Chemistry 111

Walczak

Ethanol vs. Octane: Will more energy be produced from 12 gallons of Ethanol or 12 gallons of Octane?

Octane:

C₈H₁₈ (g) + 25/2 O₂(g) Õ 8 CO₂(g) + 9 H₂O(g)

12 gal octane	4 quarts	0.9463 L	1000 mL	0.7025 g	mole octane	=279.9 moles
	1 gallon	1 quart	1 L	mL	114 g

Bonds Broken		Bonds Made
18 C-H	18x99	16 C=O	16x192
7 C-C	7x83	18 O-H	18x111
25/2 O=O	25/2 x 118
Total Broken	3838 kcal/mole	Total Made	5070 kcal/mole

Energy Released = Total Energy from Bonds Made – Total Energy from Bonds Broken

= 1232 kcal/mole

This much energy will be released for every mole of octane combusted. For the whole tank:

279.9 moles	1232 kcal	=344,837 kcal	=3.4x10^5 kcal
	mole

Ethanol:

CH₃ CH₂OH (g) + 3 O₂(g) Õ 2 CO₂(g) + 3 H₂O(g)

12 gal ethanol	4 quarts	0.9463 L	1000 mL	0.7893 g	mole octane	=779.4 moles
	1 gallon	1 quart	1 L	mL	114 g

Bonds Broken		Bonds Made
5 C-H	5x99	4 C=O	4x192
1 C-C	1x83	6 O-H	6x111
1 C-O	1x86
1 O-H	1x111
3 O=O	3 x 118
Total Broken	1129 kcal/mole	Total Made	1434 kcal/mole

Energy Released = Total Energy from Bonds Made – Total Energy from Bonds Broken

= 305 kcal/mole

This much energy will be released for every mole of ethanol combusted. For the whole tank:

779.4 moles	305 kcal	=237,717 kcal	=2.4x10^5 kcal
	mole

So, more energy is released by the combustion of a tank of octane than a tank of ethanol. The benefits of burning fuel that is “oxygenated,” such as ethanol, are not derived from it producing more energy! What reasons are there for burning ethanol or ethanol blended fuels in our cars?