Ethanol, C2H6O, is most often blended with gasoline - usually as a 10 percent mi

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 1.151×10^3 kcal/mol. How much energy is released during the complete combustion of 369 grams of heptane ? kcal Assuming the same efficiency, would 369 grams of ethanol provide more, less, or the same amount of energy as 369 grams of heptane?

b. Combustion reactions are exothermic. The heat of reaction for the combustion of 2-methylheptane, C8H18, is 1.306×10^3 kcal/mol. What is the heat of combustion for 2-methylheptane in kcal/gram? kcal/gram How much heat will be given off if molar quantities of 2-methylheptane react according to the following equation? 2 C8H18 + 25 O2-------16 CO2 + 18 H2O kcal

Explanation / Answer

a. Heat of combustion for heptane = 1.151 x 10^3 kcal/mol

moles heptane burnt = 369 g/100.21 g/mol = 3.68 mol

amount of heat released = 3.68 mol x 1.151 x 10^3 kcal/mol = 4238.3 kcal

moles ethanol = 369 g/46.07 g/mol = 8.01 mol

heat released by ethanol = 8.01 mol x 326.7 kcal/g = 2616.7 kcal

So the heat released by ethanol is lower than the heat released by heptane of the same mass.

b. heat of combustion of 2-methylheptane = 1.306 x 10^3 kcal/mol

So for 2 mol of 2methylheptane, heat released = 1.306 x 10^3 kcal/mol x 2 = 2.612 x 10^3 kcal

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