Materials Included In Kit

Additional Materials Required

Safety Precautions

Ethyl alcohol is a dangerous fire risk; it is flammable. The addition of denaturants makes ethyl alcohol poisonous. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Have students wash hands thoroughly with soap and water before leaving the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Ethyl alcohol may be disposed of according to Flinn Suggested Disposal Method #26b.

Teacher Tips

• Enough materials are provided in the kit for 30 students working in pairs. All parts of the lab can easily be completed in one 50-minute class period.
• Water is not attracted to polyethylene (there is no adhesion between the drop and the polymer). Each molecule in the water drop is attracted to the other water molecules in the drop. This causes the water to pull itself into a shape with the smallest amount of surface area, a bead (sphere). See Figure 1. All the water molecules on the surface of the bead are “holding” each other together or creating surface tension. However, the water is attracted to the glass because of the ions encapsulated in the glass. The ion–dipole attraction makes the water droplet spread on the surface and also creates an interesting attraction between the two glass plates when they are compressed.
  {13277_Tips_Figure_1}
• The surface tension of the liquid drop is directly related to the volume and density of the liquid you are testing. Water droplets will be larger in volume because of its stronger intermolecular attractions.
• Cohesive forces are the electrostatic forces hold molecules together. In water, these are primarily hydrogen bonding and dipole–dipole attraction. Adhesive forces are electrostatic forces between molecules of different substances. In the interaction of a glass tube with water, the adhesive forces between the polar water molecules and the polar Si-O bonds at the surface of the glass are greater than the cohesive forces between the water molecules.
  
  The water is pulled up the sides of the tubing until the weight of the water column just balances the total adhesive forces between the glass and water molecules (see Figure 2). The smaller the diameter of the tubing is, the greater the height of the column.
  {13277_Tips_Figure_2}

  For the water molecules at the surface away from the glass wall are attracted inward and form a downwardly curving surface, familiar to all as the meniscus formed in burets, pipets and other glass apparatus.
  
  Glass contains ions encapsulated into the silicon dioxide. Polar molecules have an affinity to the ions. If the molecules are light and have a strong dipole moment, they will rise rapidly.

• The students may have difficulty in explaining the cause of the reduced volume when water and ethyl alcohol are mixed. When 25 mL of water is added to 25 mL of water or when 25 mL of alcohol is added to 25 mL of alcohol, the final volume will always be 50 mL, as expected. In this demonstration, when the water is added to the alcohol, the final volume is about 10% less than the original volume of the two liquids. The “vanishing volume” is due to differences in packing of the solvent molecules in the mixture versus the pure substances. Molecules of ethyl alcohol actually pack together more closely with water molecules than with other alcohol molecules due to hydrogen bonding. The solvent molecules form a highly laced, 3-dimensional network held together by strong hydrogen bonds (see Figure 3). Each alcohol molecule is able to form as many as three hydrogen bonds with neighboring water or alcohol molecules. The result is an intricate lattice or network of molecules strongly attracted to one another. Looking at the molecules:
  {13277_Tips_Figure_3}
• In this demonstration, ethyl alcohol evaporates faster than water. This is due to the strong hydrogen bonding in water and to a lesser extent in ethyl alcohol. Water will form little streams and does not spread out as much as ethyl alcohol. This is due to the large surface tension in water caused by the hydrogen bonding.
• If there are not enough burets for each student group to perform step 14–20, the groups can be combined to accommodate the number of available burets. Graduated cylinders can also be substituted. Add the liquids in the same manner, then mix by stirring.

Answers to Prelab Questions

1. Which of the following should exhibit hydrogen bonding?

   a. CH₃OCH₃     b. HF     c. CH₄     d. LiH
   b. Only HF has a hydrogen bonded to a highly electronegative atom, F, and a highly electronegative atom with a lone pair of electrons, :F.

2. Rank the following in order of increasing boiling points.

   a. CO     b. C₂H₆     c. NH₃
   NH₃ > CO > C₂H₆. NH₃ is polar and exhibits hydrogen bonding. CO is polar and exhibits dipole–dipole attractions, while C₂H₆ is nonpolar and will exhibit only weaker London dispersion attractions.

3. Why is the specific heat capacity for water larger than most other liquids?

   Water, with two hydrogens available for hydrogen bonding in each molecule, has a large capacity for hydrogen bonding, requiring a large amount of energy to overcome these attractions.

Sample Data

Answers to Questions

1. Which liquid was least attracted to each of the surfaces? How could you tell that this was true?

   Water was least attracted to the plastic slide and ethyl alcohol was least attracted to glass. The least attracted liquid to a particular surface formed a bead. The more attracted spread out on the surface.

2. In step 3, which of the water drops was the flattest and widest? What does this mean about the attraction of the molecule to the surface?

   The water drop on the glass slide was the flattest. The attraction of water to glass formed the strongest attraction.

3. Can you determine whether the polyethylene is made of polar or nonpolar molecules?

   If polyethylene were polar, water, being more polar than ethyl alcohol, would spread out flatter across the surface than ethyl alcohol.

4. When you compressed the slides in step 4, which slide seemed the most difficult to separate?

   The glass slides with water between them.

5. Which liquid took the most drops to fill the microspatula in steps 9 and 10? What does this indicate about the size of the drops? How does the size of the drop relate to the attraction of the molecules to each other?

   Ethyl alcohol took the most drops to fill the microspatula and therefore had the smallest drops. The larger the intermolecular attractions, the greater the surface tension and the drop. Ethyl alcohol must have weaker intermolecular attractive forces than water.

6. When you compare the size of the molecules of water and ethyl alcohol, which is bigger? How does this compare to the size of the drops?

   Ethyl alcohol molecules are larger than water molecules, just the opposite of their drop sizes.

7. The glass capillary tube contains encapsulated ions at the surface; based on this fact, which molecules shows the greatest ion–dipole attraction?

   Since the water rose higher in the tube than did ethyl alcohol, water would have the greater ion–dipole attraction.

8. In step 24 when the drops run down the dry erase board, which liquid seemed to spread out on the surface?

   Ethyl alcohol

9. Which liquid took longer to evaporate? What does this imply about the attraction of the molecules to each other?

   Water took the longest time to evaporate. The attraction between water molecules is greater than that between ethyl alcohol molecules.

10. Which liquid has weaker intermolecular attraction and which has the stronger intermolecular attractions?

    Based on the data, water has stronger intermolecular attractions than ethyl alcohol.

11. (Optional) Speculate on the cause of the observed results when water and ethyl alcohol were mixed in step 21.

    Answers will vary. See Tips.

References

Special thanks to Mark Langella, Mahopac High Schook, Mahopac, NY, for providing Flinn Scientific with the idea and procedures for this lab.

Introduction

The forces that act between molecules, called intermolecular forces, play a significant role in many aspects of chemistry, from boiling point trends and the solubility of gases, liquids and solids to the structure of DNA and proteins. A series of experiments will be performed to investigate the effect of intermolecular forces on the properties of compounds.

Concepts

• Dipole–dipole interactions
• Surface tension
• Capillary action
• Hydrogen bonding
• Evaporation

Background

Intermolecular forces include dipole–dipole attractions, hydrogen bonding, dipole-induced dipole attraction and London dispersion forces. All of these types of forces are electrostatic in nature. Electrostatic forces arise when the molecules contain or are capable of creating areas of charge separation.

For the two compounds to be studied in this experiment, ethyl alcohol and water, all four types of intermolecular forces may exist between molecules. However, dipole–dipole interactions and hydrogen bonding play the most important roles in determining the overall properties of the compounds. Dipole–dipole interactions occur only in polar compounds. The greater the polarity of the molecules, the larger the force of attraction between those molecules.

Many compounds that contain an O—H or N—H bond exhibit a specialized form of dipole–dipole attraction called hydrogen bonding. Hydrogen bonding occurs in molecules where hydrogen is bonded to a highly electronegative atom (X). The difference in electronegativity between H and X creates a large charge separation in the bond.

If the molecule also contains a highly electronegative atom with a lone pair of electrons, this lone pair is strongly attracted to the now partial positive charge on the hydrogen atom in a neighboring molecule. This is hydrogen bonding. Both ethyl alcohol and water contain —O—H bonds capable of forming strong hydrogen bonds. Because they possess strong intermolecular attractive forces, water and ethyl alcohol have higher melting and boiling points than similar-sized nonpolar molecules. For different compounds to form solutions, the intermolecular forces between the molecules must be similar to allow for the separation and mixing of the two substances. Without this similarity, the substances will remain separated. Hence the phrase, “like dissolves like.” The interactions of noncrystalline solids and liquids are also a function of the molecular forces that occur at the surface of the solid and liquid. The attractive forces between molecules of the different substances are called adhesive forces, while those between the molecules of the same substance are called cohesive forces. These forces come into play in surface tension and capillary action. At the surface of a liquid, the only forces on the molecules are inward. Unless the molecules touch a solid surface, the liquid will contract on itself and form a spherical drop. Surface tension is a measure of the force needed to break through the surface of the drop and spread the substance out as a film. The greater the forces of attraction between the molecules of a liquid, the greater the surface tension.

Experiment Overview

The purpose of this experiment is to study the effects to intermolecular forces on the properties of water and ethyl alcohol and to determine the relative strength of these forces in each compound. Various physical tests are used to compare the relative properties of water and ethyl alcohol.

Materials

Prelab Questions

1. Which of the following molecules will exhibit hydrogen bonding?

   a. CH₃OCH₃     b. HF     c. CH₄     d. LiH

2. Rank the following compounds in order of increasing boiling points.

   a. CO     b. C₂H₆     c. NH₃

3. Water has a much higher specific heat capacity than most liquids. Explain in terms of the attractive forces between water molecules.
4. Why is ethyl alcohol used in the lab poisonous?

Safety Precautions

Ethyl alcohol is a dangerous fire risk; it is flammable. The addition of denaturants makes ethyl alcohol poisonous. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Procedure

1. Obtain two slides, one glass and one polyethylene.
2. Using a microtip pipet, add a drop of water to each slide.
3. Using a millimeter ruler, measure the drop width and height on both plates. Record these values in the data table.
4. Thoroughly dry the slides and repeat steps 2 and 3 using ethyl alcohol.
5. Thoroughly dry the glass slide and polyethylene slide. Obtain a second glass slide and another polyethylene slide.
6. Wipe off the surfaces of the four slides. Place a drop of water on one glass slide and one drop on the polyethylene slide.
7. Place the second glass slide over the first one. Place the second polyethylene slide over the first one.
8. Compress the water between the slides. Try separating the slides. Record the relative force needed to separate the slides in the Data Table.
9. Obtain a microspatula. Using a micropipet, count the number of drops of water it takes to fill the microspatula, in that the liquid is level with the sides of the scoop. Record this value in the Data Table. Note: The micro-tip pipet must be kept level for this comparison to be accurate.
10. Repeat step 9 for ethyl alcohol.
11. Obtain a disposable Petri dish. Fill the top with 10 mL of water and the bottom with 10 mL of ethyl alcohol.
12. Obtain two capillary tubes and the 15-cm ruler at the top of the Intermolecular Forces Worksheet.
13. Place one capillary tube in the Petri dish with water. Use the ruler to measure, in millimeters, the height of water in the capillary tube. Record this value in the data table.
14. Repeat step 13 with a new capillary tube for the Petri dish bottom containing ethyl alcohol.
15. Obtain a clean 50-mL buret, a 25-mL graduated cylinder, and a rubber stopper.
16. Use the 25-mL graduated cylinder to measure out 25 mL of distilled water.
17. Close the stopcock of the buret and fill the buret with the distilled water. 
18. Measure out 25 mL of ethyl alcohol.
19. Carefully and slowly add ethyl alcohol to the buret, keeping the mixing of the two liquids to a minimum.
20. Firmly, but with care, seat the rubber stopper in the end of the buret.
21. Mix the two liquids by repeatedly inverting the buret. Check the areas around the stopcock and rubber stopper to verify no air is leaking into the buret.
22. Continue inverting until no areas of mixing are observed in the buret.
23. Place the buret in the buret clamp and measure the final volume of the solution. Record this volume in the data table. Calculate the percent volume change and enter this value in the data table.
24. Use a dry erase marker to place a mark halfway up a dry erase board.
25. Fill two micropipets, one with water and one with ethyl alcohol.
26. Place a drop of each liquid at the mark. Note the path or pattern that each liquid takes to reach the bottom. Record your observations in the data table.
27. Use the micropipets to place a drop of each liquid on a glass slide. Determine the time, in seconds, for each drop to evaporate. Record this time in the data table.
28. Dispose of the ethyl alcohol as directed by the instructor.

Student Worksheet PDF

13277_Student1.pdf