Chemistry 11 Warren—NKEC
Name: ___________________________
Intermolecular Forces Lab Objective: Properties of matter such as boiling point, vapor pressure, and surface tension are best explained by the forces action between molecules (intermolecular forces). In this experiment, using the models, the nature of the intermolecular attractive forces for various substances will be examined. Part 1: IMFs of water, ethanol and acetone Predicting polarity based on models. Obtain a model set from the cart and after drawing the Lewis structure for the following compounds, you will make the model, sketch its 3-D shape using the wedge and dash notation, look at the intramolecular bonds to see if any of them are polar and then look at the shape of the molecule to see if the molecule is polar. You should now be able to predict the type of intermolecular forces and how the properties will vary between the molecules that you will be testing. Compounds to be studied: Water H2O
Ethanol CH3CH2OH
Acetone CH3COCH3
Chemistry 11 Warren—NKEC
Name: ___________________________
Part 2: Evaporation rate This experiment may be conducted as a group of four - one person per substance (water, ethanol, acetone). 1. Obtain 3 pieces of filter paper and five rubber bands to attach the paper to your thermometer. 2. Attach filter paper to end of the thermometer at the bulb with a rubber band and then dip into the test tube containing the compound to be studied (water, ethanol, or acetone). When the temperature has stabilized, record the temperature while the paper on the thermometer is still submerged in the liquid. This is the measurement for time = 0.0 min. 3. Your time starts when you pull it out of the liquid. Tape the thermometer to the lab bench so that the paper is hanging off the bench and you can read the thermometer. Note: If there is a big drop hanging from the paper when you pull it out then touch the drop to the side of the test tube to get rid of it. 4. Record the temperature with the correct significant figures every thirty seconds for 8 min. 5. Rank the compounds in terms of rate of evaporation from fastest to slowest.
1. ________________________ 2. ________________________ 3. ________________________
Chemistry 11 Warren—NKEC
Name: ___________________________
Critical Thinking Question 1: Does a substance that evaporates more quickly (thus giving a greater T) have a higher or lower force of attraction to other molecules in the liquid compared to a substance that evaporates more slowly (smaller T)? Explain.
Chemistry 11 Warren—NKEC
Name: ___________________________
Part 3: Polarization 1. There will be three burets set up with the liquids ethanol, acetone and water. 2. You will rub a balloon with a wool sock to create a static charge on the balloon. 3. Turn all the burets on so the liquids are flowing down in a stream into a beaker below. 4. Bring the balloon so that it is in front of all the liquid streams and slowly move it to the liquids until one of them starts to bend. 5. Try to determine which has the greatest to least deflection (bending). 6. When you are finished close the burets and pour the contents of the beakers back into the burets through the funnel. Observations of each chemical:
Place the chemicals (acetone, water, ethanol) in order of most deflected to least deflected. 1. ________________________ 2. ________________________ 3. ________________________
Critical Thinking Question 2: If the static electric charge is composed of negative electrons, which chemicals should be deflected more, polar or nonpolar ones?
Chemistry 11 Warren—NKEC
Name: ___________________________
Part 4: Viscosity You will study three tubes, each containing a liquid and a marble. When you invert the tube, the marble will fall to the bottom. The rate of fall depends on the viscosity of the liquid which is dependent on intermolecular forces. The falling marble must break connections as it travels down. The stronger the connections the slower the marble goes. Record the relative speed of the marble and put the substances in order of fastest (least viscous) to slowest (most viscous). If you get any that are really close, try starting them at the same time as in a race. Note: To the water was added a drop of blue food coloring and to ethanol was added one drop of red food coloring. This will not affect the viscosity of either one. Comments on liquids in tubes with marble: Ethanol (blue) Vegetable oil Water (red) Put them in order of fastest to slowest: 1. ________________________ 2. ________________________ 3. ________________________
Critical Thinking Question 3: Which should be more viscous, polar or nonpolar molecules?
Chemistry 11 Warren—NKEC
Name: ___________________________
Part 5: Surface tension and vortex When a liquid is swirled, a vortex is developed in which the surface 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. Try to swirl each of the prepared flasks with the same intensity and record the time it takes for the vortex to disappear. Which liquid appears to have greater surface tension and greater intermolecular forces? Record your answer.
Part 6: Surface tension and droplet shape Using an eyedropper, transfer one drop of each fluid onto a penny. The liquid with greater surface tension will hold more drops of liquid on the penny than the liquid with the lower surface tension. Which liquid appears to have the greater surface tension and greater intermolecular forces? Record your answer.
Chemistry 11 Warren—NKEC
Name: ___________________________
Part 7: Visualization of surface tension The surface of a liquid with strong hydrogen bonding will exhibit great tension much 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, another with isopropyl alcohol, and the third with glycerin. Sprinkle crushed pepper on the surface of both. The pepper will be more likely to float on the fluid with greater surface tension. Cover the tip of a paper clip with liquid dish soap and hold over the center of each Petri dish until a drop of soap falls into the liquid. If the surface of the liquid is under tension, the pepper will recoil towards the sides immediately (see picture). Which liquid appears to have the greater surface tension and greater intermolecular forces? Record your answer.
Clean all equipment and glassware. Return materials to the community area. Wipe down the lab bench. Do not remove your safety goggles until ALL GROUPS have finished cleaning up.
Chemistry 11 Warren—NKEC
Name: ___________________________
Part 8: Post lab questions H2 O
C3H8O
C3H8O3
1. Are all of the above chemicals polar? Rank these chemicals from least polar to most polar and justify your answer based on your models.
2. What type of intermolecular forces (LDF, D-D, H-bonds) would you expect for each molecule based on the VSEPR shape?
Chemistry 11 Warren—NKEC
Name: ___________________________
3. Is your answer in #2 consistent with your laboratory observations? Support your statements for each of the three liquids with references to specific observations you made during the lab.
4. Which of the liquids you tested (isopropyl alcohol, water and glycerol), displayed the greatest surface tension (greatest intermolecular forces)?
5. Which of the liquids you tested (isopropyl alcohol, water and glycerol), do you think will boil most easily? Why? Be specific.
6. You may have noticed mosquitoes, water striders, and other insects walking on the surface of a pond. Why don't they sink?
