Lab #11: Investigating Intermolecular Forces (mini-lab)

Transcription

1 (mini-lab) Part One: The Penny Drop Activity What effects do intermolecular forces have on surface tension? Name Materials: Pennies Water Rubbing Alcohol (ethyl or isopropyl will work fine) Dec. 14, 2016 Safety Concerns Rubbing alcohol is toxic and flammable. Care should be exercised when using. Real-World Connections Surface tension is important in the functioning of living organisms. For example, water striders (a type of insect) are able to walk on the surface of ponds. Procedure: 1. Place a penny face down on the table. 2. CAREFULLY add drops of water on top of the penny. Your goal is to put as many drops on top of the penny without any water falling onto the table. 3. Count the number of drops that the penny was able to hold. RECORD YOUR DATA. 4. Dry the penny. 5. Have each person in the group repeat the process. RECORD YOUR DATA. 6. Be sure to observe the penny from the side. 7. CAREFULLY add drops of alcohol on top of the penny. Your goal is to put as many drops on top of the penny without any alcohol falling onto the table. 8. Count the number of drops that the penny was able to hold. RECORD YOUR DATA. 9. Dry the penny. 10. Have each person in the group repeat the process. RECORD YOUR DATA. 11. Be sure to observe the penny from the side. Water Drops Alcohol Drops Trial 1 Trial 2 Average 1

2 1. Which liquid had the higher average number of drops? 2. What intermolecular forces are present in water? 3. What intermolecular forces are present in alcohol? 4. Suppose you were to try this experiment with acetone, a liquid that does not have any hydrogen bonding, is non-polar, and has a molar mass of 58.08g/mol. How would you expect the result to be in comparison to the water and alcohol? Explain your answer using at least 2 complete sentences. Part Two: Evaporation Test How does the presence of intermolecular forces affect the rate of evaporation of various liquids? Materials: Tap water Distilled water Olive Oil Hot plate Small beaker Aluminum foil Pipette Isopropyl alcohol Safety Concerns Hot plates can burn students and isopropyl alcohol is flammable. Care should be exercised and small quantities of alcohol should be used. Real-World Connections: Certain liquids must be stored in air-tight containers because they can evaporate quickly. Gasoline is an excellent example of this. Procedure: 1. Obtain a hot plate, enough aluminum foil to cover the heating surface, small beaker, a bottle of isopropyl alcohol and a bottle of distilled water. 2. Cover the top of the hot plate with aluminum foil. Crimp the edges so that the foil is secured to the top of the hot plate. Turn the hot plate to 8 and wait about 5 minutes for the hot plate to warm up. 3. With no overlap, place a small drop each of olive oil, tap water, distilled water and alcohol on the foil. Time how long it takes for each drop to evaporate. Stop at 5 minutes. Carefully observe any residue from the drop 2

3 Evaporation Test Time Observations Isopropyl alcohol Distilled water Tap water Olive oil Data Analysis: 1. What is the chemical formula and molar mass for water? Isopropyl alcohol s formula is C3H7OH (CH3CH(OH)CH3). Draw the structural formula for each. Based on the molar mass, which compound would you expect to evaporate first? Based on what you know about intermolecular forces, how do you explain the result you obtained? 3

4 2. What is the difference between tap water and distilled water? Was there an appreciable difference between the times for the two types of water? What was it (if any) and how do you explain it? Was there any difference in the residue? What was it (if any) and how do you explain it? 3. Draw structural formulas for each of the substances you evaporated on the hot plate. Chemical formulas are: Isopropyl alcohol CH3CH(OH)CH3, Water H2O, Olive oil (predominantly oleic acid) C18H34O2 [set up structural formula as CH3(CH2)7CHCH(CH2)7COOH linking the carbon atoms in a chain and adding the hydrogens and oxygens in the order given]. Describe each molecule in terms of polarity and presence/absence of hydrogen bonding and length of nonpolar portion of molecule. What would you predict about boiling points based on this analysis? 4

5 4. Write a paragraph summarizing the concepts covered in this lab and what you learned that you didn t know before. There should be a minimum of five sentences and you should use scientific language to describe your ideas. 5