PCC Rock Creek
CH 105
VIKIC
SP 06
Intermolecular Forces
Objectives
•
•
To observe how intermolecular forces of attraction affect the physical properties of a liquid.
To practice the technique of extraction to separate a liquid from a solution by using its
solubility properties in different media.
•
Materials & Equipment
•
•
•
•
•
•
•
•
Computer with interface
pressure sensor - absolute
temperature sensor
acetone
isopropyl alcohol
Aluminum Foil
rubber stopper, two hole (to fit bottle)
large beaker for water bath
•
•
•
•
•
•
•
•
250 mL Erlenmeyer flask
hot plate
graduated cylinder, IO mL
ring & ring stand
Sudan IV solution (Scarlet Red)
hexane
separatory funnel
3-125mL Erlenmeyer flask
Discussion
According to the kinetic molecular theory, the three physical states of the matter may be
explained by the speed (kinetic energy) and the distance (potential energy) between molecules
in each state. In the gaseous state, molecules are moving very fast and are far apart from each
other. So much so, that there is little or no interactions between them. That is why the
properties and behavior of gases can be explained by simple empirical laws independent of the
nature of the gas (ideal gas). On the other hand, in the liquid phase molecules are moving
slower and are closer to each other and their properties and behavior are dependent on the
nature of the liquid and the interactions between molecules. The same is true for solids where
molecules are vibrating in place next to each other.
The forces that hold liquid molecules together are called intermolecular forces of attraction.
There are three main types of intermolecular forces: London forces, dipole-dipole and hydrogen
bonding. Which intermolecular force is in effect in a particular liquid depends on the structure
and polarity of the molecule. Intermolecular forces of attraction affect many physical
properties of liquids; including, vapor pressure, solubility, boiling point, melting point,
viscosity, capillary action and surface tension.
The stronger the attractions between particles (molecules or ions), the more difficult it will be
to separate the particles. When substances melt, the particles are still close to one another
but the forces of attraction that held the particles rigidly together in the solid state have been
sufficiently overcome to allow the particles to move. When substances boil, the particles are
completely separated from one another and the attractions between molecules are completely
overcome. The energy required to cause substances to melt and to boil, and thus disrupt the
forces of attraction, comes from the environment surrounding the material. For example, if
you place a piece of ice in your hand, the ice will melt more quickly than if it is placed on a
cold counter top. The energy required to melt the ice comes from your hand, your hand gets
colder and the ice gets warmer.
Intermolecular Forces
1
PCC Rock Creek
All molecules experience the weakest of the attractions, London forces. London forces are due
to an instantaneous dipole moment created due to the movement of electrons in a molecule.
This “instantaneous” dipole creates an induced dipole in neighboring molecules. Since all
molecules have electrons, all molecules experience London Forces. The instantaneous dipole
behaves as a flashing light turning “on” and “off”. When it’s “on” it induces a dipole moment in
neighboring molecules which in turn induces dipole moments in other molecules, spreading
itself throughout the substance; flashing “on” and “off”. London forces are strongest in
molecules with large molecular weights and branched structures
Induced dipole in London forces
Polar molecules, in addition to London forces,
experience dipole-dipole interactions due to the
presence of a permanent dipole in the molecule.
Dipole-dipole interactions are stronger than London
Forces because the molecules have a permanent
dipole.
A special case of a dipole-dipole interaction is the hydrogen bond. Molecules having hydrogen
bonded to a nitrogen, oxygen or fluorine atom have an extremely polar bond. These molecules
experience an even stronger type of intermolecular force of attraction called hydrogen bonding.
This intermolecular interaction is so strong that in recent scientific papers it has been
described as having a “partial covalent character”.
Hydrogen bonding is responsible for the unexpectedly high boiling point of water (compared to
the other group 16 hydrides such as H2S, H2Se etc.), the lower density of ice compared to
liquid water (see picture on left), and the shape of many essential biological molecules (e.g.
DNA and proteins).
Hydrogen Bonding in water
Intermolecular Forces
2
PCC Rock Creek
The cells of living things are made up of many
different sorts of molecule. Two important
classes of molecule are proteins and nucleic
acids. In both of these molecules, parts of the
(very large) molecules are involved in
hydrogen bonds with other parts of the same
molecules.
Dotted line represents Hydrogen bonding
between base pairs in DNA.
In today’s experiment you will investigate how intermolecular forces of attraction affect the
vapor pressure and solubility of various liquids. You will work in pairs; half of the class will
start with part A and the other half with part B. In part A, you will determine the vapor
pressure of the two liquids at selected temperatures using temperature and pressure probes
connected to the Scientific Workshop computer program. There are six computer set ups for
this part of the experiment. In Part B, you will study the difference in solubility of liquids by
performing a liquid-to-liquid extraction. There are six separatory funnels to be used for this
part of the experiment.
Intermolecular Forces
3
PCC Rock Creek
Procedure
Part A-Determination of the Vapor Pressure of two Liquids
The higher the temperature of a liquid, the greater the kinetic energy of the molecules. When a
liquid absorbs enough energy, the molecules move with enough speed and momentum to
break into the vapor or gas phase. As more and more molecules break from the liquid state
into the vapor or gaseous state, the pressure they exert increases. This is the vapor pressure
of a liquid. The vapor pressure of all liquids is directly related to the temperature of the liquid
and to the intermolecular forces of attraction present in the liquid.
For this part of the experiment, we will measure the pressure of acetone and isopropyl alcohol
at different temperatures. To measure these, we will use temperature and pressure sensors
linked to the Science Workshop program which records and displays the data.
Water bath and Sample Preparation
1 . Use a large beaker to make a water bath (a 250 mL Erlenmeyer flask should fit in it). Start
to heat the water bath to near boiling. Do not boil the water. Maintain the temperature
above 90°C. Check the temperature occasionally as you set up the rest of the equipment.
2.
Put 5 mL of acetone in an Erlenmeyer flask. Place the Aluminum foil loosely over the
flask but DO NOT CAP COMPLETELY!! Place the flask into the hot water. Leave the
flask in the water bath until approximately 1ml of the solvent remains allowing the solvent
gas to fill the flask.
Computer Setup
Familiarize yourself with the computer setup and the computer screen. The screen will open
with a graph display that has a plot of temperature in Celsius (deg C) [x-axis] versus pressure
in kilopascals (kPa) [y axis]. Note that the scales may need adjustment. Ask the instructor or lab
assistant if you need the scales adjusted. Another screen that may be useful is the data screen;
ask instructor or lab assistant for help.
Data Recording
1. After approximately 3 minutes, quickly remove the Aluminum Foil on flask and replace it
with the temperature sensor and stopper. Promptly remove the flask from the water bath.
When everything is closed click monitor on the computer screen. When you observe the
first drop in pressure, click stop than record to begin collecting data.
2. Observe the change in temperature and pressure as the acetone cools. What is the
observed trend?
3 . Continue collecting data for about four minutes.
4. Record pressure values at four different temperatures (T1 through T4). Spread the points
from near the boiling point of the liquid to about room temperature.
5. Click on the "STOP" button to end data recording.
Intermolecular Forces
4
PCC Rock Creek
6. Slowly remove the temperature sensor stopper to allow air to enter the flask.
7. Dispose of the remaining acetone by rinsing the acetone down the drain with a large
volume of water following. Air-dry the flask.
Repeat the entire procedure with isopropyl alcohol instead of acetone.
Part B- Extraction of a liquid by using Solubility differences: liquid-liquid
extraction.
Extraction is one of humankind's oldest chemical operations. The
preparation of a cup of coffee or tea involves the extraction of flavor and
odor compounds from dried vegetable matter with hot water. Aqueous
extracts of bay lean stick cinnamon, peppercorns, and cloves are used as
food. A commonly used form of extraction is the liquid/liquid extraction.
In this process the solubility characteristics of the substance to be
isolated are used to separate it from a liquid mixture.
The extraction is performed by placing the mixture containing the
substance to be isolated into a separatory funnel (see Figure 1) . Then a
carefully selected solvent, which only dissolves the product to be isolated,
and inmiscible with the rest of the mixture, is added to the funnel. This
will produce two distinct inmiscible layers or phases.
Figure 1- Separatory Funnel
The mixture is shaken as demonstrated in Figure 2, and the substance to be isolated will
preferentially dissolved in the added solvent, leaving the other components of the mixture
behind.
Things To Think About: Which solvent will be your top layer and
which solvent your bottom layer?
The layer containing the isolated product is evaporated and the
solid product is recovered.
The solvent used for extraction should be carefully selected to
meet the following criteria: it should readily dissolve the
substance to be extracted; it should have a low boiling point so
that it can be readily removed by evaporation; it should not react
with the solute or the other solvent; and it should not be
flammable or toxic. In addition, it should not be miscible with
the second layer.
Although the isolated substance is
preferentially soluble in the selected solvent it will still be somewhat
soluble in the original solvent, therefore, several small extractions
are better than one large one.
Figure 2: Proper way of
holding and shaking a
In this part of the experiment, you will be extracting the red dye
separatory funnel.
Intermolecular Forces
5
PCC Rock Creek
Sudan IV also known as Scarlet Red, from a solution that is 50:50 water and ethanol. The
solvent chosen for this extraction is hexane.
Intermolecular Forces
6
PCC Rock Creek
1 . Take a ring support for the separators funnel and attach it to the lab stand. Place the
separators funnel in it as shown in figure 1. Make sure that the stopcock is closed
(horizontal position). Pour in 20ml of Sudan IV solution. Record your observations.
2.
Add 10ml of hexane to the mixture in separators funnel. Record your observations.
3.
Put the stopper in the top of the funnel securely. Securely hold the funnel and invert it a
few times as shown in fig. 2. With the stopcock pointing UP and away from everyone,
carefully open it (the handle should be parallel with funnel.) to release any pressure. IF
THIS IS UNCLEAR ASK YOUR INSTUCTOR OR LAB STAFF BEFORE ATTEMPTING IT.
There will probably be a gentle hiss of escaping gas. CLOSE THE VALVE. GENTLY swirl
the mixture with the funnel still inverted and VENT again. Repeat process until no more
gas is released.
4.
Place the funnel in the support ring. Wait until the two liquids separate. If the interface
between the two liquids isn't sharply defined, it means that the system (liquids; solute;
impurities) is prone to form an emulsion, droplets of one liquid trapped in the other as in
salad dressing. Shake the two liquids together as vigorously as possible without forming
an emulsion. Record your observations.
5.
Let the mixture rest in the support ring, until the liquids separate.
6.
Take three 125 mL Erlenmeyer flask. Label them "A", "B" and "C".
7.
Place Erlenmeyer flask(A) under funnel. Remove stopper from separatory funnel. Open
stopcock slowly and drain bottom layer. Stop just after you get to the interface.
6.
Now pour the top layer out of the top into flask(B).
7.
Put the contents of flask (A) into the funnel. Add 10ml hexane and repeat steps 3 through
5. This time put the bottom layer back into flask (A) and the top layer into flask(C).
Compare the color of each flask. Record your observations.
Waste Disposal
Dispose all left over reagents in the waste container under the hood.
Intermolecular Forces
7
PCC Rock Creek
Prelaboratory Questions: Intermolecular Forces
Date
Name
1) Use the sources on the internet (www.chemfinder.com), CRC Handbook of Chemistry and
Physics or the Merck Index and complete the following table:
Name of
Compound
Molecular
Mass (g/mol)
Density
(g/mL)
Structure
(Kekulé structure)
Intermolecular
Forces
Vapor
Pressure
(mmHg)
Water
Acetone
Isopropyl
alcohol
Hexane
SudanIV
(scarlet red)
2) How is the vapor pressure of a liquid affected by temperature? Why?
Intermolecular Forces
8
PCC Rock Creek
Prelaboratory Questions: Intermolecular Forces
Date
Name
3) Without looking for the answer in a reference text, place the first three compounds in order
of increasing vapor pressure at a given temperature. Please explain your reasoning.
________________< ____________<______________
4) Compare your predicted results with values reported in the handbooks or your text.
Vapor Pressure from Literature Values
water
___________
isopropyl alcohol
___________
acetone
___________
Temp = ______
your prediction from (Q#2 )
________________< ____________<______________
according to literature
________________< ____________<______________
If your prediction does not match literature, explain.
5) What is the vapor pressure of any liquid at its boiling point?
6) Do you expect Sudan IV to be soluble in organic solvents or in aqueous solutions? Please
explain your reasoning.
Intermolecular Forces
9
PCC Rock Creek
Data and Questions: Intermolecular Forces
Make sure to discuss the following points in your notebook and report:
Part A- Determination of Acetone and Isopropyl Alcohol Vapor Pressure
•
Trend of vapor pressure with temperature.
•
Temperature where the highest vapor pressure of each liquid was recorded.
•
Comparison of acetone and isopropyl alcohol’s pressure at the same temperature.
•
Which of the two liquids had the highest vapor pressure at a given temperature?
•
Does your experimental data agree with your expected results? Explain.
•
Which liquid do you expect to have the highest heat of vaporization, acetone of
isopropyl alcohol? Why?
Part B- Extraction of a liquid by using solubility differences: liquid-liquid extraction.
•
Why is the Sudan IV in a mixture of ethanol and water instead of just plain water?
•
Why is hexane chosen as the solvent for this extraction?
•
Suggest another solvent that may have been used instead. Explain your choice.
•
Identify the components of the two layers formed in step 2. How did you make this
determination?
•
Identify the components of the mixture collected in flasks a, b and c. How did you
make this determination?
•
How could you “prove” that those are the correct components of each flask?
•
What was the purpose of step 7? Was it necessary?
Intermolecular Forces
10