Download Experiment: Evaporation and Intermolecular Attractions

Experiment: Evaporation and Intermolecular Attractions
Purpose
The purpose of this lab activity is to study temperature changes caused by the evaporation of
several liquids and relate the temperature changes to the strength of the forces of attraction
between molecules of the liquid.
Introduction
When temperature sensors are removed from a liquid, evaporation occurs. Evaporation is an
endothermic process that results in a temperature decrease for the surrounding air. The
magnitude of a temperature decrease is related to the strength of the intermolecular forces of
attraction in that the greater the decrease in temperature the greater the attractive force
strength.
You will encounter two types of organic compounds called “hydrocarbons” – alkanes and
alcohols. The two alkanes used in this experiment are pentane, C 5H12, 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 will be used in this
experiment. The molecular structure of alkanes and alcohols will be examined for the presence
and relative strength of the two intermolecular forces – hydrogen bonding and dispersion
forces.
Pre-Lab Exercises
You will need to complete the title, purpose, storyboard of the procedures, develop a prelab
table for the lab (*see instructions and example below) and, copy the data table below
into your lab book as a prelab. As in the past, if the pre-lab assignment is not completed you
will not be permitted to participate in the lab.
Complete a pre-lab table before beginning the experiment. The name and formula are
given for each compound and should be given in the first column on your table. Draw a
structural formula for a molecule of each compound and include that in the second column.
Determine the molar mass of each compound and put that in the third column. Next, examine
each molecule for the presence of hydrogen bonding. Before hydrogen bonding can occur, a
hydrogen atom must be bonded directly to an N (nitrogen), O (oxygen), or F (fluorine) atom
within the molecule. Indicate whether or not each molecule has hydrogen-bonding capability in
the pre-lab table in the forth column.
Prelab Table:
Substance
Formula Structural Formula
Ethanol
C2H5OH
1-propanol
C3H7OH
1-butanol
C4H9OH
Methanol
CH3OH
Pentane
C5H12
n-hexane
C6H14
Molecular Hydrogen
weight
Bonding?
Data/prediction tables:
Substance
Ethanol
1-propanol
1-butanol
Methanol
Pentane
n-hexane
Ti (C)
Tf (C)
T (C)
Predicted T
Explanation
Materials:
Ethanol, C2H5OH
1-propanol, C3H7OH
1-butanol, C4H9OH
Methanol, CH3OH
Pentane, C5H12
n-hexane, C6H14
Procedure
Part I: Equipment Setup
1. Wrap the ends of two thermometers with triangular pieces of filter paper secured by small
rubber bands as shown in the diagram. Roll the filter paper around the sensor tip in the
shape of a cylinder.
2. Hold one thermometer in the ethanol well and the second thermometer in the 1-propanol
well.
3. Cut 2 pieces of masking tape, about 10-cm long, to be used to tape the sensors in
position during Data Recording
Part II: Data Recording Procedure
1. After the thermometers have been in the liquids for at least 45 seconds record the
temperature as the initial temperature of each liquid.
2. Simultaneously remove the sensors from the liquids and tape them so the sensor tips
extend 5 cm over the edge of the table top
3. Monitor the temperatures, when they each reach the lowest number and start to rise,
record the lowest temperature that is seen as Tfinal.
Analyzing The Data
1. Record the initial and final temperature for ethanol and 1-propanol. Subtract the initial
temperature from the final temperature to determine the temperature change during
evaporation.
2. Based on the change in temperature values you obtained for ethanol and 1-propanol,
plus information in the PRE-LAB chart, predict the size of the change in temperature
value for 1-butanol and methanol.
3. Compare the hydrogen-bonding capability and molecular weight to those of ethanol and
1-propanol. Record your predicted value and explain how you arrived at this answer in
the space provided. It is not important that you predict the exact change in temperature
value; simply estimate a logical value that is higher, lower, or between the previous
change in temperature value.
4. Dispose of the filter paper from the previous trials as directed by your teacher.
5. Test your predictions by repeating the date recording procedure using 1-butanol for one
thermometer and methanol for the other thermometer.
6. Using your measured change in temperature values, predict the change in temperature
values for acetone and hexane. Compare the hydrogen-bonding capability and
molecular weight of acetone and hexane to those of the previous four liquids. Record
your predicted change in temperature value and explain how you arrived at your answer.
7. Test your prediction by repeating the data recording procedure using acetone for one
thermometer and hexane for the other thermometer.
Post Lab Questions (to be answered in your lab report)
1. What are Intermolecular forces and how do they affect physical states?
2. Which of the alcohols (first 4 substances tested) studied had the strongest intermolecular
forces of attraction? The weakest intermolecular forces? Explain using the results of this
experiment.
3. Which of the alkanes (last two substances tested) studied had the strongest
intermolecular forces of attraction? The weakest intermolecular forces? Explain using
the results of this experiment.
4. Using excel, plot a graph of T values of JUST the four alcohols versus their respective
molecular weights. Plot molecular weight on the x-axis and T on the y-axis. When
entering the information put the 4 alcohols in MM order lowest to highest. How does
molecular weight correspond to T?
5. Compare the substances that exhibit H bonding to those that do not. How does the
presence of H bonding (strong attractive force) seem to affect the temperature (energy
absorbed) for evaporation?