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Name: Michael
Status: student
Grade: 9-12
Country: USA
Date: Fall 2009


Question:
In regards to your response in Alcohol vs Water Evaporative Cooling,

http://www.newton.dep.anl.gov/askasci/chem07/chem07113.htm :

The original question was "When rubbing alcohol evaporates, it 'feels' colder than water. Is this because it requires more energy for a phase change than water?" Your response was that rubbing alcohol requires more energy for changing from liquid to gas, due to both the phase change (heat of vaporization) as well as temperature change (specific heat capacity) while in the liquid phase. Where does the different volatilities of the two compounds come into this picture? I thought the increased volatility of rubbing alcohol (and higher tendency for evaporative cooling) was the main reason why it feels colder to the skin than water does.



Replies:
This is the answer to which you are referring:

"You are correct in thinking that the difference in "feel" is a function of the heat of vaporization. Iso-propyl alcohol (the main component of rubbing alcohol) has a dH = 44 kJ/mol versus the dH of water which is 41 kJ/mol. Vaporizing alcohol requires more energy. However, another factor is the specific heat of iso-propyl alcohol is 154 J/mol.K, that of water is 76 J/mol.K. This means for every degree temperature change in alcohol, a lot more energy is absorbed."

When you say rubbing alcohol feels cooler than water, you are reporting the experience that more heat is removed from the body by rubbing alcohol than by water.

Volatility is the capacity of a substance to change from a liquid to a gas. This characteristic is expressed by the heat of vaporization factor. So your conclusion is accurate. This explains why the evaporation of rubbing alcohol feels cooler than the evaporation of water.

Sincere regards,

Mike Stewart


I have read the original answer in the archives, and I won't argue with its facts and figures. However, I think it may be misleading. It quotes the molar enthalpies of vaporization of water and isopropyl alcohol, notes that the value is higher for isopropyl alcohol, and lets it go at that. What this means in English is that a molecule of isopropyl alcohol absorbs more energy (heat) when it evaporates than a molecule of water does.

However, the molecules aren't the same size. The molar mass of water is 18 g/mol, while isopropyl alcohol is 60 g/mol, more than triple water's. And note that their enthalpies of vaporization aren't very different--that means that when 1 gram of water evaporates, it absorbs nearly three times as much heat as 1 gram of isopropyl alcohol evaporating.

Now, isopropyl alcohol boils at a lower temperature than water, and it is more volatile than water at lower temperatures as well. This means that its equilibrium vapor pressure is higher at all temperatures. The effect of this is that if you have equal amounts of isopropyl alcohol and water on your skin, much more of the alcohol will evaporate over a short period of time. This means the evaporation process will consume more energy per second. Although evaporating equal masses of alcohol and water will require much more energy in the case of the water, it consumes energy faster in the case of the alcohol.

Richard Barrans, Ph.D., M.Ed.
Department of Physics and Astronomy
University of Wyoming


Hi Michael,

Unfortunately "volatility" is a confusing term because it is used colloquially to mean two different things. It is often used to distinguish whether a substance has a lower boiling point, and it is also used to mean that a substance requires less energy in order to volatilize. Moreover, volatility is also a function of how strong the intermolecular attractive forces are in the liquid. As such, we should talk about boiling point versus evaporation, and the energy requirements for the process.

While it is true that alcohol has a lower boiling point than water (and will volatilize sooner given the same conditions), it is also true that alcohol requires more energy become vapor (as mentioned, because of its higher specific heat and enthalpy of vaporization).

Since we are essentially talking about a non-equilibrium condition of evaporating a liquid (rather than having it boil), then the issue is not whether alcohol reaches its boiling point sooner, but rather how much energy is required to convert a liquid to gas. In this case, alcohol requires more energy and therefore extracts more heat from our skin and so feels cooler.

Greg (Roberto Gregorius)
Canisius College



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