFs, which can be used to explain miscibility.
Table 1: Complete the table below. In the boxes for the VSEPR diagrams (see Figure 3 as a guideline):
• identify the most polar bond(s) by placing δ+ and δ- charges on their atoms
• list bond dipoles
VSEPR Structures (include any molecular dipole)
IMF(s) Present in
pure sample
Bond Dipole(s)
Water
Hexanes
C. Which IMF(s) exist(s) between hexanes and water?
D. Use your answer for C to explain A.
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2-Butanone
1-Butanol
1-Propanol
Methanol
VSEPR Drawings
Line Drawings
Table 2: Complete the table below. When drawing the VSEPR structures:
• identify the most polar bond(s) by placing δ+ and δ- charges on their atoms
• list bond angles
Bond Angles
IMF(s)
Present
Miscible
in Hexanes?
Tube 6
 Yes
 No
Tube 7
 Yes
 No
Tube 8
 Yes
 No
Tube 9
 Yes
 No
Miscible
in Water?
Tube 2
 Yes
 No
Tube 3
 Yes
 No
Tube 4
 Yes
 No
Tube 5
 Yes
 No
A. Looking at the structure of methanol can you rationalize your observations using IMFs on the miscibility
with water and hexanes?
B. Looking at the structure of 1-propanol can you rationalize your observations using IMFs on the miscibility
with water and hexanes?
C. Based on your answers for A and B, 1-butanol should be
 miscible
 immiscible (in water)
D. Rationalize any discrepancy between your answer for C and your observation in Table 2. using IMFs.
Bonus Question:
1-butanol is __________________and 2-butanone is ___________________in water.
miscible |immiscible
miscible |immiscible
Rationalize this relative miscibility using IMFs.
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Part 2 – Using Structure to Rationalize Relative Boiling Points
Table 3: Your TA will give you the compounds and boiling points to be studied. You are expected to evaluate the structure
of compounds given and correctly assign the boiling points. When drawing the VSEPR structures (see Figure 3 as a
guideline), identify the most polar bond(s) by placing δ+ and δ- charges on their atoms and use this information to identify
and if needed draw any molecular dipole.
Compound
Assigned BP (°C)
IMF(s) Present
VSEPR Structure
Name
Name
Name
E. Based on their structures and the IMFs present can you rationalize why these compounds vary in their boiling
points?
Hint: what are the key structural differences? How do these differences influence the strength of IMFs?
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Table 4: Your TA will give you three boiling points. You are expected to evaluate the structure of compounds given and
correctly assign the boiling points. When drawing your line drawings, identify the most polar bond(s) by placing δ+ and
δ- charges on their atoms and use this information to identify, and if needed, draw any molecular dipole.
Compound
Assigned BP (°C)
IMF(s) Present
Line Drawing
CAS#
CAS#
CAS#
F. Based on their structures and the IMFs present can you rationalize why these compounds vary in their boiling
points?
Hint: what are the key structural differences? How do these differences influence the strength of IMFs?
Lab Partner(s):
References
Worksheet
/15
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CHEM 201 Experiment #5 Flowchart
• Label ______________ 1 through 9.
• What goes in each test tube?
1
2
3
4
5
• Stopper and ________ the test tubes.
• Record _____________ in Table_____________.
• What goes in each test tube?
6
7
8
9
• Stopper and ________ the test tubes.
• Record ____________ in Table ___.
• Match/Rationalize the _______________________ of the __________ given in the
PowerPoint to complete Tables _____ and _______.
• Hand in worksheet by ______________________________.
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