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 2-methylhexane, C7H16, is 1.150×103 kcal/mol. How much energy is released during the complete combustion of 324 grams of 2-methylhexane ? kcal Assuming the same efficiency, would 324 grams of ethanol provide more, less, or the same amount of energy as 324 grams of 2-methylhexane?

b. Combustion reactions are exothermic. The heat of reaction for the combustion of pentane, C5H12, is 838.7 kcal/mol. What is the heat of combustion for pentane in kcal/gram? kcal/gram How much heat will be given off if molar quantities of pentane react according to the following equation? C5H12 + 8 O2----------5 CO2 + 6 H2O _____ kcal

Answer 1

The heat of combustion of 2-methylhexane is 1150 Kcal/mol (given)

Complete combustion of 324 grams of 2-methylhexane:

324 grams of 2-methylhexane (molar mass = 100 g/mol) = 3.24 moles

Amount of energy release = 3.24 * 1150 Kcal/mol = 3726 Kcal

Heat of combustion of ethanol is 326.7 Kcal/mol

324 grams of ethanol (molar mass 46.07) = 7.03 moles

Amount of heat generated by ethanol combustion = 7.03 * 326.7 Kcal/mol = 2296.7 Kcal

Since the heat generated by 2-methylhexane is 3726 Kcal, ethanol provides less amount of energy at the same amount of mass.

b. 838.7 Kcal/mol in Kcal/gram

Molar mass of C5H12 = 72.15 grams/mol

Heat of combustion in Kcal/gram = 838.7 / 72.15 = 11.624 Kcal/gram

Heat evolution using the equation: C5H12 + 8 O2----------5 CO2 + 6 H2O

This would be done by bond enthalpies. The bond energy on the LHS would be consumed, while the bond energy on the LHS would be released.

Bond energies: C-C (348), C-H (413), O=O (495), C=O (799), O-H (463) (All values in KJ/mol)

Total bond energy on LHS: 12(413)+4(348)+8(495) = 10308 KJ/mol

Total bond energy on RHS: 5[2(799)] + 6[2(463)] = 13546 KJ/mol

Total energy released (heat given off) = Energy on RHS - Energy on LHS = 13546 KJ/mol - 10308 KJ/mol

Total energy released = 3238 KJ/mol = 773.9 Kcal/mol

Since molar quantity is used, total heat release is 773.9 Kcal.