Name:_____________________
Heat of Vaporization 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.
Substance
Molar mass (g/mol)
% of bonds that are polar # of polar bonds # of total bonds
Strongest intermolecular forces (hydrogen bond, dipoledipole, or London dispersion forces)
Rank polarity 1 = least 5 = most
Hyp #1 prediction 1 = fastest 5 = slowest
Hyp #2 prediction 1 = fastest 5 = slowest
Water
Ethanol
Pentane
Acetone Ethylene glycol American Association of Chemistry Teachers
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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 Substance
Seconds to evaporate
Hyp #1 prediction 1 = fastest 5 = slowest
Hyp #e prediction 1 = fastest 5 = slowest
Actual 1 = fastest 5 = slowest
Water Ethanol Pentane Acetone Ethylene glycol
American Association of Chemistry Teachers
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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?
American Association of Chemistry Teachers
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