Chemical Bonding Introductory Investigation

Prelab:

1. What type of attractions hold atoms together as a molecule?

a. intramolecular (bonds within the molecules)

b. intermolecular (attractions between molecules)

2. What type of attractions hold molecules together as a liquid or solid?

a. intramolecular

b. intermolecular

3. What 2 things determine the strength of chemical bonds?

4. In a bond between phosphorous and fluorine, which element pulls the shared electrons closer to its nucleus?

Will this make that atom more negative or positive?

Surface tension is the force that pulls adjacent molecules or atoms on the surface of a liquid together. The greater the surface tension, the more difficult it is to penetrate the surface of a liquid. Hydrogen bonding, a strong intermolecular force, increases surface tension by bonding surface molecules to each other. In this activity you will investigate the behavior of liquids to compare the strength of intermolecular forces between the molecules of the liquid.

Part 1: Surface tension and vortex: When liquid is swirled, a vortex is developed in which the level of the center of the liquid is substantially below the surface level of the perimeter. The greater the surface tension, the longer the vortex will remain after you have stopped swirling the container. Swirl each of the two flasks (water and isopropyl alcohol) to create the vortex. After setting the flask down, time until the vortex can no longer be seen. Repeat this two more times and determine the average time for each flask. Which liquid has the greater surface tension? Explain using your data.

Part 2: Surface tension and droplet shape: Using a dropper pipet, transfer on drop of each fluid (water, isopropyl alcohol, and oil) to a sheet of wax paper. Sketch a diagram of what the droplets look like from the side. How do the drops compare? Which of the liquids has the greatest surface tension (attraction between molecules)? Explain.

Part 3: Surface tension and impermeability: Liquids with strong intermolecular bonding will be less penetrable than those with weaker intermolecular bonding. Try to float a paper clip on both water and isopropyl alcohol by gradually lowering a dry paper clip into each liquid on a cradle fashioned from another paper clip. Which liquid appears to have the greater surface tension and greater intermolecular forces?

Part 4: Visualization of surface tension: The surface of a liquid with strong hydrogen bonding will exhibit great tension like the head of a drum that has been pulled tight. If a drumstick ruptures the head of a drum, the sides recoil under the tension. In a similar manner, if a chemical ruptures the surface tension of a fluid, the “skin” of the liquid will recoil away from the point where the chemical was applied. Fill one petri dish with water and the other with isopropyl alcohol. Sprinkle crushed pepper on the surface of both. Cover the tip of a toothpick with liquid dish soap and hold over the center of each petri dish until the drop of soap falls into the liquid. Describe what happens when the soap hits each liquid. Which liquid appears to have the greatest surface tension? How can you tell?

Part 5: Comparing surface tensions of liquids: Obtain two pennies. Carefully add drops of water to the top of each penny until the penny will not hold any more drops without spilling. Record the number of drops of water the penny held. Repeat this two more times and record the average number of drops of water the penny held. Now do the same thing only this time use isopropyl alcohol instead of water. How does isopropyl alcohol compare to water? Why do you think there is this difference?

Part 6: Paper chromatography: Capillary action is a process by which water is pulled to a different part of a substance such as paper. Obtain about a 20 cm length of chromatography paper and draw a pencil line across the width of the paper about 4 cm from one end. Then make a dot with a black overhead marker just above the line to the left of center. Repeat this with a green or red marker placing the dot to the right of center. Place the chromatography paper in a beaker of water so that the water level is just below the pencil line. Hold the paper in this position for 3 – 4 minutes. What might have been the cause of the observations you made? Why was there a difference between the two markers?

Part 7: Cat's Meow

This activity is used to arouse interest in a common substance: milk. Students are asked to form a hypothesis about the behavior of milk as household detergents act upon it.

1.Pour milk into a petri dish to a depth of 1 cm. (1/2 inch).

2.Add a couple of drops of four, differently colored, water-soluble food coloring near the edge of the container. Arrange the drops so that they are in positions equivalent to 3, 6, 9, and 12 on a clock (0º, 90º, 180º, and 270º).

3.Dip the tip of a toothpick in detergent. Touch the surface of the milk in the center of the petri dish and hold the toothpick in place for a while. What happens?

4.Experiment with the milk and toothpick. How is it possible that the fairly quiet pan of milk is now exhibiting such activity? Suggest a hypothesis that might explain the phenomena that you observed.

5.Repeat steps 1 – 4 except substitute low fat milk or dilute the original milk in half with water. How does what happened in step 5 compare to the original observations? Suggest a possible explanation.

Average Composition of Milk

water 87 %
total solids 13 % proteins (casein) 3 - 4 %
lipids (triglycerides) 3.5-5 %
sugars (lactose) 4.5-5 %