Conceptual Challenge Problems
These rigorous, thought-provoking problems integrate conceptual learning with problem solving and are suitable for group work.
CP19.A (Section 19.6) Most automobiles run on internal combustion engines, in which the energy used to run the vehicle is obtained from the combustion of gasoline. The main component of gasoline is octane, C8H18. An automobile manufacturer has recently announced a chemical method for generating hydrogen gas from gasoline and proposes to develop a car in which an H2/O2 fuel cell powers an electric propulsion motor, thus eliminating the internal combustion engine with its problems (for example, the generation of unwanted by-products that pollute the air). The hydrogen for the fuel cell would be directly generated from gasoline on board the vehicle. There are two steps in this hydrogen generation process:
(i) Partial oxidation of octane by oxygen to carbon monoxide and hydrogen
(ii) Combination of carbon monoxide with additional gaseous water to form carbon dioxide and more hydrogen (the water gas shift reaction)
(a) Write the chemical equation for the complete combustion of 1 mol octane.
(b) Write balanced chemical equations for the two-step hydrogen generation process. How many moles of H2 are produced per mole of octane? (Remember that water is a reactant in the two-step process.)
(c) By combining these equations, show that the net overall reaction is the same as in the combustion of octane.
(d) Assuming that the entire Gibbs free energy change of the H2/O2 fuel cell reaction is available for use by the electric propulsion motor, calculate the energy produced by a fuel cell when it consumes all of the hydrogen produced from 1 mol of octane. Compare this energy with the Gibbs free energy change for the combustion of 1 mol of octane.
(Note: The Gibbs free energy of formation, ΔG°f , for C8H18 (l) is 6.14 kJ/mol.)
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