CHEM 2010 Chattanooga State Molecular Representatives & Resonance Chemistry Questions

I already did the melting point determination, just wanted someone to go over it and make it sound better. For the molecular representation assignment, there’s 11 questions.

CHEM 2010 Lab
Molecular Representations and Resonance
Name: ______________________________
Grade: ____________
1. Build and then draw all of the constitutional isomers of C5H12 as Lewis, bond-line,
partially-condensed, and condensed formulas.
Lewis
Structure
Bond-line
structure
Partiallycondensed
formula
Condensed
formula
2. Which of the following is not an isomer of the others?
a. …
b. …
1
CHEM 2010 Lab
Molecular Representations and Resonance
3. The Lewis Dot Structure of one form of the amino acid glycine is shown. Where should
formal charges be placed to make the structure completely correct?
a
4. Consider the structure below. Indicate any formal charges to complete the structure.
aidColorallsp3hybridizedatomsblue
H
H
H
lbcolorallsp2hybridizedatomspink
O
c colorall sp hybridizedatomsgreen
N
C
H
H
C
O
H
H
C
H
H
C
H
S
H
O
N
C
H
C
C
H
H
H
O
N
C
H
C
H
N
C
C
C
H
H
H
H
C
C
N
N
C
H
O
H
2
CHEM 2010 Lab
Molecular Representations and Resonance
5. Consider the resonance structures below. For each pair:
•
•
•
Use proper curved arrows to illustrate how the resonance structures on the left could
be moved to generate the structures on the right.
Indicate the type of arrow pushing pattern you have used.
indicate which of the structures is more important.
H
H
4
a.
f
N5
3
een
1 2 C
H3C
H
6
H
N
C
CH3
H3C
CH3
d
ye
b.
a
c.
d.
3
CHEM 2010 Lab
Molecular Representations and Resonance
6. Write the three best resonance forms for the anion with the formula –CH2NO2. Then
answer the following questions.
a. How many resonance forms have a formal charge of -1 on the C?
b. How many resonance forms have a formal charge of +1 on the N?
ANS:
ummmm
7. Draw the two best two resonance forms for the formula CH3N3. Do not write a ring
structure.
a. How many pairs of nonbonding electrons are there?
b. How many structures have a +1 charge on only one N atom?
c. How many structures have a -1 charge on only one N atoms?
ANS:
sie
H
H
C
H
N
N
N
H
H
C
N
N
N
H
4
CHEM 2010 Lab
Molecular Representations and Resonance
8. Draw the reaction between phenol and sodium hydroxide to produce sodium phenolate
and water (Google it!). Include all formal charges and lone pairs.
9. Draw all resonance structures of the conjugate base phenolate ion using proper curved
arrow notation to convert each unique resonance structure into the next.
10. The molecule 4-cyanophenol (pKa = 7.7) is approximately 125 times more acidic than
phenol (pKa = 9.8). Use resonance logic to propose a complete explanation why that is.
Your answer should be complete with any resonance structures that support your
argument along with proper curved arrow notations for converting each into the next.
5
Melting Point Determination:
Identifying an Unknown by Mixture Melting Point
THEORY
The melting temperature of a compound is the temperature at which it changes from a solid
to a liquid. This is a physical property often used to help identify compounds or to check the
purity of a compound. The melting temperature is related to the amount of kinetic energy that
one adds to a solid substance to overcome the intermolecular attractions that maintain its solid
state under given conditions.
It is very difficult, however, to find an exact melting point. Because it is a thermodynamic
process, when a substance begins to melt, a dynamic equilibrium is established within which the
substance exists in both solid and liquid form. Because the energy transferred to this system is
not used entirely to convert the solid to a liquid, a single temperature value is commonly not
reported, but rather a temperature range.
Thus, melting temperatures are usually reported as values with a range of 2–3°C. Though
grammatically incorrect, it is still referred to as a melting point (mp). Melting temperature is not
a unique physical property of a substance, but it does help you understand more about the
substance. It can also help determine the purity of a substance that you have synthesized.
A common use of melting point in identifying a substance is called the ‘mixture melting point’
technique. A small bit of the unknown is mixed thoroughly with a bit of a pure substance. If the
melting point range tested is sharp and matches that of the pure substance then the unknown is
reasonably assumed to be that pure substance. If the mixture melting point range is broad and
depressed in comparison to that of the pure compound then the unknown acted as an impurity
and they are not the same.
Adapted from: Organic Chemistry with Vernier
CHEM 2010 LAB
Melting Point Determination
2
Pre-Lab
See eLearn for the detailed Pre-lab assignment. Review all material and take informative notes as
described. Submit the pre-lab material to the appropriate Dropbox prior to lab.
EXPERIMENTAL OBJECTIVES
In this experiment, you will
• Determine the melting point range of pure benzoic acid and pure urea.
• Determine the melting point range of a mixture of each known with an unknown to identify the
unknown by mixture melting point logic.
MATERIALS
Benzoic acid
Urea
Unknown solid
Melt Station apparatus
Glass capillary tubes (one closed end)
Spot plate
Spatula
SAFETY CONSIDERATIONS AND KEY PHYSICAL DATA:
1. Research the web and use a legitimate chemical manufacturer site such as Sigma-Aldrich,
Fisher, or Flinn Scientific to find the Safety Data Sheets for benzoic acid and urea.
Benzoic acid
Molecular formula
Bond-line formula
C Ho 02
EIN
Urea
CHAND
HIN
can
NHz
cause skin
Safety and handling
concerns
very corrosive
and toxic
Literature melting
point
122.30C
1332
Literature Reference
Fisher sci
fisher sci
ant eye irritation
2. Consider the theoretical melting temperatures listed above and the fact that the Mel-temp will
need to cool down to 10-15 degrees below the melting temp of the next sample between runs.
What order should you test your samples in to use time efficiently?
CHEM 2010 LAB
Melting Point Determination
1
EXPERIMENTAL PROCEDURE
** DO NOT BEGIN THIS EXPERIMENT UNTIL YOU HAVE VIEWED
ALL PRE-LAB VIDEOS, REVIEWED THE MELT STATION USER
MANUAL, AND ARE FAMILIAR WITH SAFE USE OF THE DEVICE.
I. Test the Melting Temperature of Pure Benzoic Acid and Pure Urea
1. Take a picture of the plate and insert in below in your Data and Observations then label
it as to which ample is in which spot. I suggest placing one sample in the upper right
corner of the place and the other in the upper left.
2. Obtain a small amount of pure benzoic acid and pure urea. If one or both of your
provided compounds is in a crystalline form, transfer a bit to a watch glass first and use
the spoon portion to grind gently until any crystalline material is crushed into a fine
powder. Take care to clean your spatula between samples as to not contaminate anything.
Place a small bit of the finely crushed samples into the appropriate well of your spotplate.
3. Pack one capillary tube 3-4 mm (~1/8 inch) deep with each sample. As each is prepped,
lay them to the side of your spot plate, near the well containing that sample to not mix
them up. Again, using the labeled image you inserted below to keep track.
4. Turn on your Melt Station by plugging in the AC power and turn the temperature knob to
the cooling fan. The Bluetooth® LED will blink red.
5. Follow instructions from the Getting Started section of the User Manual to connect your
apparatus to the data collection App on your iPad.
a. Bluetooth Connection – locate and open the Vernier Graphical Analysis™ app on
your iPad (NOT the GW version). Launch Graphical Analysis then click or tap
Sensor Data Collection.
b. You will need to select the proper product code found on the face of the apparatus
in lower corner.
6. Using the Product –
a. Carefully place a prepared capillary tube into one of the three slots in the
aluminum heating block of the Melt Station. You can tilt the Melt Station toward
you slightly for a better look at the heating block.
b. Tilt the Melt Station up or down slightly to get the best view of the solid sample
through the viewing lens.
c. Click Collect in the app to begin data collection.
CHEM 2010 LAB
Melting Point Determination
2
d. On the Melt Station, turn the control knob to the Rapid Heat area. The red LED
will come on, indicating the Melt Station is heating. Rapid Heat will warm your
solid sample at a rate of >10°C/min.
e. Observe the temperature vs. time graph. When the temperature is within about
10°C of the expected melting temperature of your solid sample, turn the control
knob to that temperature, slowing the heating rate to ~1.5°C/min.
f. Carefully observe your sample. At the first indication of the solid melting, note
and record the temperature. When the entire solid has melted, note and record the
temperature. The examine line can be used to help mark these spots while
monitoring the substance melting. Screenshots or taking quick pics of the iPad
screen with your phone are also quick ways to caprture these temps without
looking away from the sample for more than a couple of seconds.
g. Stop data collection. The run is automatically stored. On the Melt Station, turn the
control knob to the Fan/Cooling setting. The blue LED will come on, indicating
that the Melt Station is cooling.
h. Prepare a second solid sample to test. Observe the temperature of the heating
block in the meter. After the heating block cools to a suitably low temperature, at
least 10 degrees below your samples expected melting point range, you can begin
heating the Melt Station again.
7. Repeat any runs that you questioned based on heating rate, observed melt characteristics,
correlation between theoretical and observed melting point range, etc.
8. Turn down the power level to the fan setting and allow the block to cool or turn off the
apparatus and unplug it. Be careful of the hot surfaces near the heating block.
II. The ‘mixture melting point’ method to determine the identity of an
unknown
9. To each of the pure samples already in your well plate, add a small bit of the unknown.
Make the mixture approximately 10-30% unknown, it is not necessary to weigh anything,
just eyeball it! Use a spatula to mix thoroughly, crushing into a fine powder.
10. Fill a capillary tube with each mixture. Keep separate and clearly labeled.
11. Observe and record the melting temperature of each mixture using the techniques
outlined above. If you choose to place both mixture samples in the apparatus and heat
once, be prepared to record the melting points without having to look away from the
samples for more than a glance.
CHEM 2010 LAB
Melting Point Determination
3
Data and Observations
A. Take a picture of your spot plate, insert it below, and label it as you work to follow where
each sample is placed.
Urea
Benzoicacid
unknown
unknownB
A
B
B. Melting point data for pure samples and mixtures with Unknown ________
Melting Point Data
Run 1
Run 2 (as needed)
Benzoic acid
122.2 123.4
Urea
132.6 134.2
Benzoic
acid/Unknown
mixture
122.8 123 8
Urea/Unknown
mixture
96.0
124.8
Run 3 (as needed)
CHEM 2010 LAB
Melting Point Determination
4
Post-lab Data Analysis
1. Based on your melting temperature data, both theoretical and experimental, comment on the
relative purity of the ‘pure’ benzoic acid and urea samples.
My experimental data was very
close to the theoretical value
so the samples were very pure
2. What is the identity of your unknown, benzoic acid or urea? Include the unknown ID letter (A or
B) that you tested and thoroughly explain the logic used to identify your unknown.
melting
Benzoic acid Urea’s
while Benzoic
point was lowered
acid had no change My unknown
was
B
3. A student was consistently recording sample melting points as too high and a bit broad in range.
What technique mistake might they be making?
Identifying
beginning
the sintering as the
of the
melting
4. Suppose that you have three unlabeled bottles that contains either malonic acid or maleic acid.
You, being a responsible organic chemist, would like to label this bottle. Consider their pure
melting points, and the techniques used in this experiment, to describe how you would identify
the contents of each bottle using melting point determination.
I would
use
the
internet to
determine the melting paints
of both compounds then take
samples of each battle and
determine their melting point
melt station
a
with