Name:
Per:
Date:
Lab # 14: Heating Curves and Boiling Points
Chemistry 1
.
Heating Curve of Water
Temperature, oC
140
E
120
100
D
D:
E:
80
60
40
C
C:
20
B
0
-20
-40
B:
A: S (solid)
Heat
at16
Constant
0 2040608010
1214
18202224Rate
26283032343638404244
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Watch the phases of water animation on these web sites:
http://mutuslab.cs.uwindsor.ca/schurko/animations/waterphases/status_water.htm and
http://www.alchemical.org/thermo/other/heatingcurve.swf
1.
The heating curve above shows the temperature in the sample as a block of ice at -40°C is heated at a
constant rate. State all the phase(s) of water present along each leg (solid, solid + liquid, etc.) along with
what is happening to the water. Leg A is done for you.
A: solid warms from -40oC to 0oC
B:
C:
D:
E:
2. Label each leg to show what phase is present (S = solid, L = liquid, G = gas, and combinations)
3. During which legs are phase changes occurring?
Although we are still heating the sample during phase change, what happens to the temperature of the
sample as the phase is changing? Why?
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4. Describe what is happening on a sub-microscopic level (as if you could see the molecules) to the water
molecules in the liquid during leg C. Use a full sentence.
5. What is occurring during Leg D, a chemical change or physical change?
6. Which drawing best represents what is happening during Leg D:
or
a.
or
b.
or
c.
d.
Explain.
7. Draw a heating curve for isopropanol (C3H8O, rubbing alcohol) and label all the phases present (solid, solid
+ liquid, etc.) along each leg. Start with C3H8O(s) at -150ºC and heat up to 100ºC. Isopropanol
freezing/melting point = -89ºC; boiling point = 82ºC.
Temp,oC
Heating Curve for Isopropanol
100 −
50 −
0−
-50 −
-100 −
-150 −
Heat at constant rate
On a separate sheet of paper draw a heating curve for methanol (CH3OH) from -120oC to 100oC and label
phases present along each leg. Freezing point = -94 oC, Boiling point = 65 oC.
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Name:
Per:
Date:
Lab: Boiling Points of Alcohols
Each team will be assigned one alcohol to study. Your team will determine the boiling point.
1. Obtain a 250-mL beaker and fill it with 150-mL of very hot tap water. This beaker will be used as a “water
bath.” If you have been assigned to find the boiling point of butanol, you will use an empty beaker as an
“air bath”—just place the empty beaker on the hot plate with the test tube inside.
2. Put the beaker of water on a hot plate, and turn the hot plate to a medium setting.
3. Use a Beral pipette to transfer about 4 mL of your assigned alcohol into a small test tube.
4. Clamp your test tube with a utility clamp as demonstrated by your teacher and lower the test tube and into
the water bath.
5. Clamp a thermometer to the ring stand so it is suspended in the center of the liquid. Record your first
substance________________ and initial temperature_________.
6. Stir the water bath as the temperature rises.
7. Observe the temperature of the alcohol as it rises. The boiling point is observed when the temperature
of the liquid stops rising and remains constant. Do not allow the alcohol to boil away.
8. In the first “Boiling Point” column in your data table, record “your value” for your observed boiling point of
the alcohol. Record your answer on the board.
9. For each alcohol, calculate the average boiling point from the class data on the board.
Data Table 1 : Alcohol Type: _____________________
Time (min)
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
7.5
8.0
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Temperature (°C)
3
Name
Formula
Methanol
CH3OH
Ethanol
CH3CH2OH
n-Propanol
CH3CH2CH2OH
Isopropanol
CH3CHCH3OH
n-Butanol
CH3CH2CH2CH2OH
# of
Carbons
Your value
Class average
Analysis and Questions.
1. Using your data, prepare a temperature vs. time graph by placing time on the x-axis and temperature on the
y-axis.
2.
Using the class data, prepare a graph of boiling points vs. # of carbon atoms. Place the number of carbon
atoms on the x-axis. The x-axis scale should range from 1 to 6 carbon atoms (although you will initially
plot just 1-4 carbons). The y-axis values should extend up to 160°C.
3. Plot the average boiling points, but leave out isopropanol. (All alcohols tested have carbon atoms arranged
in a straight line, except isopropanol. So isopropanol doesn’t fit the general trend.)
4. Draw the best-fit line through the data. The line should not extend past your data points.
5. Describe the relationship between the number of carbon atoms and the boiling point of these alcohols.
6. The pattern of boiling points among these straight alcohols allows you to predict boiling points for other
alcohols.
a. Using your graph, estimate the boiling points of n-pentanol (CH3CH2CH2CH2CH2OH) and n-hexanol
(CH3CH2CH2CH2CH2CH2OH). To do this, extend the trend of your graph line by drawing a dashed line
from your best-fit line. This procedure is called extrapolation. Then read your predicted boiling points
for the 5- and 6-carbon alcohols on the y-axis.
predicted boiling point for 5-carbon pentanol :
predicted boiling point for 6-carbon hexanol:
b. Compare your predicted boiling points to actual values provided by your teacher.
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7. You learned that a substance’s boiling point depends in part on its intermolecular forces, or attractions
among its molecules. For the alcohols you have studied, what is the relationship between these attractions
and the number of carbon atoms in each molecule?
8. Challenge question 1: First, determine which of the following molecules are polar and which are nonpolar.
Write “P” or “NP” next to the structure. Second, decide which of the molecules can hydrogen bond. Write
H-bonding next to the structure. Then, place the following molecules in order of increasing boiling point.
_______________ < _______________ < _______________ < ______________ < ______________
9. Challenge question 2: The accepted boiling point of methane (CH4) is -164∘C. Compare this to the
observed boiling point of methanol (CH3OH). Why is the boiling point of methane so much lower?
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