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I found an article that explains why ethanol causes gas to be more volatile, which causes the problem of vapor-locking in carburetor-equipped cars that previously did not have this problem running 100 percent gasoline. http://www.chemcases.com/converter/converter-24.htm
Ford Model T's had carburetors that could be adjusted to run on ethanol or gasoline. I wonder how that worked. Maybe the Model T was not prone to vapor lock, because it had no fuel pump (just gravity feed) and the engine did not generate as much heat under the hood as a Squarebird or other big-block cars.
Ford Model T's had carburetors that could be adjusted to run on ethanol or gasoline. I wonder how that worked. Maybe the Model T was not prone to vapor lock, because it had no fuel pump (just gravity feed) and the engine did not generate as much heat under the hood as a Squarebird or other big-block cars.
Ethanol Containing Fuel - A Volatility Paradox:
Ethanol containing fuels have a higher volatility - a higher composite vapor pressure than corresponding MTBE-containing fuels. You can see this data. But, as shown by its boiling point and vapor pressure, ethanol itself is much less volatile than MTBE.
So why would a gasoline with 10% ethanol be more volatile - have a higher vapor pressure than a similarly constituted gasoline with 10% MTBE?
The answer is based on deviations 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. 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 MW 46 it has a boiling point of 86 degrees Celsius, far above what we would predict of a material of that low molecular weight. Propane, MW 44, is a gas! We 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.
But in hydrocarbon solution, the ethanol molecules are separated from each other by the preponderance of nonpolar, hydrocarbon molecules! Ethanol is soluble, but each polar, hydrogen bonding molecule cannot find the ready association with another ethanol that increases boiling point and lower volatility. Ethanol acts more like it has a MW of 46! Its partial vapor pressure is a lot higher than we predict from Raoult's Law. This deviation from linear, "ideal" behavior that increases ethanol containing gasoline volatility, is a common phenomenon in chemistry.
So why would a gasoline with 10% ethanol be more volatile - have a higher vapor pressure than a similarly constituted gasoline with 10% MTBE?
The answer is based on deviations 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. 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 MW 46 it has a boiling point of 86 degrees Celsius, far above what we would predict of a material of that low molecular weight. Propane, MW 44, is a gas! We 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.
But in hydrocarbon solution, the ethanol molecules are separated from each other by the preponderance of nonpolar, hydrocarbon molecules! Ethanol is soluble, but each polar, hydrogen bonding molecule cannot find the ready association with another ethanol that increases boiling point and lower volatility. Ethanol acts more like it has a MW of 46! Its partial vapor pressure is a lot higher than we predict from Raoult's Law. This deviation from linear, "ideal" behavior that increases ethanol containing gasoline volatility, is a common phenomenon in chemistry.
Ahem!
but I did not understand any of that 
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