Lab 4
Name_____________________________
Intermolecular Forces
Pre-Lab Assignment
This written pre-lab is worth 3 points of your lab report grade and must be turned in to your lab instructor
before class begins.
1. Read the entire lab handout.
2. Make a table listing the names and chemical formulas of all chemicals used in this lab. For organic
chemicals, draw the line structure.
3. Make a table listing the suspected safety hazards in one column and the safety precautions that should be taken
to protect yourself from these hazards in the other column.
4. Write a brief summary of each experiment that will be performed today.
5. On the data table on page 4, draw the Lewis structures for each molecule. Your instructor will initial this page
before you begin lab.
6. Review how to determine molecular shape and molecular polarity before coming to lab. If you come to lab not
knowing how to do this, you may not finish which will affect your lab report grade.
Experimental Questions
Which types of compounds have the strongest intermolecular forces?
How can certain physical properties give an indication of the strength of IMFs of compounds?
Learning goals
Relate molecular polarity to the intermolecular forces of the molecule.
Relate intermolecular forces of the molecule to physical properties such as evaporation and viscosity.
Background
In today’s lab we will examine the attractive forces that hold molecules together and the disruptive forces that break
them apart. The forces between molecules that hold molecules together are called Intermolecular Forces (IMF) and
are comprised of London dispersion forces (LDF), dipolar forces, and hydrogen bonding (H-bonding). The forces
that break molecules apart are related to the temperature of the object. You will explore the effect of polarity on the
strength of IMF and how the IMF is related to physical properties such as melting point, boiling point, state of
matter, and viscosity of liquids.
All material is held together by attractive forces but there is always some disruptive force present that can break it
apart. When an object is a solid at a given temperature it means that the attractive forces must be greater than the
disruptive forces. When something is a gas at any temperature it means that the disruptive forces much be much
greater than the attractive forces. Finally when the forces of disruption and attraction are on about the same level the
substance will exist as a liquid. There are only two ways that elements are held together: sharing of electrons (as in
covalent bonding) or by plus to minus charge or partial charge interactions (as in ionic compounds or intermolecular
forces).
Cohesive forces are attractive forces between particles. They are what hold particles together. They are weak in
gases and get progressively stronger as we go from liquid to solid. Recall that the force of attraction decreases with
increasing distance of separation or decreasing charge.
There are forces that result from particle motion that work in opposition to the cohesive forces. They are known as
the disruptive forces (forces that tend to separate molecules). The two main types of disruptive forces are the
motion of atoms or molecules or the repulsion of like charges. All atoms are always in motion. Externally an atom
or molecule can travel in a straight line (called translational motion) and it can spin (called rotational motion).
Internally when atoms are bonded together they also will move back and forth relative to one another (this is called
vibrational motion). The bonds are like little springs that can move slow or fast but are always in motion. There is an
energy associated with each of the three types of motion called vibrational, rotational and translational energy and
all of these are forms of kinetic energy. The temperature of an object is proportional to the average of all its kinetic
energies.
T ~K.E. = ½ m v2 (m = mass and v = velocity)
This means that at the same temperature all objects have the same average kinetic energy. At the same temperature
smaller objects must have a faster velocity and larger objects must have a lower velocity for them to have the same
average kinetic energy. Remember that the faster an object is moving the easier it is to break away from attractive
General Chemistry
Lab 4: Intermolecular Forces
2
forces and the slower the object is moving the easier it is to be trapped by the attractive forces. Solids, since they are
not moving or rotating, the only motion left is vibrational.
As stated above, disruptive forces are associated with kinetic energy, which is a measure of the movement of the
particle and is associated with the temperature of the sample. We all know that at the same temperature, heavier
molecules move slower. When the disruptive forces are greater than the cohesive forces, we can have a change of
state (e.g. liquid → gas). It takes energy to make this transition, and since the energy is used to evaporate the
molecules, the remaining liquid gets cooler. This is called evaporative cooling.
Procedure
My lab partner is: ________________________________________
Read lab instructions on the next two pages. Record your data on the Data Collection pages, pp. 4-7.
Experiment 1: IMFs of water, hexane, ethanol and acetone (at your bench)
Part 1: Predicting polarity based on models
Obtain a model set from the cart and after drawing the Lewis structure for the following compounds, you will make
the model, sketch its 3-D shape using the wedge and dash notation, look at the intramolecular bonds to see if any of
them are polar and then look at the shape of the molecule to see if the molecule is polar. You should now be able to
predict the type of intermolecular forces and how the properties will vary between the molecules that you will be
testing.
Compounds to be studied:
Water H2O
Hexane CH3(CH2)4CH3
Ethanol CH3CH2OH
Acetone CH3C(=O)CH3
H2O Water
Lewis Structure
3-D Sketch of Model
Polar Bonds (Yes or No)?_________________
Polar or Nonpolar Molecule? _______________
CH3(CH2)4CH3 (C6H14) Hexane
Lewis Structure
3-D Sketch of Model
Polar Bonds (Yes or No)?_________________
Polar or Nonpolar Molecule? _______________
CH3CH2OH (C2H6O) Ethanol
Lewis Structure
3-D Sketch of Model
Polar Bonds (Yes or No)?_________________
Polar or Nonpolar Molecule? _______________
O
CH3 C CH3 (C3H6O) Acetone
Lewis Structure
3-D Sketch of Model
Polar Bonds (Yes or No)?_________________
Polar or Nonpolar Molecule? _______________
Pre-Lab: Instructor Initials ___________
General Chemistry
Lab 4: Intermolecular Forces
Part 2: Evaporation rate
This experiment may be conducted as a group of four - one person per substance (hexane, water, ethanol, acetone).
1. Obtain 4 pieces of filter paper and five rubber bands to attach the paper to your thermometer.
2. Attach filter paper to end of the thermometer at the bulb with a rubber band
and then dip into the test tube containing the compound to be studied (hexane,
water, ethanol, or acetone). When the temperature has stabilized, record the
temperature while the paper on the thermometer is still submerged in the
liquid. This is the measurement for time = 0.0 min.
3. Your time starts when you pull it out of the liquid. Tape the thermometer to
the lab bench so that the paper is hanging off the bench and you can read the
thermometer. Note: If there is a big drop hanging from the paper when you
pull it out then touch the drop to the side of the test tube to get rid of it.
4. Record the temperature with the correct significant figures every thirty
seconds for 8 min.
Data Table 1: Rate of cooling due to evaporation
Water
T ( C)
Acetone
T ( C)
Ethanol
T ( C)
Observations/Comments
Hexane
T ( C)
0.0 min
0.5 min
1.0 min
1.5 min
2.0 min
2.5 min
3.0 min
3.5 min
4.0 min
4.5 min
5.0 min
5.5 min
6.0 min
6.5 min
7.0 min
7.5 min
8.0 min
8.5 min
Critical Thinking Question 1: Does a substance that evaporates more quickly (thus giving a greater T) have a
higher or lower force of attraction to other molecules in the liquid compared to a substance that evaporates more
slowly (smaller T)? Explain.
3
General Chemistry
Lab 4: Intermolecular Forces
4
Experiment 2: Additive Attractive Forces of Hydrocarbons and Alcohols (Station 2)
This station will demonstrate the effect of length and type of force relative to the state of the material (solid, liquid,
gas) at the same temperature (room temperature).
 Pick up each Erlenmeyer or Florence flask and swirl it.
 Make a note of the state (solid liquid gas) of the material and qualitatively comment on its viscosity if it is a
liquid.
 Note also the number of carbons (chain length) it takes to have a liquid, viscous liquid, and solid and note the
forces involved.
Figure 1: Erlenmeyer flasks that contain compounds of Just C & H’s
methane
butane
pentane
hexane
mineral oil
(paraffin oil)
paraffin wax
CH4
C4H10
C5H12
C6H14
~ C18H38
~ C24H50
CH4
CH3(CH2)2CH3
CH3(CH2)3CH3
CH3(CH2)4CH3
CH3(CH2)16CH3
CH3(CH2)22CH3
Figure 2: Florence flasks that contain compounds that have One OH per C :
1,2-ethandiol
1,2,3-propantriol
Official Name 
methanol
methyl alcohol
Common Name 
ethylene glycol
glycerol
(wood alcohol)
(anti-freeze)
(glycerin)
Alternative name 
Formula 
CH4O
C2H6O2
C3H8O3
OH OH
OH OH OH
OH
Structural `
Formula 
H C C H
H C C C H
H C H
H
H H
glucose
dextrose
(blood sugar)
C6H12O6
OH OH OH OH OH
H C C
C
C C C H
H H
H
H H O
H H H
These alcohols (carbon compounds with OH groups) have several names so use the name in bold
Data Table 2: Observations on hydrocarbons and alcohols
Substance
C & H only
One OH per C
State of Matter
Number of
Carbons
Viscosity / Notes
General Chemistry
Lab 4: Intermolecular Forces
Observations or Comments on the “C&H” series, on the “one OH per C” series, and comparisons of the two
series:
Critical Thinking Question 2: Which of the two series (“C&H” or “one OH per C”) seems to have greater
IMFs? How did you reach that conclusion?
Critical Thinking Question 3: Within each series, does there seem to be any pattern to the strength of IMFs?
If so, what is the pattern?
Experiment 3: Polarization (Station 3)




There will be four burets set up with the liquids ethanol, acetone, water, and hexane.
You will rub a plastic rod (or pipe) with a cloth or rabbit fur to create a static charge in the rod.
Turn all the burets on so the liquids are flowing down in a stream into a beaker below.
Bring the rod horizontal so that it is in front of all the liquid streams and slowly move it to the liquids until
one of them starts to bend.
 Try to determine which has the greatest to least deflection (bending). When you are finished close the
burets and pour the contents of the beakers back into the burets through the funnel.
Observations of each chemical.
Place the chemicals (acetone, water, ethanol, hexane) in order of most deflected to least deflected.
1.
2.
3.
4.
Critical Thinking Question 4: If the static electric charge is composed of negative electrons, which chemicals
should be deflected more, polar or nonpolar ones?
5
General Chemistry
Lab 4: Intermolecular Forces
6
Experiment 4: Viscosity (Station 4)
You will study a number of tubes, each containing a liquid and a marble. When you invert the tube, the marble
will fall to the bottom. The rate of fall depends on the viscosity of the liquid which is dependant on
intermolecular forces. The falling marble must break connections as it travels down. The stronger the
connections the slower the marble goes. Record the relative speed of the marble and put the substances in order
of fastest (least viscous) to slowest (most viscous). If you get any that are really close, try starting them at the
same time as in a race.
Note: To the water was added a drop of blue food coloring and to ethanol was added one drop of red food
coloring. This will not affect the viscosity of either one.
Comments on liquids in tubes with marble:
hexane
ethanol
mineral oil (also called paraffin oil and it would be similar to vegetable oil)
water
Put them in order of fastest to slowest.
1.
2.
3.
4.
Critical Thinking Question 5: Which should be more viscous, polar or nonpolar molecules?
Clean all equipment and glassware. Return materials to the community area. Wipe down the lab bench. Return the
key to the front desk. Do not remove your safety goggles until ALL GROUPS have finished cleaning up. Obtain
instructor initials before leaving the lab.
Instructor Initials ___________
General Chemistry
Lab 4: Intermolecular Forces
Data Analysis
Results Table 1: Evaporative cooling
Lowest T
Fluid
Initial T (°C)
(°C)
T
(°C)
Water
Acetone
Ethanol
Hexane
Note: For Temperature change ( T), take the lowest T
minus the initial T. If the temperature dropped it would be
reported as negative
Questions
Answer on a separate sheet and attach to the lab report. These may be typed or neatly and legibly handwritten.
1.
A. For part B of Experiment 1, rank the compounds in terms of rate of evaporation from fastest to slowest.
B. Which has the strongest IMFs according to this data?
C. Explain your results on the basis of intermolecular forces.
2.
To get a sense of why the two series (the enclosed flasks in Experiment 2) had similarities even though different
forces were involved, we are going to try to give a qualitative feel to the size of the IMFs .
Let’s start with a hypothesis that each IMF can be added separately to get a total IMF. If we let each LDF (C-H)
=1, each dipole (C=O) = 7, and each H-bond (OH) = 11 and ignore the C-C connections, we should be able to
add up all the forces involved and put that sum in the last column on Results Table 2 on the next page. Note: if
a compound has just CH’s then just count the number of H’s; otherwise look at the Lewis structure.
A. As IMFs increase, should boiling points increase or decrease? Explain.
B. Do the “Total IMF” numbers follow the trend of increasing boiling points? Does this calculation seem to be
a reliable estimation of IMFs?
3.
Fill in the rest of Results Table 2 on the next page. Please use the abbreviations at the bottom of the table for
the three IMF forces: LDF, dipole-dipole, H-bonding. For viscosity, use the words like high, low or N/A.
Summary Writing Assignment
Answer on a separate sheet and attach to the lab report. These may be typed or neatly and legibly handwritten.
Answer the experimental questions in grammatically correct English sentences. Use your data to concisely
explain your answers (support answers with experimental evidence).
A. Which types of compounds have the strongest intermolecular forces?
B. How can certain physical properties give an indication of the strength of IMFs of compounds?
7
General Chemistry
Lab 4: Intermolecular Forces
8
Results Table 2: Summary of data for chemicals used in this lab arranged according to boiling point.
Melting
point
Boiling point
Methane
MM=16.0
-183
-164
Propane (C3H8)
MM= 44.1
-190
-42
Butane
MM=58.1
-138
-0.5
Pentane
MM=72.1
-130
36
Acetone
MM=34.1
-95
56-57
Methanol
MM=32.0
-98
65
Hexanes
MM=86.2
-95
69
Ethanol
MM=46.1
-117
78.5
Water
MM=18.0
0
100
Ethylene glycol
MM=62.1
-12.9
197.3
Glycerol
MM=92.1
17.8
290
Mineral oil
MM=~254
not available
260-330
Paraffin wax
MM=339
47-65
Substance
Strongest
IMF
state
Viscosity
Total IMF
Add the IMFs C-H=1
Dip=7 H-bond=11
4(1)= 4
5(1)+11= 16
Greater than
400
Not available
Glucose
(decomposes
149-152
7(1)+1(7)+5(11)= 69
MM=180
before it
boils)
LDF= London Dispersion Forces Dip-Dip = dipole-dipole H-bond = H-bonding
Note: in doing the total IMF in the last column we have neglected the weight of the compounds and this only works
for linear carbon compounds. Remember that heavier compounds move slower and therefore feel the attractions
greater.
Lab Report
Staple IN ORDER:
This handout with original data and data analysis (tables filled in).
Answers to Questions and Summary Writing Assignment neatly handwritten or typed.