Lab: Heat of Vaporization

Lab: Heat of Vaporization Submitted by: FOR THE TEACHER Andrew Lyman-Buttler Thomas Edison High Summary School in Minneapolis, In this lab, students test whether a substance’s heat of vaporization Minnesota is determined by its molar mass, the strength of its intermolecular Thanks to: forces, or both. Flinn Scientific

Resource Type Grade Level Lab High school

Objectives By the end of this lesson, students should be able to  define heat of vaporization.  explain the that molar mass does not influence a substance’s heat of vaporization.  explain the influence of intermolecular forces on heat of vaporization.

Chemistry Topics This lesson supports students’ understanding of  Intermolecular forces  Boiling point  Heat of vaporization

Time Teacher Preparation: 20 minutes Lesson: 40 minutes

Materials For each group:  Hot plate  Aluminum foil  Pipette  Safety goggles  Samples of each of the following chemicals: o Water o Ethanol o Pentane o Acetone o Ethylene glycol

Safety  Always wear safety goggles when working with chemicals in a laboratory setting.  Use caution when working with hot laboratory equipment such as hot plates. Teacher Notes  Students will each need a lab sheet as well as a table with the formula and structure of each chemical.  If the students’ data does not match either hypothesis, have them double check their prelab work to make sure their initial rankings were correct.

FOR THE STUDENT Student Activity Sheet: Heat of Vaporization

Lesson Prelab Questions 1. Define heat of vaporization. 2. If a substance’s heat of vaporization depends on its molecular mass, which would you expect to vaporize faster, a heavy molecule or a lighter one? This will be hypothesis one. 3. If a substance’s heat of vaporization depends on its molecular mass, which would you expect to vaporize faster, a molecule with strong intermolecular forces, or weak intermolecular forces? This will be hypothesis two. 4. Based on these two hypotheses you just made, fill in the table below, ranking each substance from fastest to slowest vaporization based on each of the two predictions made above. These predictions will be tested during the lab.

Strongest % of intermolecular Rank bonds that forces polarity Hyp #1 Hyp #2 Molar are polar (hydrogen 1 = prediction prediction Substance mass # of polar bond, dipole- least 1 = fastest 1 = fastest bonds dipole, or (g/mol) 5 = 5 = slowest 5 = slowest # of total London most bonds dispersion forces)

Ethanol

Pentane

Acetone Ethylene glycol

Purpose To determine whether a substance’s heat of vaporization depends on its molar mass, on the strength of its intermolecular forces, or both.

Safety Always wear safety goggles when working with chemicals in a laboratory setting.

Materials  Hot plate  Aluminum foil  Pipette  Safety goggles  Samples of each of the following chemicals: o Water o Ethanol o Pentane o Acetone o Ethylene glycol

Procedure 1. 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. (The hot plate should not be turned on yet!) 2. Turn the hot plate to 100 and wait about five minutes for the hot plate to warm up 3. Have one group member place a small drop of the chemical you are testing on the foil. (Hint: some of the liquids have very little surface tension, so they tend to run out of the pipette even if you don’t squeeze it. Be careful with these.) Have a different group member use a stopwatch to time how many seconds it takes for the drop to evaporate completely. 4. If five minutes pass and a drop still hasn’t evaporated, just write “>five min.” and stop timing. Record your data. 5. Repeat this process with the other chemicals. 6. Turn off and unplug the hot plate.

Results/Observations Hyp #1 Hyp #e Actual Seconds to prediction prediction Substance 1 = fastest evaporate 1 = fastest 1 = fastest 5 = slowest 5 = slowest 5 = slowest Water

Ethanol

Pentane

Acetone

Ethylene glycol

Analysis 1. Which hypothesis is a better fit for your data? 2. Does either hypothesis fit your data perfectly? If not, which molecule(s) did not fit? 3. What factor(s) do you think explain the vaporization time of ethylene glycol? 4. Discuss the validity of your experiment. Do you believe your results? Why or why not? How did your results compare with those of other groups? (i.e. did you get your molecules in the same order as others?) How good were your lab techniques and your following of directions?