Download Lab Evaporation and Intermolecular Attraction

Lab Evaporation and Intermolecular Attraction
Purpose Investigate the relationship of dispersion forces and hydrogen bonding forces in intermolecular
attractions.
Discussion:
In this experiment, temperature probes covered with filter paper are placed in various liquids.
Evaporation occurs when the probe is removed from the liquid's container (see figure 1). This
evaporation is an endothermic process that results in a temperature decrease. The magnitude of a
temperature decrease is, like viscosity and boiling temperature, related to the strength of intermolecular
forces of attraction. In this experiment, you will study temperature changes caused by the evaporation of
several liquids and relate the temperature changes to the strength of intermolecular forces of attraction.
The temperature change is greater if more evaporation occurs (weaker intermolecular forces). You will use
the results to predict, and then measure, the temperature change for several other liquids.
You will encounter two types of organic compounds in this experiment — alkanes and alcohols. The two
alkanes are pentane, C5H12, and hexane, C6H14. In addition to carbon and hydrogen atoms, alcohols also
contain the -OH functional group. Methanol, CH3OH, and ethanol, C2H5OH, are two of the alcohols that we
will use in this experiment. You will examine the molecular structure of alkanes and alcohols for the presence
and relative strength of two intermolecular forces—hydrogen bonding and dispersion forces. Dispersion forces
exist between any two molecules and generally increase as the molecular weight of the molecule increases. A
hydrogen bond can occur when, in one molecule, a hydrogen atom is bonded directly to an N, O, or F atom
(the donor) and that hydrogen is attracted to a lone pair on an N, O, or F atom in another molecule (the
acceptor). Both a donor and an acceptor must be present for a hydrogen bond to occur.
Materials
Warning:
The chemicals used in this experiment are flammable and poisonous. Avoid inhaling their vapors. Avoid
contacting them with your skin or clothing. Be sure there are no open flames in the lab during the experiment.
Notify your teacher immediately if an accident occurs.
1. Setup the CBL and calculator
 Plug temperature probes into channel 1 and 2 on the CBL.
 Turn on the calculator, press APPS and choose Datamate
 Select 1 setup
 Curser down to mode: and press enter
 Select time graph
 Select change time settings
 Enter 3 as the time between samples in seconds
 Enter 80 as the number of samples
 Select advanced
 Select change graph settings
 Select CH1-Temp(C) ( it will change both)
 Enter 5 for Ymin
 Enter 30 for Ymax
 Enter 5 for Yscl
 Select OK
 Select OK again
 Select OK one more time
2. Wrap probes 1 and 2 with pieces of filter paper and secure them with a rubber band. Roll the filter paper
around the probe tip in the shape of a cylinder.
3. Fill the test tubes just before use as the liquid may evaporate quickly.
4. Position probe 1 in an upright manner in the ethanol test tube. Position probe 2 in the 1-propanol test tube
in the same way.
5. Prepare 2 pieces of masking tape, each about 10 cm long to be used to tape the probes in position on the
table after step 6.
6. After the probes have been in the liquids for at least 45 seconds, select start on the calculator to begin data
collection. Monitor the temperature for 15 seconds then simultaneously remove the probes from the liquid
and tape them so the probe tips extend 5cm over the edge of the table.
7. When data collection is complete after 4.0 minutes it will show a graph. Determine the maximum and
minimum temperature either by tracing the graph or pressing enter, quit and viewing the list by pressing
STAT edit, CH1 temp is in L1 and CH2 temp is in L2
8. For each liquid subtract the minimum temperature from the maximum temperature to determine t, the
temperature change during evaporation.
9. Dispose of the filter paper in the trash.
Data Table
Substance
ethanol
1-propanol
1-octanol
pentane
methanol
hexane
T1 (oC)
T2 (oC)
∆T (oC)
Post Lab
Substance
Formula
Molecular Weight Lewis Dot
Structure
Hydrogen
bonding
Yes or No
Alkane or
Alcohol
ethanol
1-propanol
1-octanol
pentane
methanol
hexane
Discussion Questions:
1. The change in temperature tells you about how readily a sample evaporates. Circle the appropriate response
to each statement below.
Large ∆T values mean that the sample
does
does not
evaporate readily.
Small ∆T values mean that the sample
does
does not
evaporate readily.
2. Correlate your answers to how readily a sample evaporates to the strength of the intermolecular forces in
that molecule. (again, circle the appropriate response to complete the statement.)
If the sample evaporates readily, then the relative strength of the IMFs in the molecule is: (circle one)
strong
weak.
If the sample does not evaporate, then the relative strength of the IMFs in the molecule is: (circle one)
strong
weak.
3. Plot a graph of t values of the four alcohols versus their perspective molecular weights. Plot molecular
weight on the horizontal axis and t on the vertical axis. What is the relationship between mass and
intermolecular force?
Conclusion- write one.