Intermolecular Forces and Properties of SubstancesObjectives
This lab has two parts. In successfully completing this lab you will:
Understanding the properties of substances is crucial to designing laboratory experiments. Many properties of liquids and solids are affected by the intermolecular forces experienced by these substances. The type and strength of the intermolecular forces a molecule will experience is largely related to the polarity of the molecule. For lab this week we review polarity, and explore intermolecular forces. We will also investigate how intermolecular forces affect various properties of substances.Experiment Part 1: Polarity and Intermolecular Forces
Using the PhET simulation “Molecule Polarity”.
Adapted from “Intermolecular Forces and Molecules” by Ted Clark and Julia Chamberlain. (CC by 4.0) Link to original file here.
Part 2: Intermolecular Forces and Properties
Polarity affects the type and strength of intermolecular forces a substance will experience. In turn, the strength of intermolecular forces affect some properties of the substance. We will now investigate this further by looking at the different types of intermolecular forces, various substances, and their properties. Please use the data provided to answer the questions. Then, use your observations to make predictions about properties of substances.
Intermolecular Forces and Evaporation
In order for molecules to evaporate from the surface of a liquid, they must have enough kinetic energy (obtained from collisions with neighboring molecules) to break the attractions (intermolecular forces) they have for their neighbors. When the molecules escape from the gas phase, they carry this kinetic energy away with them. Therefore, evaporation is an endothermic process that cools the remaining liquid.
Figure 1: Evaporation of liquid particles.
Molecules that are not very sticky (i.e. those that have small intermolecular forces) can evaporate rapidly, causing the temperature of the liquid to drop quickly. Conversely, molecules with strong intermolecular forces will not experience much evaporation and little cooling will be observed.
The change in temperature during a two minute evaporation period was recorded for several substances and organized in the table below. Use the following data to answer the next few questions:
Table 1: Change in Temperature During Evaporation and Total Number of Electrons in Various Hydrocarbons.
|Compound||Change in T during evaporation (0C)||Total number of electrons in compound|
1. The compound with the strongest intermolecular forces is ________. The compound with the weakest intermolecular forces is ________.
2. All of these compounds are polar/nonpolar (circle one). The predominant intermolecular force found in these substances is ________.
3. Based on the data above, the more/less electrons a substance has, the stronger the __________.
Compare two liquids: acetone (C3H6O) and 2-butanone (C4H8O).
4. Of the two liquids above, ________ would have the stronger intermolecular forces.
5. Of the two liquids above, ________ would show the greater change in temperature during evaporation in a two minute time period.
Intermolecular Forces and Boiling Point
The boiling point is the temperature at which the liquid and gas exist in equilibrium. At this temperature the liquid molecules have gained enough energy to overcome the attractive intermolecular forces and escape into the gas phase. Also at this temperature, the gas molecules have lost enough energy to succumb to the intermolecular attractive forces and condense into the liquid phase. Use the following table to answer the questions below:
Table 2: Structures, Molecular Weight and Boiling Points of Several Substances.
|Compound||Structure||Molecular Weight (g/mol)||Boiling Point (0C)|
1. Pentane is a polar/nonpolar molecule. The predominant intermolecular force in pentane is LDF/D-D/H-Bonding. (Circle one)
2. 1-butanol is a polar/nonpolar molecule. The predominant intermolecular force in 1-butanol is LDF/D-D/H-Bonding. (Circle one)
3. 2-butanone is a polar/nonpolar molecule. The predominant intermolecular force in 2-butanone is LDF/D-D/H-Bonding. (Circle one)
4. Rank the three principle intermolecular forces in order of weakest to strongest.
5. The stronger the intermolecular force, the lower/higher the boiling point. (Circle one)
6. Pentane, 1-butanol and 2-butanone share an intermolecular force that is approximately the same strength for all three compounds. What is the intermolecular force, and why is it approximately the same strength for all three compounds. Explain.
7. Draw Lewis Structures for acetone (C3H6O), butane (C4H10) and isopropanol (C3H8O) and determine the predominant intermolecular force in each compound. Use the structures to predict the boiling points given the data below:
|Compound||Boiling Point (0C)|
Please show all work below.
8. Explain your answers for Question 7. Why did you place each compound where you did?
9. Draw Lewis Structures of the following compounds and determine their predominant intermolecular forces:
1. ethane (C2H6)
2. ethylene glycol (CH2OHCH2OH)
3. propane (C3H8)
4. ethanol (CH3CH2OH)
5. dimethyl ether (CH3-O-CH3)
Use the structures to predict the boiling points given the data below:
|Compound||Boiling Point (0C)|
Please show all work on next page.
Lewis Structures for Question 9:
10. Explain your answers for Question 9. Why did you place each compound where you did?
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