Question

Ethanol, C₂H₆O, is most often blended with gasoline - usually as a 10 percent mix - to create a fuel called gasohol. Ethanol is a renewable resource and ethanol-blended fuels, like gasohol, appear to burn more efficiently in combustion engines. The heat of combustion of ethanol is 326.7 kcal/mol. The heat of combustion of heptane, C₇H₁₆, is 1.151×10³ kcal/mol. How much energy is released during the complete combustion of 309 grams of heptane?

Assuming the same efficiency, would 309 grams of ethanol provide more, less, or the same amount of energy as 309 grams of heptane? More, less, or the same amount of energy? Please type the answer! :)

Answer 1

C₇H₁₆ has the molecular weight of 100 g/mol

Ethanol (C₂H₆O) has the molecular weight of 46 g/mol

So, 309 g of ethanol = 309/46 moles = 6.72 moles

and 309 g of heptane = 309/100 moles = 3.09 moles

Thus the energy released by 6.72 moles of ethanol = 326.7 x 6.72 = 2195.42 kcal

and the energy released by 3.09 moles of heptane = 1.151 x 10³ x 3.09 = 3556.59 kcal

Clearly the heat released by heptane is greater than that of ethanol. So, heptane will provide more energy than ethanol if the efficiency remains same.