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 473 grams of 2-methylhexane ? kcal Assuming the same efficiency, would 473 grams of ethanol provide more, less, or the same amount of energy as 473 grams of 2-methylhexane?

Answer 1

Heat of combustion of 2-methylhexane = 1.1510³ kcal/mol

molar mass of 2-methylhexane = 100 g

mass of 2-methylhexane= 473 g

number of moles of 2-methylhexane =  

=  

= 4.73 moles

energy released during combustion of 473 g of 2-methylhexane =(number of moles of 2-methylhexane)(Heat of combustion of 2-methylhexane per mole)

  = (4.73)(1.1510³ kcal)

= 5439.5 kcal

Heat of combustion of ethanol = 326.7 kcal/mol

molar mass of ethanol = 46 g

mass of ethanol = 473 g

number of moles of ethanol =  

=  

= 10.28 moles

energy released during combustion of 473 g of ethanol = (number of moles of ethanol )(Heat of combustion of ethanol per mole)

  = (10.28)(326.7 kcal)

= 3358 kcal

From the above caluculations energy released during combustion of 473 g of 2-methylhexane is greater than energy released during combustion of 473 g of ethanol