Download lab 15: hydrocarbons

LAB 15: HYDROCARBONS:
STRUCTURE & PROPERTIES
PURPOSE: To discover the physical and chemical properties of alkanes, alkenes, and
aromatic hydrocarbons.
To identify an unknown hydrocarbon by comparing it to known samples.
SAFETY CONCERNS:
Always wear safety goggles.
Hydrocarbons are flammable.
Bromine is toxic and should be used in the hood. Observe waste disposal procedures.
HYDROCARBONS:
Hydrocarbons are compounds made primarily of carbon and hydrogen. An alkane is a
hydrocarbon containing only single bonds. An alkene contains one or more carbon-carbon
double bonds. Alkynes have one or more carbon-carbon triple bonds. Aromatic
hydrocarbons contain a circular pattern of double and single bonds where the double bonded
electrons can travel completely around the ring.
Saturated hydrocarbons contain no double bond or triple bonds. Alkanes are saturated.
Unsaturated hydrocarbons contain double or triple bonds. Alkenes, Alkynes, and Aromatic
hydrocarbons are all unsaturated.
Examples:
Alkane
Alkene
Alkyne
Aromatic
(saturated)
(unsaturated)
(unsaturated)
(unsaturated)
Propane
Propene
Propyne
Benzene
H
H
H H H
H C C C H
H
H
H
C C
H H H
C H
H H
H
H C C C H
H
H
C
C
C
C
C
C
H
H
H
Alcohols are hydrocarbons that contain an –OH bonded to a regular carbon. Phenols are
aromatics that contain the alcohol group.
Alcohol
Phenol
H
H
H
H C O H
H
Methanol
(Methyl alcohol)
H
C
C
C
C
H
CH105 Lab 15: Hydrocarbons (F15)
C
C
O H
H
Phenol
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PHYSICAL PROPERTIES:
Volatility: Because of the small difference in electronegativities between carbon (C=2.5) and
hydrogen (H=2.1), hydrocarbons are characteristically nonpolar. Since hydrocarbons do not
have partially negative or partially positive atoms there is minimal attraction between
hydrocarbon molecules. Thus, hydrocarbons require less energy to evaporate or vaporize than
do polar compounds. Small hydrocarbons require less energy to vaporize than large ones. They
are highly volatile.
Odor: The volatile nature of hydrocarbons causes them to vaporize and move through the air.
These air-borne molecules reach our noses and we smell them. Many hydrocarbons have
characteristic odors.
Solubility: The nonpolar character of hydrocarbons allows them to have minimal attraction to
other hydrocarbons but causes them to repel polar compounds such as water.
Density: Nonpolar hydrocarbons have minimal attraction to each other so hold together
loosely. They have low density. Polar compounds, on the other hand, attract each other and bind
close. Water has a lot of hydrogen bonding and is particularly dense.
CHEMICAL PROPERTIES:
Combustion: Hydrocarbons easily combust in the presence of oxygen. They are commonly
used for fuel. Wood, fuel oil, gasoline, diesel, and candle wax are all common flammable
hydrocarbons fuels.
CH4
methane
+
2O2
CO2
oxygen
carbon dioxide
+
2H2O
water
Not all hydrocarbons will react with the same amount of oxygen so when burning in a normal
atmosphere they may appear very different. Aromatic hydrocarbons, for example, burn very
dirty in that they undergo incomplete combustion and produce a lot of soot. Small hydrocarbons
and alcohols are more likely to completely combust and burn cleanly, with very little or no soot.
Bromination: The double bond of an alkene reacts with halogens to form alkyl halide
compounds. The pi bond (second bond) of the double bond is weak and breaks more easily than
the sigma bond (first bond).
H
H
C C
H
+
Br2
CH3
propene (colorless)
bromine (brown)
Br Br
H C C H
CH3
H
1,2-dibromopropane (colorless)
Bromine (Br2) is a brown liquid so if a reaction occurs in which the Br2 splits apart and the Br’s
bond with carbons we will see the brown color disappear.
Aromatic compounds do not react with halogens in the same way. The double bonds of an
aromatic compound are stronger and so do not easily break.
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CH105 Lab 15: Hydrocarbons (F15)
Oxidation: Some hydrocarbons are easily oxidized with potassium permanganate, KMnO4.
H
H
+
C C
H
KMnO4
CH3
potassium permanganate (purple)
propene (colorless)
HO OH
H C C H
CH3
H
+
MnO2
manganese (IV) oxide (brown)
1,2-propandiol (colorless)
Potassium Permanganate is purple so if a reaction occurs in which the KMnO4 oxidizes the
carbons we will see the purple KMnO4 change to brown MnO2.
CH105 Lab 15: Hydrocarbons (F15)
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PROCEDURES:
ACTIONS:
NOTES:
I. PHYSICAL PROPERTIES:
A. Volatility:
1. Obtain three small beakers of identical1 size. Label2
them #1, #2, #3.
2. Into beaker #1 put 5 mL of water.
Into beaker #2 put 5 mL of ethanol.
Into beaker #3 put 5 mL of hexane.
3. Carefully weigh each beaker on the electronic
balance3 and record the masses on your report sheet.
4. Weigh each beaker again every 15 minutes4 and
record the masses.
1
The beakers must have identical
surface areas so that each of the liquids
has the same amount exposed for
evaporation.
2
Some beakers have a white patch
where labels can be written with
pencil. Otherwise use a grease pencil
to clearly distinguish your beakers.
3
Use the same balance for each
measurement to avoid errors.
4
Continue working on other parts of
the lab and come back every 15
minutes to weigh the beakers in this
part.
5
5. On the report sheet make a graph of the total mass
lost by each substance every 15 minutes. Draw a
straight line through the graph points to show the
linear relationship for the evaporation of each liquid.
Compare to determine relative volatility.5
A volatile liquid evaporates very
easily.
6
Never smell a chemical directly but
instead gently “waft” the vapors
toward your nose.
7
B. Odor:
6. Waft the vapors6 of each of the 3 beakers used in
Part I and record your observations on the report
sheet. 7
We are using hexane to represent the
odor of all hydrocarbons although not
all hydrocarbons smell exactly the
same. Compare the general properties
of the odor of water, hydrocarbon, and
alcohol.
8
C. Solubility in Water:
1. Obtain 4 stoppered test tubes, each containing 2 mLs
of water.8
Measure the 2 mLs for the first tube
with a graduated cylinder and then
eyeball the levels of the rest to match.
9
2. Into tube #1 put 5 drops ethanol. (C2H5OH)
Into tube #2 put 5 drops of hexane. (C6H14)
Into tube #3 put 5 drops cyclohexene. (C6H10)
Into tube #4 put 5 drops toluene. 9 (C6H5CH3)
Stopper the tubes and shake each to mix.
3. Check each tube for layers. Add more drops of a
particular reagent if you can’t determine the solubility
with the amount given. 10
4. Record your observations about the solubility of
each hydrocarbon in water. Use ‘S’ for soluble, and
‘I’ for insoluble. Save these tubes for use in Part
ID.
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CH105 Lab 15: Hydrocarbons (F15)
We are using toluene, (methyl
benzene) as a representative of the
family of aromatics. It is less toxic
than benzene because it is not as flat
(planar) so therefore does not
intercollate as readily into our spiral
DNA.
10
Sometimes the light optics due to
curvature of the test tube looks like a
layer when it is not. Compare to plain
water in a tube. If you are still not
certain that an insoluble layer is
forming then add a couple more drops
of reagent and see if your layer gets
bigger.
D. Density:
11
5. For each insoluble hydrocarbon in Part IC observe the
positioning of the hydrocarbon layer relative to water. The more
dense substance will be on the bottom.
The glassware must
very dry. If any water is
present your results may
not be accurate.
6. Record the density of each hydrocarbon relative to water on the
report sheet. Report M for more dense than water, and L for
less dense than water.
7. Dispose of hydrocarbons in the designated waste containers.
E. Solubility in Other Hydrocarbons:
1. Obtain 3 dry11 stoppered test tubes, each containing 5 drops of
hexane (C6H14).8
2. Into tube #1 put 5 drops ethanol. (C2H5OH)
Into tube #2 put 5 drops cyclohexene. (C6H10)
Into tube #3 put 5 drops toluene. (C6H5CH3)
Stopper the tubes and shake each to mix.
3. Check each tube for layers. Add more drops of a particular
reagent if you can’t determine the solubility with the amount
given. Record your observations about the solubility of each
hydrocarbon in hexane (a typical hydrocarbon solvent).
4. Dispose of hydrocarbons in the designated waste containers.
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You will be given a
sample of an unknown
hydrocarbon that you
will
endeavor
to
characterize by class.
You unknown will be
either an alkane, an
alkene, or an aromatic.
II. CHEMICAL PROPERTIES:
A. Combustion (Instructor Demo):
1. Line up 5 clean and dry11 watch glasses about 6 inches apart in
a fume hood.
2. Onto watch glass #1 put 10 drops ethanol. (C2H5OH)
Onto watch glass #2 put 10 drops of hexane. (C6H14)
Onto watch glass #3 put 10 drops cyclohexene. (C6H10)
Onto watch glass #4 put 10 drops toluene. (C6H5CH3)
Onto watch glass #5 put 10 drops unknown hydrocarbon.12
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Observe
if
the
hydrocarbons
ignited
rapidly or if they were
slow to ignite. Observe
if there is a “clean”
flame or a “sooty”
flame.
3. Quickly go down the row and ignite each liquid with a burning
wood splint and compare the colors and types of flames
produced by each hydrocarbon as they burn together. 13
4. Record your observations.
CH105 Lab 15: Hydrocarbons (F15)
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B. Bromination:
1. Obtain a dry11 white spot plate.
2. Into well #1 put 3 drops ethanol. (C2H5OH)
Into well #2 put 3 drops hexane. (C6H14)
Into well #3 put 3 drops cyclohexene. (C6H10)
Into well #4 put 3 drops toluene. (C6H5CH3)
Into well #5 put 3 drops unknown hydrocarbon.
3. Move your plate to a fume hood14 and into each well drop 1 drop
of Bromine (Br2) solution15. Stir to mix if needed and record
any results.
4. Dispose in the designated waste containers. Bromine must go
into “bromine waste”.
C. Oxidation:
1. Obtain a clean white spot plate.
2. Into well #1 put 3 drops ethanol. (C2H5OH)
Into well #2 put 3 drops hexane. (C6H14)
Into well #3 put 3 drops cyclohexene. (C6H10)
Into well #4 put 3 drops toluene. (C6H5CH3)
Into well #5 put 3 drops unknown hydrocarbon.
3. Into each well drop 1 drop of Potassium Permanganate (KMnO4)
solution16. Stir to mix if needed. Record any results.
4. Dispose in the designated waste containers.
D. Unknown Hydrocarbon:
5. You have observed the behavior of an unknown compound in
combustion, bromination, and oxidation. On the report sheet
characterize your unknown as an alkane, alkene, or an aromatic
and then explain why you believe it to be so.
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CH105 Lab 15: Hydrocarbons (F15)
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Bromine is highly
toxic. Keep the bottle
stoppered and do this
reaction only in the
fume hood. You may
choose
to
wear
protective gloves.
If
you spill bromine
inform your instructor
immediately for help
with proper clean up.
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Bromine is soluble
both in water and in
hydrocarbon solvents.
If we use a solution of
Bromine dissolved in
water here we would
have two layers since
hydrocarbons
are
insoluble in water. By
using bromine dissolved
in a hydrocarbon we can
mix the bromine with
our hydrocarbons more
easily.
16
KMnO4 is soluble in
water but is not soluble
in hydrocarbon solvents.
We do not have the
option of using a
hydrocarbon solvent as
we did with Br2 that
would
allow
easy
mixing with our sample.
Because KMnO4 is
dissolved in water here
we have two layers that
must be stirred or
shaken to mix.
LAB 15: HYDROCARBONS
NAME_____________
DATE______________
PRE LAB EXERCISES:
1. Draw the structural formula for each of these hydrocarbons:
Ethanol
Hexane
Cyclohexene
Toluene
2. Match the following terms with the structures they represent:
____ Alcohol
A.
C.
H
H
C C
H
____ Alkene
E.
H
H
CH2 CH3
CH3
H
H
H H H H
H C C C C H
H H H H
H
____ Alkane
B.
D.
H H H OH
H C C C C H
H H H H
____ Alkyne
H
H C C C H
H
____ Aromatic
3.___ Black soot is given off when certain hydrocarbons are burned because __
A.
B.
C.
D.
the fuel is undergoing complete combustion in the presence of oxygen.
carbon dioxide and water mix to give a black powder.
there is not enough oxygen present in the environment to combust that much fuel.
more than one of these.
4.___ Several drops of a bromine solution were added to a liquid. The red-brown color of the bromine
immediately disappeared. The compound may have been
A. a saturated hydrocarbon
B. a cycloalkane
D. an alkane
E. an alkene
5.
C. an aromatic hydrocarbon
Complete and balance the following equations:
A.
H
H
C C
H
C H
H
H
+ Br2
B.
C5H12
+
O2

CH105 Lab 15: Hydrocarbons (F15)
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CH105 Lab 15: Hydrocarbons (F15)
LAB 15: HYDROCARBONS
NAME___________________
PARTNER_________DATE___
REPORT:
I. PHYSICAL PROPERTIES:
A. Volatility:
a. Mass at Start
b. Mass lost in zero min
c. Mass at 15 minutes
d. Total g’s lost from beginning
e. Mass at 30 minutes
f. Total g’s lost from beginning
g. Mass at 45 minutes
h. Total g’s lost from beginning
i. Mass at 60 minutes
j. Total g’s lost from beginning
#1 + Water
#2 + Ethanol
#3 + Hexane
Mass in grams
Mass in grams
Mass in grams
Og
Og
Og
(a-c)
(a-c)
(a-c)
(a-e)
(a-e)
(a-e)
(a-g)
(a-g)
(a-g)
(a-i)
(a-i)
(a-i)
Mass lost by Water (#1), Ethanol (#2), and Hexane (3#)
Total
Mass
Lost
From the
Beginning
(in grams)
0
15 (a-c)
30 (a-e)
45 (a-g)
60 (a-i)
Time (in minutes)
Volatility Results Summary: Rank the substances in order of decreasing volatility.
Most volatile
>
>
A. On the given structures of ethanol label any partially
positive (+) or partially negative (-) atoms.
B. Draw a dotted line (…..) to show Hydrogen bonding
H H
H C C O H
H H
Least volatile
H H
H O C C H
H H
forming between the 2 ethanol molecules given.
Explanation/Analysis:
Water
Why did each of the three liquids behave as they did?.Be Specific .
Ethanol
CH105 Lab 15: Hydrocarbons (F15)
Hexane
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Water
Ethanol
Hexane
An Alcohol
A Hydrocarbon
B. Odor
Ethanol
Hexane
Cyclohexene
Toluene
An Alcohol
An Alkane
An Alkene
An Aromatic
C. Solubility
in Water
D. Density
Compared to Water
E. Solubility
in Hydrocarbons
Physical Properties Results Summary:
_____1.
As a general rule, most hydrocarbons are _____ than water.
A. more volatile
B. less volatile
C. there is no general rule
_____2.
As a general rule, hydrocarbons are _____ in water and _____in other hydrocarbons.
A. soluble, soluble
C. insoluble, soluble
B. insoluble, insoluble
D. soluble, insoluble
_____3.
As a general rule, most hydrocarbons are _____ than water.
A. more dense
B. less dense
C. there is no general rule
II. CHEMICAL REACTIVITY:
Ethanol
Hexane
Cyclohexene
Toluene
An Alcohol
An Alkane
An Alkene
An Aromatic
A. Combustion
B. Bromination
(rxn w/ Br2)
C. Oxidation
(rxn w/ KMnO4)
D. Unknown
# _____
Belongs to what family of compounds? (circle one)
Alkane
Alkene
Explanation:
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CH105 Lab 15: Hydrocarbons (F15)
Aromatic
Unknown
# _________
Chemical Reactivity Results Summary:
_____4.The identity of the black soot that is given off when certain hydrocarbons are burned is ___
A. impurities that were present in the original sample.
B. a mixture of carbon monoxide and carbon dioxide..
C. elemental carbon.
D. none of these.
_____5.Which of the following generally gives the most soot when burned?
A. Alcohols
B. Alkanes
C. Alkenes
D. Aromatics
_____6.List all of the following that generally undergo an immediate reaction with Br2?
A. Alcohols
B. Alkanes
C. Alkenes
D. Aromatics
_____7.List all of the following that generally undergo an immediate reaction with KMnO4?
A. Alcohols
B. Alkanes
C. Alkenes
D. Aromatics
Write balanced chemical equations to represent the following reactions:
9. Complete Combustion of Hexane: (Use structures not formulas for hexane)
10. Reaction of Cyclohexene with Bromine: (Use structures not formulas for cyclohexene & product)
Related Exercises:
5.___ Two structures are identical when
A. they have the same molecular formula
B. they have the same combustion products
C. they can be superimposed on each other.
6.___ Do the following structural formulas represent the same or different molecules?
H Br
H H
H H
A. same
H C C H
H C C Br
H H
H H
B. different
H C C H
H Br
C. not enough information
CH105 Lab 15: Hydrocarbons (F15)
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CH105 Lab 15: Hydrocarbons (F15)
LAB 15: HYDROCARBONS: Worksheet
Name ____________
1. Draw structural formulas and give names for the 2 isomers of C3H7Br.
Name
Name
2. Draw structural formulas and give names for the 4 isomers of C3H6Br2.
Name
Name
Name
Name
3. Draw structural formulas and give names for the 2 isomers of C4H10.
Name
Name
4. Draw structural formulas and give names for the 3 isomers of C5H12.
Name
Name
Name
5. Draw structural formulas and give names for the 3 isomers of C2H2Br2
Name
Name
CH105 Lab 15: Hydrocarbons (F15)
Name
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6. Draw structural formulas and give names for the 5 cyclopentane isomers of C5H8Br2.
Name
Name
Name
Name
Name
7. Draw structural formulas and give names for the 5 isomers of C6H14.
Name
Name
Name
Name
Name
14.___ For geometric (cis-trans) isomerism to occur, must a double bond be present in the
structure?
A. Yes
B. No
C. Not enough information
15.___ Do the carbon atoms in an ethene have freedom of rotation?
A. Yes
B. No
C. Depends on the temperature.
16.___ Do the carbon atoms in a cyclopentane have freedom of rotation?
A. Yes
B. No
C. Depends on the temperature.
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CH105 Lab 15: Hydrocarbons (F15)