Posted by Anonymous on Monday, May 18, 2009 at 2:32am.
I did a lab and got a bunch of questions about the lab.
Here is what my lab was about:
Properties of alcohols:
First, I wrapped the bulb of a thermometer with a tissue secured with a wire. Then I squeeze enough methanol onto the tissue to completely saturate. At the same time, another person starts the stopwatch, reads the temperature and, and records it. Then I fanned the tissue-covered thermometer bulb with a piece of cardboard or other stiff paper. After one minute, read and record the final temperature. And we repeat doing these steps for the other alcohols.
Here are my questions:
1. Why did the temperature change during the experiment?
My guess: As we fanned, the temperature is lowered as heat transfers to the surroundings?
2. Assume that the three alcohols have approximately the same molar enthalpy of vaporization. What can you say about the relative rates of evaporation of the three alcohols?
My guess: Their relative rates will approxiamately be the same?
3. Consider the answer to question 2, what can you conclude about the relative strength of intermolecular forces existing in the three alcohols?
My guess: The relative strength of intermolecular forces existing in the three alcohols are pretty much the same as their enthalpy of vaporization are the same?
4. Suggest a way to make this experiment more quantitative and controlled.
My guess: Repeat the procedures for methanol with water and more other alcohols?
5. How can this experiment help explain why small chain alcohols have a warning label indicating that they are flammable?
My guess: Small chain alcohols have lower enthalpies of vaporization which allow them to be vaporized in lower temperatures.(I don't really know the answer)
6. A mixture of 70% 2-propanol(isopropanol) and 30%water is sold as rubbing alcohol, which may be used to help reduce a fever. Explain how this process works.
I only know the enthalpy of vaporization of water is lower than the enthalpy of vaporization of 2-propanol.(Don't really know the answer)
7. Why do you suppose that 2-propanol is a component in some producs used to soothe sunbrned skin?
No ideas on this.
It's really too many questions on this. Thanks for your patience! I really appreciate that.
Chemistry - DrBob222, Monday, May 18, 2009 at 2:10pm
1. A good answer but it leaves out a couple of important points. One is that it takes energy to evaporate the molecules; i.e., move the molecules from the liquid state to the vapor state. Only the faster molecules make the escape. So the average energy of the liquid state decreases and that results in a lower energy/temperature.
2.I assume you answer is a good one but it would have been nice for you to include the names of the other alcohols you used.
3. Sounds ok to me but the REAL test is that the temperature should be lowered by the same amount for the different alcohols.
4.You didn't describe the experiment in enough detail for me to comment.
5. Small chain compounds vaporize more easily than long chain compounds and more vapor more easily MAY mean more of a hazard from combustion BUT not all vapors are combustible. For example, CCl4 is easily vaporized but it isn't combustible.
6. Think of the skin as being the alcohol soaked mesh on the end of the thermometer. As alcohol is rubbed on the body, the alcohol evaporates, the alcohol cools, the body cools, and the patient's fever subsides.
7. Because the evaporation of the alcohol cools the sunburned skin.
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