| [GMCnet] Ethanol in gasoline [message #186303] |
Wed, 03 October 2012 13:32  |
emerystora
Messages: 4442
Registered: January 2004
Karma:
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Senior Member |
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We seem to go over and over this topic every year for the last several years.
Recently it was posted that ethanol in gasoline is really not bad and should not cause vapor lock because ethanol has a higher boiling point than gasoline.
This is incorrect and is typical reasoning that one who doesn't have training in chemistry can easily assume. Unfortunately this overlooks what happens when different chemicals are mixed. Mixtures often take on entirely different properties including boiling points.
Here is something that Larry Davick and I posted about a year ago referring to a post that I made in 2010.
>
> On Aug 24, 2011, at 6:43 PM, Larry Davick wrote:
>
>> From ask . com
>>
>> "Alcohol has a boiling point of 72 degrees C or 173 degrees F, which is only slightly lower than the boiling point of water."
>>
>> If we can assume that gasoline has a higher boiling point than alcohol, then we can be assured that blended gasoline will boil at some temperature above 173 degrees F, at sea level.
>>
>> But it need not boil to vaporize. You can dry your laundry on the line in winter - even ice evaporates - check out the old ice in your freezer. (My freezer has no old ice - I'm careful to rotate it through an alcohol bath from time-to-time.)
>>
>> Larry Davick
>> Fremont, California
>> The Mystery Machine
>> '76 (ish) Palm Beach
>
> Larry
>
> using the boiling point of Ethanol by itself to predict the boiling point of a mixture with gasoline can be very misleading.
>
> Here is something that I had posted in June of last year:
>
> ------------
> When it comes to predicting the boiling points of mixtures it get very complicated. Adding more ethanol doesn't necessarily mean that the boiling point will get closer toward the alcohol's boiling point.
> Mixtures of gasoline and ethanol deviates from Raoult's Law caused by variation in intermolecular forces in pure alcohol and in hydrocarbon solution.
>
> Most gasoline components follow Raoult's Law - that is their individual component vapor pressure is the product of their pure component vapor pressure times their mole fraction (or in other words - their percentage in the mixture). This Raoult 's Law behavior allows us to predict the vapor properties of most gasoline blends - each component will contribute according to its concentration in the final blend.
>
> But consider pure alcohol. At molecular weight 46 it has a boiling point of about 174 deg F. (only at sea level) , far above what we would predict of a material of that low molecular weight. Propane, with a molecular weight of 44, is a gas! One can explain the high pure ethanol boiling point and low vapor pressure by the intermolecular forces due to hydrogen bonding between ethanol molecules. The hydrogen of one molecule and the electron pair on oxygen on a second molecule attract each other and additional energy - more heat - is required to separate the molecules to form a gas - to boil. And ethanol has a high dipole moment - a skewed electron distribution that establishes an additional intermolecular attraction.
>
> Note that this boiling point stated above is only for anhydrous ethanol (without water). The ethanol that is used to blend gasoline is hydrous ethanol which contains 4 to 7 % water. Trying to get closer to anhydrous ethanol would make it much more expensive to manufacture.
> But in hydrocarbon solution, the ethanol molecules are separated from each other by the preponderence of nonpolar, hydrocarbon molecules. Ethanol is soluble, but each polar, hydrogen bonding molecule cannot find the ready association with another ethanol molecule that increases boiling point and lowers volatility. Its partial vapor pressure (and thus lower boiling point) is a lot higher than one would predict from Raoult's Law. This deviation from linear, "ideal" behavior that increases ethanol containing gasoline volatility, is a common phenomenon in chemistry.
>
> This is probably getting to be much more technical than most of the people on this site want to hear about or know so I would just suggest that we just drop this direction of this thread.
>
> Emery Stora
> 77 Kingsley
> Santa Fe, NM
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