Tennessee Tech Chemistry Computer Resources

Introduction

The experiment demonstrates the interactions between hydrophobic and hydrophilic substances. Hydrophobic substances repel water – the name comes from the Greek word, “Hydros”, for water, and, “phobos”, for fear. Hydrophilic substances (the Greek stem, “philic”, means “love”) attract, or love, water.

A polar molecule has a separation of positive and negative charges, while a non-polar molecule has no areas of prevalent positive or negative charges (it is neutral). Water is a polar molecule, having positive charge around the hydrogen and a negative charge around the oxygen. Because water is polar, other polar molecules will be attracted to it; these are the molecules we describe as hydrophilic. Conversely, hydrophobic molecules are non-polar/non-charged.

Soap molecules are very interesting in that they contain both hydrophobic portions and hydrophilic portions. We refer to these molecules as AMPHIPATHIC. These molecules typically have a polar head and a long, hydrophobic tail. Shaving cream is a foam consisting of molecules of soap and air.

Food coloring consists of dye molecules that are dissolved in water, so they must be hydrophilic. When these dyes are added to shaving cream, the dye molecules can only interact with the polar heads of the soap molecules, and not with the hydrophobic tails. This therefore limits their mobility. Molecules in paper are composed of molecule of cellulose (a polymer of glucose). Cellulose contains multiple polar hydroxyl functional groups that can interact readily with water, so paper is very hydrophilic. Because both the food coloring dyes and paper molecules are hydrophilic, they will interact readily with one another and the dyes will spread easily across the paper to form very colorful patterns in a much different way than they interacted with the shaving cream. This creates a marbling effect on the paper that is clearly distinct from the pattern seen on the shaving cream.

Materials

• Shaving Cream
• Food Coloring
• Squeegees
• Wax Paper
• Small pieces of card stock paper (3×5 index cards)
• Toothpicks
• Paper plates (optional)

Procedure

1. Place a large sheet of wax paper on a solid surface (table) to make it easier to clean up.
2. Place a small mound of shaving cream on the wax paper or a paper plate. Add three to four drops of food coloring to the shaving cream and swirl it with a toothpick. Be careful not to overswirl. If all of the colors blend together into a new color the final product will not be as attractive.
3. Place a 3×5 index card on the shaving cream mound and press down lightly. Remove the card and use a squeegee to remove excess shaving cream. Observe the marbled paper product!
4. The mound of shaving cream can be re-used by re-swirling it. You can add a new drop of food coloring or leave it as-is. You can color the back of their card or another card. Swirl the colors around some more to see how it chages the marbled pattern.

Safety

This experiment is relatively safe. All products can be purchased from a grocery store and there are no major safety issues.

Disposal of Waste Products

No special requirements. Everything can be discarded through normal disposal procedures. Mix the shaving cream with water and flush down the drain.

Introduction

Multi-colored slime can be created using a solution of poly-vinyl alcohol (PVA) and borax. This experiment demonstrates the ability of borax to form chemical cross-links between molecules of the PVA polymer. The chemical structures below show a molecule of the PVA polymer on the left and a molecule of borax (borate ion) on the right.

When these molecules are mixed together, hydrogen bonds are formed between the hydroxyl groups of both the PVA and borate ion. This “sticks” them together and creates the slimy texture to the mixture.

Slime is a non-Newtonian fluid that dilate, or expand, under stress. Similar substances with these properties include quicksand, starch solutions and Silly Putty. Dilatant materials tend to have some unusual properties.

If you slowly pull on the material, it will flow and stretch. If you are very careful, you can form a thin film. But if you pull sharply (inducing high stress), the material will break. If you pour the material from its container and tip the container slightly upwards, the gel will siphon itself.

Materials

• 2 L soda bottles (remove the label)
• Small disposable cups
• Plastic spoons or wooded stirrers
• 4% Poly-Vinyl Alcohol (PVA) solution
• 4% Borax solution
• Food coloring
• ZipLoc Bags
• Wax Paper or Tarp for easy cleanup
• 0.5 M Hydrochloric Acid (HCl) (optional)
• 0.5 M Sodium Hydroxide (NaOH) (optional)

Procedure

1. Prior to the demonstration, prepare the Poly-Vinyl Alcohol (PVA) solution by adding 4 g of PVA to 100 mL of hot, deionized water (70°C). You may use a microwave to heat the water. Put the 4% PVA solution into 2 L soda bottles for storage. Prepare the 4% borax solution by mixing 4 g Na²B⁴O⁷ in 100 mL of deionized water.
2. At the demonstration, perform all mixing of PVA and borax on the wax paper or tarp. You will mix the two solutions together in a 10:1 (PVA:borax) ratio in the small cups provide. Add two to four drops of food coloring of the desired color. Mix well using the plastic spoon or wooden stirrers. A gel should form immediately.
3. Demonstrate the flexibility and pliability of the gel.
4. Optional: Adding acid (HCl) to the slime breaks the crosslinking and produces a liquid with lower viscosity. If you add a base (NaOH), the process is reversed and the slime is regenerated.
5. The slime can be taken home by storing it in a ZipLoc bag.

Safety

Wear eye protection and latex gloves when handling slime.

Disposal of Waste Products

All waste products may be safely disposed of in the garbage.

Introduction

This is a fun demonstration that teaches students how to build simple molecular models using colored candies. It is relatively simple to prepare and set up, and easy to maintain. The basic principles of building small molecular models of organic compounds can be easily demonstrated. The basics of single, double and triple bonds can be demonstrated.

Materials

• Toothpicks
• Soft, jellied candies of a variety of colors
• A large bowl or ziploc bags to contain the candy atoms
• 3×5 Index Cards
• Markers or Sharpies

Procedure

1. Place all of the jellied candies in a large bowl or ZipLoc bags. You may opt to separate the candies by color, but it is not necessary.
2. Place the toothpicks on the table for easy access.
3. Decide ahead of time a small collection of potential models for participants to construct, and draw their names and chemical structures on the 3×5 index cards. The best molecules to build in this project are small organic molecules (the simpler the better). Planar aromatic compounds also work the best, since they can be built and rested on the table for stability (the candies are often heavy enough to not stay upright in, for example, a tetrahedral carbon atom).Examples of good molecules to use for this demonstration include:
   • Water (H₂O)
   • Acetylene (C₂H₂)
   • Carbon Dioxide (CO₂)
   • Carbonic Acid (H₂CO₃)
   • Ethane
   • Ethanol (CH₃CH₂OH)
   • Ethylene
   • Glucose or Fructose (C₆H₁₂O₆)
   • Methane
   • Propane
   • Ribose
   • 20 Amino Acids
   • Palmitic Acid
   • alpha-Linoleic Acid
   • Cyclohexane (boat and chair conformations?)
4. A single toothpick represents a single bond, two toothpicks represents a double bond, and three toothpicks represent a triple bond.

Safety

This experiment is relatively safe. It uses edible candies. However, because the candies will tend to be handled by many people, eating them is not recommended and should be discouraged. Also, the toothpicks can be sharp, so care should be taken to avoid poking oneself with them.

Disposal of Waste Products

No special requirements. Everything can be discarded through normal disposal procedures.

Introduction

This is a simple and fun, hands-on activity that demonstrates the basic concepts of computer-based molecular modeling. Using molecular mechanics, small molecules may be modeled by treating the atoms as balls and the bonds connected them as springs. We can then use a set of equations, which we refer to as a force field to place these atoms in the correct geometrical arrangement and calculate a relative energy that we can use to compare different conformations of these molecules.

The basic components of a typical molecular mechanics force field include:

E_relative = E_{covalent interactions} + E_{non-covalent interactions}

The covalent and non-covalent interactions can further be broken down into the following components:

E_{covalent interactions} = E_{bond-stretching} + E_{angle-bending} + E_torsion

E_{non-covalent interactions} = E_{electrostatic} + E_{van der Waal’s}

The equations used for the covalent interactions are Hook’s law (spring harmonics). Equations used for the electrostatic interactions is Coulomb’s law, and the equations used for the van der Waal’s component is the 6-12 Lennard-Jones potential.

Materials

• Windows, Macintosh or Linux Notebook PC>
• Avogadro Molecular Modeling Software (available free of charge here)
• 3×5 Index Cards
• Sharpies or colored markers

Procedure

1. Place one to three notebook computers on a table and have Avogadro already running as participants approach. It may also be advisable that the Desktop space is clear of files as well, in the event that Avogadro is closed or accidentally crashes during the dmeo and must be restarted.
2. Decide ahead of time a small collection of potential models for participants to construct, and draw their names and chemical structures on the 3×5 index cards. The best molecules to build in this project are small organic molecules.Examples of good molecules to use for this demonstration include:
   • Water (H₂O)
   • Acetylene (C₂H₂)
   • Carbon Dioxide (CO₂)
   • Carbonic Acid (H₂CO₃)
   • Ethane
   • Ethanol (CH₃CH₂OH)
   • Ethylene
   • Glucose or Fructose (C₆H₁₂O₆)
   • Methane
   • Propane
   • Ribose
   • 20 Amino Acids
   • Palmitic Acid
   • alpha-Linoleic Acid
3. Allow participants a few minutes to play with the software and familiarize themselves with building small molecules. You may want to explain that the best way to build molecules is to start with their carbon chain, and build everything as if all atoms are carbons first. Then, modify specific atoms in functional groups to oxygen or nitrogen or whatever is necessary. Hydrogens do not need to be explicitly added (the software adds those automatically).
4. Once a molecule is built on the screen, the molecule can be optimized to its correct geometry through a process called energy minimization. This process applies the force field and moves the atoms to their optimum positions based on the lowest energy conformation (you are minimizing the energy to its lowest point – hint: draw them an energy diagram as an example).Go to the EXTENSIONS > MOLECULAR MECHANICS menu option and select SETUP FORCE FIELD. Use the MMFF94 force field, change the number of steps to 2,000, change the algorithm to CONJUGATE GRADIENT, and the convergence criterion to 1 x 10^-7, and press OK. Run the procedure by going to EXTENSION > OPTIMIZE GEOMETRY and the molecule should appear in its lowest energy conformation.If it does not appear in its lowest energy conformation, that means that the computer put the molecule into a LOCAL ENERGY MINIMA, as opposed to the GLOBAL ENERGY MINIMA. Molecular mechanics energy minimization can only lower energy, it can not increase energy – so you are stuck in a local energy minima. To get out of this, you need to physically move some atoms and re-minimize to recalculate the new energy.
5. A simple experiment that can be done to illustrate the concept of minimization and local vs. global energy minima, is to have them build cyclohexane. This is easy to build as they just draw six carbon atoms and connect them all in a ring. Have them rotate this molecule around a bit to observe that the atoms are more or less arranged flat (not correct).Apply the energy minimization procedure, as specified above, to optimize the geometry. Most of the time, the computer will obtain the correct, “chair”, conformation of cyclohexane. Occasionally, some computers may minimize to the, “boat”, or, “twist-boat”, conformation – they are stuck in a local minima.See if the participant can convert between the boat and chair conformations of cyclohexane by moving atoms around and re-minimizing.

Safety

There are no safety issues with this demonstration.

Disposal of Waste Products

There are no waste disposal issues with this demonstration.

Introduction

Some artists use the way paint moves on a surface to produce interesting shapes and designs. Many artists paint on a canvas, a type of fabric that is very absorbant. Before painting on canvas, most artists treat the surface of the canvas so that it does not absorb as much liquid. The artist Helen Frankenthaler did not prepare her canvas in this way. Frankenthaler used the absorbant property of the canvas to create interesting shapes and patterns. To make a painting, she would tack a canvas onto the floor and pour the paint directly onto the surface. She would let the way the paint moved over the canvas to help decide what the picture would be.

In this activity, students paint with water over marker designs on coffee filters. This produces different shapes and beautiful works of art. Chemically, what is happening is that the filter is made up of a special type of paper that absorbs water very easily. Paper towels are made of a similar type of paper. The colors in the markers dissolve, or are soluble in, water. When the water is painted onto the coffee filter, the colors dissolve in the water. As the paper filter absorbs the water, the dissolved colors move with the water and create the resulting color patterns.

Materials

• Two circular white coffee filters
• One pipe cleaner
• Water-based markers of various colors
• Clean scrap paper with nothing on it
• Paintbrush
• Paper towel
• Water (for rinsing)

Procedure

1. Place the coffee filters on top of a piece of scrap paper. Use several different colored markers to create a design or pattern on each coffee filter. Note that this design will be changed when the directions in step three are carried out.
2. Place both coffee filters on another piece of scrap paper.
3. Dip the paintbrush in the water and paint over the designs with the wet brush. Be certain to rinse the brush in the water several times while you are painting with the water. Watch how the designs change.
4. Fold the pipe cleaner in half. Hold the pipe cleaner about 2 cm from the fold and twist two times. This will leave a small loop.
5. Scrunch one of the coffee filters along an imaginary line down the middle of the filter to produce one set of butterfly wings.
6. Place this filter inside the open ends of the pipe cleaner, centering it close to the twisted end.
7. Repeat step five with the other coffee filter. This is the second set of the butterfly’s wings. Place it above the first filter, inside the open ends of the pipe cleaner.
8. Twist the two pieces of the pipe cleaner together about 4 cm from the open end of the pipe cleaner. This will hold the two filters in place.
9. Turn down the ends of the pipe cleaner to look like antennae.
10. Thoroughly clean the work area and wash your hands.

Drying Hint: Place the folded end of the pipe cleaner in the end of a straw. Balance the straw in a glass or jar. More than one butterfly can be dried in the same jar this way. The coffee filters can be dried on a paper towel before scrunching them and putting them in the pipe cleaner.

Safety

This experiment is relatively safe. All products can be purchased from a grocery store and there are no major safety issues.

Disposal of Waste Products

No special requirements. Everything can be discarded through normal disposal procedures.

Introduction

The iodine clock reaction is a classical chemical clock demonstration that displays chemical kinetics in action. It was first discovered by Hans Heinrich Landolt in 1886, so many texts may refer to it as the Landolt Reaction. In this experiment, two colorless solutions are mixed. At first, there is no reaction, and after a given period of time, the solution turns dark blue.

This time period is termed the, “clock period”, and it is the amount of time that the mixture remains colorless until the change to dark blue. This can be calculated according to the following equation:

0.003 sM² / [IO₃^–]₀ [HSO₃^–]₀The procedure provided here should result in a clock period of approximately 9 seconds. Note that if the [HSO₃^–]₀ is three times greater than the [IO₃^–]₀, the reaction will not produce a color change. The overall experiment can be described by the following series of reactions:

IO₃^– + 3 HSO₃^– –> I^– + 3 SO₄^-2 + 3 H⁺

IO₃^– = 8 I^– + 6 H⁺ –> 3 I₃^– + 3 H₂O

I₃^– + HSO₃^– + H₂O –> 3 I^– + SO₄^-2 + 3 H⁺

2 I₃^– + starch –> starch-I₅^– complex (blue)The generation in of I₃^– in the presence of starch generates the blue starch color change. The triiodide ion is consumed by any remaining bisulfite ion, which prevents starch complexation. So the color change occurs when the bisulfite ion is consumed.

Materials

• 0.1 M KIO₃
• 1% starch solution
• 0.25 M NaHSO₃
• deionized water
• ice bath
• graduated cylinder, 100 mL
• beakers, 400 mL and 600 mL
• Syringes
• stopwatch

Procedure

1. Add the KIO₃^–-starch solution to a 400 mL beaker containing 100 mL 0.1 M KIO₃, 50 mL 1% starch solution, and 100 mL deionized water (labeled SOLUTION A). Add 20 mL of 0.25 M NaHSO₃ and 130 mL of deionized water to a 600 mL beaker (labeled SOLUTION B).
2. During the demonstration, add SOLUTION A to SOLUTION B. You may use syringes to mix the two solutions together. Note the amount of time required to observe the change in color to dark blue. When these solutions are mixed, the [KIO₃] = 0.025 M and the [NaHCO₃] = 0.013 M.
3. The effect of concentration on the rate of the reaction can be measured by varying the concentrations of the starting solutions. Prepare some variants of the KIO₃ mixture at 0.04 M and 0.02 M and repeat the experiment, noting the difference in the amount of time required for the color change to take place.

Safety

KIO₃ is a strong oxidizing agent and NaHSO₃ is a strong reducing agent. Avoid mixing solid or concentrated solutions of NaHSO₃ with KIO₃. Eye protection and latex gloves should be worn in this experiment.

Disposal of Waste Products

For disposal of waste products, combined all of the solutions used in this experiment with solid sodium thiosulfate until the mixture is no longer blue. The resulting clear mixture can then be discarded by flushing down the drain upon the addition of water.

References

1. Mitchell, R.S. Iodine Clock Reaction. J. Chem. Educ., 1996, 73 (8), 783.