CHEMICAL BONDS

USING POLYMERS TO UNDERSTAND CHEMICAL BONDS



AUDIENCE: HIGH SCHOOL STUDENTS

TIME FRAME:

ST-UP: 5 MINUTES

ACTIVITY: 40 MINUTES

CLEAN UP: 5 MINUTES


PUERTO RICO SCIENCES EDUCATION STANDARDS AND EXPECTATIVE


STANDARDS: Structure and levels of organization of the matter

Expectative: Matter and their interactions Q.CF1: Estructura y propiedades Indicadores: ES.Q.CF1.EM.10, 14 y 17

Objectives: Through the study of the chemical bonds, students can:


  1. COMPARE THE DIFFERENCES BETWEEN THE DIFFERENT TYPES OF CHEMICAL BONDS.

  2. RELATE THE VARIOUS APPLICATIONS OF POLYMERS IN OUR DAILY LIVES.

  3. QUANTIFY THE AMOUNT OF WATER THAT CAN BE INCORPORATED INSIDE A POLYMER NETWORK.

  4. ANALYZE THE EFFECTS OF THE AMOUNT OF CROSSLINKER ON THE FINAL POLYMER STRUCTURE AND QUANTIFIES THE SAME.

  5. CONCLUSION ON THE EFFECT OF DIFFERENT SOLVENTS IN PHYSICALLY BONDED POLYMERS.


VOCABULARY AND DEFINITIONS:

  1. Polymers

  2. Chemical Bonds

  3. Physical Bond

  4. Ionic Bond

  5. Covalent Bond  

SCIENTIFIC BACKGROUND

NOTE: This module is designed to visually show students the physical differences between various types of molecular bonds. It is recommended that this activity should be performed in two consecutive class periods, although depending on the group size it could be done in one.


Polymers are defined as long chains composed of repetitive units [POLY-many; MER-unit]. Although, there is a common belief that polymers are straight chains, they are in reality more like spaghettis in a bowl (see Figure 1). Chains are all entangled and try to face themselves.    When  there  are  no  specific  interactions  between  chains  and  they just

entangle, you can consider this a physical bond. These bonds are weak in nature and can be easily disrupted by a solvent where the polymer feels more comfortable in.


However, sometimes chains contain chemical moieties that with the help of an additional molecule (known as the crosslinker) can be either chemically or ionically crosslinked.   An ionic bond occurs when the additional molecule is an ion in nature (such as Ca+2and Mg+2) (see Figure 2). Notice that these ions are divalent (two charges) in nature. This is important because it allows them to interact with two chains at a time. These bonds can be disrupted with solvents of high ionic strength (high salt/ion content).


Figure 1. LEFT: Representation of entangled polymer chains. RIGHT: Analogy with a bowl of spaghetti.




Figure 2. LEFT: Representation of sodium alginate in water. Notice chains do not interact with each other. RIGHT: Representation of calcium alginate. Notice that calcium ions function as “connecting bridges” between chains.


Covalently bonded polymer chains occur when a chemical reaction takes place during the formation of a polymer chain and a difunctional monomer (meaning that it could react in two points of the molecule) is used. This di-functional monomer can potentially bind two different chains creating a mesh. These types of polymers do not dissolve in any solvent but can swell in some cases hundreds of times their dry weight (see Figure 3).

Figure 3. LEFT: Representative molecular structure of a poly (ethylene glycol) crosslinked hydrogel. The subscripts n, m and x represent the molecular weight of the poly(ethylene glycol) tethered chain, the methacrylate backbone of one chain, and the methacrylate backbone of a secondary chain joined by the crosslinker, respectively. RIGHT: Idealized representative model of a poly (ethylene glycol) crosslinked hydrogel.



Recomendation Videos: http://www.youtube.com/watch?v=Jw83nWI9jCA

www.youtube.com/watch?v=85XmStwDdJo


MATERIALS

  1. Polystyrene cups

  2. Acetone (pure, the one used to take out acrylic nails)

  3. Beakers (any size will do but preferably 25mL)

  4. Water

  5. Alginate

  6. Calcium chloride

  7. Diapers

  8. Scissors

  9. Glass beakers (various sizes)

  10. Syringe (5mL)

  11. Magnetic stirrer

  12. Stirring plate

  13. Balance

  14. Plastic spoon

  15. Graduated cylinders

  16. 100mL volumetric flask

  17. Phosphate buffer pH=7

  18. Sodium chloride (table salt)

METHODS

There are three steps to this module. First you will demonstrate physical bonds, then ionic bonds, and finally chemical bonds.


PHYSICAL BONDS (ALL LEVELS)

  1. Take three poly (styrene) cups and place them on top of a beaker. See Figure 4.


      Figure 4.  Experimental configuration for styrofoam cup.

  1. Measure 25mL of acetone, water, and juice in three different graduated cylinders.

  2. The student should pour water in the first beaker, and write down his/her observations.

  3. Then pour the juice and write down the observation.

  4. Finally pour the acetone. The cup should dissolve. They should write down their observations.


QUESTIONS (ALL LEVELS)

  • Write down what happened to the cup when you pour each liquid.

  • What do you think happened to the polymer when you put acetone? IONIC BONDS

  1. Pre-prepare a 2% w/w (2g polymer in 100g water ~100mL) solution of sodium alginate in distilled water. You can prepare this on an Erlenmeyer Flask. Tap water should not be used since it contains plenty of ions that will interfere with the process. Stir until all solid is dissolved. This may take a while. You can put a bit of food coloring as an option.

  2. Pre-prepare a 0.5M (110.98 g/mol) solution of calcium chloride in distilled water. 250 mL should be more than plenty.

  3. Load the polymer solution in a 5mL syringe without needle.



Figure 5.  Sodium alginate loading in syringe



4. In 50 mL beakers put 25 mL of the calcium chloride solution. In another beaker put distilled water. 
5.Drip drop by drop the polymer solution in the calcium chloride solution (See Figure 6)

Figure 6.  Method of creating calcium alginate beads by drop-by-drop method.


  1. Let the beads stand in the solution for 10min.

  2. With the spoon take them out of solution for the student to see and touch.

Figure 7.  Calcium alginate bead separation from solution using a spoon or a filter.


  1. Now take the same polymer solution and drip it into the distilled water solution. Allow the students to observe what happened.

  2. Another alternative is to take microscope slides, dip them in the polymer solution, and then put the slide in the calcium chloride solution.

  3. Let it stand for 10min.

  4. Allow the students to peel off the membrane.


QUESTIONS

  • Was the polymer solution a liquid, gas, or solid?

  • What happened when the drop of polymer solution fell in the cup of pure water?

  • What happened when the drop of polymer solution fell in the calcium chloride solution?

  • When you touched the beads, were they soft or hard?


  • They should be allowed to prepare an experimental design. See below.

  • Ask them to select one of these variables: polymer concentration and calcium chloride concentration.

  • From the selected variable let them prepare (for high school) three concentrations a. Polymer – 2% , 0.5%, 0.1% w/w

b.   Calcium chloride concentrations – 1M, 0.1M 0.01M

  • For the variable that was will not be varied select a value and keep it constant.

  • Prepare the beads as described in the previous section.

  • After the beads are prepared, separate them from the calcium chloride solution.

  • Prepare a phosphate buffer solution pH=7.

  • Pour approximately 10mL of buffer solution into a beaker. One of the beakers should contain only water.

  • Each student or group of students should take their beads and label them according to the way they were prepared.

  • Take beads and put one on each beaker. Make sure that for each preparation condition you have 3 repetitions. This will teach the students that science must be reproducible.

  • Label each beaker.

  • Start a timer and observe how much time it takes for the beads to dissolve. Some of them may take more than one day.


EXPERIMENTAL DESIGN


An experimental design is a statistical tool used to develop a guide for information gathering. Planning experiments in science ensure that variations between more than one variable is measured and observed. It also provides a way to minimize work, time, and money.


For this particular exercise students are asked to vary one variable and keep a second constant. This is the simplest version of an experimental design. Table 1 contains the complete version of the experimental design. Students will do a column or a row. Each experiment should be labeled appropriately. For example, if there are enough students (at least 9) each can take a row or a column, do the experiment, measure, and at the end the teacher can fill out the table with the observations (times collected). Then they can discuss why the values are different.

Table 1. Experimental Design. The cells in the center will be filled with the times that took the polymer to dissolve.

Polymer Concentration


2%


1%


0.1%

Calcium Chloride Concentration

1M




0.1M




0.01M





REFLEXION AND QUESTIONS


  • Which variable you selected and why?

  • What do you expect will be the behavior of the bead depending on the variable you selected?

  • When you collected the time, which beads dissolved the fastest? Explain why?


CROSSLINKED POLYMERS (ALL LEVELS)

  1. Each student/group should take 1 or 2 diapers.

  2. Allow students to open the diapers and cut the center very carefully.

  3. Inside the diaper there is cotton padding and embedded within there something that look like “dust”. This is the absorbent polymer.

  4. Have the students collect this dust in a beaker or cup. Make sure that the cotton padding is left behind. Depending on the brand of diaper you may need more than 2 diapers to collect enough polymer.

  5. Try to collect at least 200mg. This does not have to be exact.

  6. Divide the 200mg into three different beakers or cups. One should have ~100mg, another ~75mg, and the other ~25mg.

  7. Using a pipette or any device to measure small volumes of water measure 5mL or water. This water can be tap water.

  8. Pour 5mL into each vial. Using a spoon or a stirrer, stir the water and let the polymer absorb it.

  9. If the water is readily absorbed, add an additional 5mL.

  10. Continue adding 5mL until you observe that the water is no longer absorbed and looks in excess.

  11. Record the amount of water that was absorbed by each group.


QUESTIONS


  • Let them observe and ask them which vial absorbed the highest amount of water.

  • What happened to the size of the polymer? Did it grow?


  • Calculate how many times the polymer is capable of absorbing its own weight in water


Sample calculation


Mass of polymer in cup = 0.1g

Total volume of water absorbed = 25mL Density of water = 1.0 g/mL




Vwater  = 1 g/mL *(25mL) = 25g


Water to polymer mass ratio = mass of water/mass of polymer 25g Water/0.1 g polymer = 250 g water/g polymer

THE POLYMER IS CAPABLE OF ABSORBING 250 TIMES ITS OWN WEIGHT IN WATER!!!


This is just an example, you can be surprised that it could be much more.


ADDITIONAL QUESTIONS


  • Calculate how much water can be absorbed per gram of polymer.

  • Is the polymer capable of absorbing more water than its original weight?

  • How is this possible?

  • If you use different brands of diapers, was the water absorbed per gram of polymer similar/different?

  • Do you think is there is a difference between the efficacy of the different diaper brands?


TROUBLESHOOTING


Polymer will not dissolve.

This normally occurs if the ionic strength of the buffer is not enough to disrupt the ionic bonds.  You can solve this by adding table salt to the buffer solution.


The amount of water absorbed by each group of polymers is different.

This is likely to occur because you are adding 5mL of water without knowing when to stop. Sometimes it could be possible that you add 5mL when in reality it only needed 1 mL.  This is ok, just make sure that the differences are not astronomically big.


The polymer dust is difficult to obtain.

Each diaper will have a small amount of polymer. Try to shake it in order to get as much as you can.

Referencias Bibliográficas


Bhalerao, Rajeev S. Brownian Motion for the School-Going Child. Rep. no. TIFR/TH/04-

31. Print.


Fowler, Michael. “Brownian Motion”. Virgina University, 1 Aug. 2008. Web. 21 May 2012.  http://galileo.phys.virginia.edu/classes/152.mf1i.spring02/BrownianMotion.htm.

Química II Educación Media Editorial Santillana Encyclopedia.com (Covalent Bond):


Books on Amazon.com:


Internet: http://www.ur.mx/cursos/diya/quimica/jescobed/esteq2.htm http://www.ur.mx/cursos/diya/quimica/jescobed/estequio.htm http://www.oei.org.co/fpciencia/art08.htm

http://www.encyclopedia.com/topic/covalent_bond.aspx Encyclopedia.com (Ionic Bond): http://www.encyclopedia.com/topic/ionic_bond.aspx Wikipedia (Chemical Bonds): http://en.wikipedia.org/wiki/Chemical_bond Encyclopædia Britannica:

http://www.britannica.com/EBchecked/topic/41549/atom/260972/Atomic-bonds