Chemical Bond Comparison of Straight Gasoline and Gasohol

Every reasonable person recognizes the importance of travel in today's world. People must make a living. Much travel is done by automobile, and today's automobiles mostly use gasoline. For some time now, much available gasoline has ethanol from natural sources such as corn added to it. Gasoline often contains in the neighborhood of 10 percent ethanol. Although this is thought to "green up" America, what effect has this had on mileage per gallon of fuel consumed?

A Unique Method of Comparison

Although mileage studies are one way to compare traditional gasoline with gasohol, such studies can produce variable results. There is another approach that might be taken that produces pretty much absolute results, even if the results aren't in terms of miles per gallon. When a fuel is burned in oxygen, the bonds between atoms are broken, liberating energy, whereas new bonds are formed, the bonds of products of the reaction. The energy of the reactants, when subtracted from the energy of the products, should result in a net energy, the energy of combustion.

The Task

In this article, we want to representatively compare the enthalpies (heat energy produced) of straight gasoline with that of ethyl alcohol, the alcohol used in gasohol. What we will calculate will not be miles per gallon of fuel consumed, because there are complications in trying to do that. Such treatment goes way beyond the scope of this article. Rather, a simple energy comparison between the two fuels will be used to indicate whether gasoline or gasohol is superior.

Gasoline - Choosing a Representative Molecule

Gasoline is not such a simple beast. Gasoline is not a pure compound, but a complex collection of hydrocarbons. Nevertheless, it is not an unfair approximation to choose a representative species to compare the burning of gasoline with the burning of gasohol. Gas is often evaluated in terms of "octane number," the reference substance being the straight-chain molecule, normal heptane, or n-heptane, chemical formula,

CH₃-CH₂-CH₂-CH₂-CH₂-CH₂-CH₃

Burning this substance perfectly in oxygen, follows the reaction,

CH₃-CH₂-CH₂-CH₂-CH₂-CH₂-CH₃ + 11 O₂ → 8 H₂O + 7 CO₂

The bond table we need can be written,

C-H = 99 Kcal / Mole¹
C-C = 83 Kcal / Mole
O::O = 118 Kcal / Mole (covalent)
H-O = 111 Kcal / Mole
HO-H = 18 Kcal / Mole
C-O = 85 Kcal / Mole
OC-O = 110 Kcal / Mole

The numbers become,

16 (99) + 6 (83) + 11 (118) - 8 (111) - 8 (18) - 7 (110) - 7 (85) = 983 Kcal / Mole

Since only one molecule of n-heptane is burned in the process, this is the final number for the process.

Burning Ethanol

Burning ethyl alcohol, or ethanol, chemical formula C₂H₅OH can also have an energy for the process calculated. The reaction is,

C₂H₅OH + 3 O₂ → 3 H₂O + 2 CO₂

The bond table is,

C-H = 99 Kcal / Mole
C-C = 83 Kcal / Mole
C-OH = 91 Kcal / Mole
O::O = 118 Kcal / Mole (covalent)
H-O = 111 Kcal / Mole
HO-H = 18 Kcal / Mole
C-O = 85 Kcal / Mole
OC-O = 110 Kcal / Mole

The numbers?

5 (99) + (83) + (91) + (111) + 3 (118) - 3 (111) - 3 (18) - 2 (85) - 2 (110) = 357 Kcal / Mole

Initial Comparison

At first glimpse, a comparison of the numbers for n-heptane and alcohol, separate, suggests adding ethanol to n-heptane would lower the energy of the reaction considerably, making it undesirable.

However, we must convert the energy per mole to energy per liter if we are to make a fair comparison. The equation to convert these numbers is,

Energy / Liter = (Energy / Mole) x (Moles / Gram) x (Density in Grams / Milliliter) x 1000 Milliliters / Liter

Conversion Results

For n-heptane,

Energy / Liter = 983 x 0.00861 x 0.684 x 1000 = 5789 Kcal / Liter

For ethanol,

Energy / Liter = 357 x 0.02174 x 0.789 x 1000 = 6124 Kcal / Liter

Interesting Results!

The interesting results are that the two fuels are significantly alike in energy per liter produced. Before one concludes gasohol is superior to straight gasoline, however, one must recognize the approximate methods utilized, and the assumptions made. In addition, there may be additional factors that increase or decrease the desirability of adding ethanol to gasoline for fuel consumption.

¹ We will ignore the slight differences in types of hydrogen to carbon bonds.

Note:If any errors are detected in this treatise, please notify the author at a special email address dedicated to the purpose, VinceSciGuy at gmail.com. The author will correct any verifiable mistakes.

References and Resources:

Michigan State University - Bond Dissociation Energies

Online Introductory Chemistry - Heat Changes During Chemical Reactions