Exploring Automobile Fuels
There are three main types of fuels used for automobiles – gasoline, diesel, and ethanol (which is often added to gasoline to increase the amount of renewable energy in the gasoline – the ethanol is produced from corn). In this case study we will explore these types of fuels and the amount of energy stored in each. Gasoline and diesel are mixtures of hydrocarbons. For simplicity, we will assume one generic formula for each. Gasoline can be approximated as 2,2,4-trimethylpentane (also known as iso-octane, C8H18). Diesel fuel can be generalized as cetane (hexadecane, C16H34). The formula for ethanol is C2H6O.
1. Draw bond-line (skeletal) structures of each compound: a. Iso-octane b. Ethanol c. Cetane
2. Write balanced chemical equations for the reactions of each compound with oxygen gas to produce carbon dioxide and water.
c. Ethanol Name _________________________ We can use the energy stored in the bonds of reactants and products to get a rough calculation of the energy released by a fuel. We use this equation… Hrxn = n(bonds broken) – m(bonds made) n, m = number of moles of each bond Where the enthalpy of a reaction (a measure of energy) is related to the enthalpy of all the bonds broken and made. A bond enthalpy is the amount of energy required to break a bond between two atoms (typically in the gas phase). Breaking a bond always REQUIRES energy and making a bond releases energy. An overall negative value for H means energy is released by the reaction (exothermic). A positive value means energy must be added to the reaction (endothermic). For a nice overview of this process, see (https://www.youtube.com/watch?v=0mLFSDId6pQ) For the balanced reactions you gave in #2, Hrxn is… a. Iso-octane = -5060.5 kJ/mol b. Cetane = -9920.5 kJ/mol c. Ethanol = -1255 kJ/mol
3. Sadly, we don’t buy fuel by mole, we purchase fuel by volume (typically gallons in the US). In order to compare these fuels in the same manner we purchase them, we should calculate the amount of energy released by volume (per liter), rather than per mole. To do that you will need the density of each compound at room temperature. They are: iso-octane density = 0.692 g/mL molar mass = 114.22 g/mol cetane density = 0.770 g/mL molar mass = 226.41 g/mol ethanol density = 0.789 g/mL molar mass = 46.07 g/mol For each compound, calculate the amount of energy (in kJ) per liter of fuel. a. Iso-octane b. Cetane c. Ethanol
4. Which fuel delivers the most energy per liter? ________________ Bond Mean Bond Enthalpy (kJ/mol) Bond Mean Bond Enthalpy (kJ/mol) C-H 413 O2 495 O-H 463 C-C 348 C=O 799 C-O 358 Name _________________________
5. If 1 L of each fuel are fully combusted, how many moles of CO2 are produced? a. One liter of iso-octane produces… b. One liter of cetane produces… c. One liter of ethanol produces…
6. In an electric car, 33.7 kWh is considered equivalent to a gallon of gasoline. One gallon is 3.785 liters and one kWh is equal to 3.6 x 103 kJ. a. Is this a reasonable equivalent, based on your answers to #3? b. How many kWh represents 1 liter of gasoline? What is this in kJ/L? Name _________________________
7. On average, about 12 moles of methane (CH4) is required to generate 1 kWh of electricity. How many moles of CO2 are produced to generate the number of kWh you calculated in #6b? 8. Which fuel delivers the most energy (in kJ) per mole of CO2?