Many physical properties are determined by the intermolecular forces of attraction existing between the
molecules of a given compound. These intermolecular forces are weaker than the bonding forces that
exist between the atoms in a compound and are primarily due to an uneven distribution of the electron
cloud existing in a molecule. This uneven charge distribution can be permanent, as in the dipole
moment caused by polar bonds and 3-D geometry (dipole-dipole interactions), or temporary, caused by
the temporary distortion of the electron cloud in non-polar molecules (London dispersion forces). The
very strong dipole-dipole interaction that occurs only between O, N, F and the H atoms bonded to them
is called hydrogen bonding. Hydrogen bonding is extremely important in many systems and primarily
responsible for the unique properties of water. In addition, there can be interactions between ions and
polar molecules. In discussions of physical properties, ion-ion interactions may also be considered. In
general, intermolecular forces can be ranked in order of increasing strength as follows: London
dispersion forces, dipole-induced dipole, dipole-dipole, hydrogen bonding, ion-dipole, ion-ion. There
are many exceptions to this trend, depending on the strength of the dipole and the size of the molecule.
When trying to rank compounds in terms of a physical property such as boiling point, the stronger the
intermolecular forces between the molecules of the compound under consideration, the more energy is
required to overcome those attractions. As a result, boiling point increases with increasing
intermolecular forces. The same situation is true for melting point: the higher the intermolecular forces
the higher the melting point. In both cases, increasing the temperature gives the molecules more kinetic
energy and overcomes the forces attracting the molecules to each other.
When considering vapor pressure, the opposite is true. This may seem counter-intuitive at first, but
consider that the stronger the forces between molecules in the liquid phase the harder it is for some of
those molecule to leave the liquid phase and enter the vapor phase. Thus, stronger intermolecular forces
mean lower vapor pressure. Since vapor pressure increases with increasing temperature, a higher
temperature is required to make the vapor pressure equal atmospheric pressure and a higher boiling
point is the result.
The first part of this experiment looks at a very common application of intermolecular forces,
chromatography. Chromatography is one of the most popular laboratory techniques in use today. The
name comes from the Greek words for “color” and “writing”, referring to the bands of color that were
produced when using this technique to separate plant pigments, one of the earliest applications.. The
basic principle of chromatography is that the substance of interest, the analyte, will separate into its
component parts when dissolved in the solvent, more normally called the mobile phase, and passed
across the supporting substance, the stationary phase. The components present in the analyte are
carried across the stationary phase at different rates depending on many factors. Two of the factors
include the composition and polarity of the mobile phase, the stationary phase and the analyte.
Individual components are characterized by the Rf value, the retention factor. The Rf value compares
the distance traveled by the component to the distance traveled by the solvent and always has a value
less than 1.0.
Distance spot travels from the original spot point
Distance solvent travels from the original spot point
There are many different types of chromatography, classified by the nature of the mobile and stationary
phases, as well as by the type of detection device used. Examples are HPLC, high performance liquid
chromatography, often used in biological applications where both mobile and stationary phases are
liquid and the detection device is a UV spectrometer; HPLC-FTIR, a liquid chromatography application
with an infra-red spectrometer as a detector; GC, gas chromatography, where the mobile phase is a gas
and the stationary phase is a liquid and GC-MS, gas chromatography with a mass spectrometer as a
detection device. In addition to the instrumental techniques mentioned above, there are also many types
of column and sheet chromatography, where the stationary phase is paper or a finely-divided solid and
the mobile phase is a liquid. Detection techniques used range from visual inspection to various types of
stains to collecting the analyte components and analyzing them in spectrophotometers. The mobile
phase can travel across the stationary phase in various ways. Some techniques let the mobile phase
move down the stationary phase (descending), some let the mobile phase travel up the stationary phase
(ascending) and some travel across or through the stationary phase. This experiment will examine
ascending paper chromatography where filter paper is the stationary phase and various acetone-water
solutions are the mobile phase and travel up the paper.
One part of this experiment studies the inks/dyes from various overhead transparency pens. The ink is
spotted near the bottom of the paper, placing them in solvents of different composition and seeing how
the ink components separate and move up the paper. One solvent is a solution of 20% acetone in water,
while the other is a solution of 80% acetone in water. Since the ink contains colored components,
detection and identification of the components is done by simply looking at them.
Another part of the experiment the movement of the halide ions (F-, Cl-, Br-, I-) in 80% acetone. Since
the halide ions are colorless, the filter paper will be sprayed with a solution of silver nitrate. Silver
nitrate reacts with the halide ions to form silver halides which are light sensitive. Over a period of time,
different colored spots will appear for each of the halide ions spotted. An unknown contain two of the
halide ions will also be analyzed and identified, using color and Rf values.
The second part of the experiment will look at the effect of intermolecular forces on vapor pressure.
The effect of intermolecular forces on vapor pressure will be studied in two different ways. The effect
of increasing molecular size, of increasing London forces, will be studied using pentane, hexane,
heptane and octane. These molecules are simple alkanes, containing only carbon and hydrogen, and
thus have only London dispersion forces.
The effect of dipole-dipole interactions on vapor pressure will be studied using ethanol, ethyl acetate and
acetone. Structures for the compounds are given below.
Part 1 – Overhead Pen Inks/Dyes
Use your 400-mL beaker and a second 400-mL beaker at your station and label one “20%
Acetone” and the other “80% Acetone”. Pour 15 mL of each solvent into the corresponding
beaker and cover with a piece of aluminum foil to let the air inside saturate with solvent vapors.
Obtain two pieces of the chromatography paper that are folded into 5 columns. For each piece of
paper, at the very top of each column use pencil to write, “Red”, “Orange”, “Black”, “Green”,
and “Blue”. In each column, exactly 1 cm from the bottom edge, use the “EXPO Vis-à-vis” pens
to put a small spot of ink in the correct column.
When the ink is dry (important), fold the paper back to make a 5-sided vertical column with the
spots at the bottom. Make sure the ends of the paper do not overlap. Remove the aluminum foil
from the first beaker and put the paper column gently in the beaker. Make sure the spots are
above the solvent level. Put the aluminum foil back over the beaker. Repeat with the second
beaker and the second piece of paper. Record the time the papers are placed in the beakers.
Observe the papers as you do the next part, but do not move the beakers once the process has
begun otherwise you cause false movement of solvent due to splashing.
Remove the papers when the solvent has risen to within 1 cm of the top of the paper, recording
the time when they are removed. Label the papers with the solvent used and let them dry on the
counter. Staple the papers to the left hand page closest to the data in the lab notebook.
Part II - Halide Determination
Use your 600-mL beaker. Place 15 mL of 80% acetone in the beaker and cover with a piece of
aluminum foil. Let the beaker stand covered while you prepare the filter paper so that the air
inside becomes saturated with solvent vapors.
Take a rectangular pieces of filter paper from the box and draw a pencil line parallel to the
bottom of the long edge, about 1.5 cm from the edge. Mark with a pencil the location of the
spots to be applied. Touch the paper only on the sides or top.
Locate the halide standards and use the attached capillary tubes to apply the halide standards,
applying the spot just above the line as shown in the movie. Keep the tube in a vertical position
and keep your finger on the top of the tube in order to make a small spot on the paper. Obtain a
halide unknown from your instructor and spot on the same paper.
With clean, dry hands staple the right side of the paper to the left to form a cylinder. Be sure the
two edges do not touch so that the solvent will not overlap and ruin the separation.
Remove the foil and place the paper cylinder inside, being careful that the solvent does not touch
any of the spots. Do not splash or slosh the solvent around so that it travels up the paper by any
means other than capillary action. Record the time the paper is placed in the beaker.
Remove the paper when the solvent is about1.5 cm from the top of the filter paper. Do not let
the solvent run to the top of the paper. It takes around 20 – 30 minutes for the solvent to rise to
the top. Record the time the paper is removed.
Open the cylinder and allow to air dry. Put the paper in the spray box and apply silver nitrate
solution from the spray bottle. Be sure the fan is on and the paper thoroughly saturated with
solution. Remember that silver nitrate will blacken your skin, so avoid any contact! Use your
forceps to transfer the paper to a paper towel
Make note of the colors as the spots appear. All silver halides darken when exposed to light, but
I- gives up its electrons most easily and therefore appears first. It may require several hours or
even a couple of days for the F- spots to appear. Taking the paper outside, even if it is cloudy,
will speed up the process.
When the spots have all appeared, calculate the Rf values for all the spots and identify the
unknown by comparing both color and Rf. Staple the dried chromatograph to a left hand page in
Part III: Vapor Pressure (Do this part in groups and keep vapors under the hoods!)
Locate the 10 cm filter paper strips, the Scotch tape, the multimeter with temperature probe, a
stopwatch and the test tube rack containing the reagents in stoppered test tubes.
Fold three filter paper strips over the end of the temperature probe and tape in place.
Make sure the multimeter is in the temperature mode and turned on (red button in upper right).
Wait until the temperature reading has cooled to about 24°C (your fingers will warm it up), then
record the initial temperature reading.
Remove the stopper from one of the test tubes, insert the filter paper covered probe into the test
tube and remove it, starting the stopwatch. Make sure to record which regent you use.
Record the temperature after 45 seconds have passed.
Repeat this process with each of the reagents.
Make sure the Scotch tape is removed from the probe since acetone and ethyl acetate will start to
dissolve the tape, making it sticky.
A typical data table for this part would include the name of the compound, initial temperature, final
temperature and time
Part II: Halide Chromatography
Calculate the Rf factor for each halide ion.
Calculate the Rf factor for the spots in your unknown. Use color and R f to identify the ions
present in the unknown.
Part III: Vapor Pressure
Calculate the change in temperature for each of the compounds.
Calculate the rate of evaporation by dividing the change in temperature by the time. Faster
evaporation rates should occur in compounds with lower intermolecular forces and higher vapor
Your report table should include the unknown number and identity of the ions present in the halide
unknown for Part II, as well as the rate of evaporation for each compound in Part III.
Other results will be given as part of the Discussion section.
The more polar mobile phase is the 20% acetone solution (since it is 80% water). Paper is made
from cellulose which contains a lot of oxygen atoms and OH groups. Write a short paragraph
discussing which solvent best separated the ink components from the Vis-a-Vis pens. Consider
in your discussion the relative attraction of the ink components for the solvent and for the paper.
Does your discussion support the statement that these pens are water soluble?
Did any colors in the ink pens move differently using the two different mobile phases? Which is
more polar, the blue dye molecule or the yellow dye molecule?
Write a short paragraph discussing the relative movement of the individual halide ions in the
80% acetone solvent. Which ions moved the most/least and why? Consider what you know
about size and attractive forces between the ions, the paper and the solvent to explain the relative
The lower the intermolecular forces, the more easily a compound leaves the liquid phase, the
faster the rate of evaporation, and thus the higher the vapor pressure. Rank the non-polar alkanes
– pentane, hexane, heptane and octane - in terms of increasing vapor pressure. Explain your
ranking in terms of the molar mass and intermolecular forces present.
Rank the polar molecules ethanol, acetone and ethyl acetate in terms of increasing vapor
pressure. Explain your ranking in terms in terms of the intermolecular forces present, and the
effects (if any) of molar mass.
How would you rank the relative strengths of the intermolecular forces present in the non-polar
and polar molecules discussed in questions #4 and #5? Briefly explain if your data supports this
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