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Final Exam Solution

We can use hf of 2h2og to represent the amount of

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Unformatted text preview: = -­‐457 kJ/mol (10 points) Yes, burning H2 and O2 is spontaneous at 298K. e) Use the ΔG° and ΔH° that you calculated to calculate the maximum efficiency of this fuel cell. We can use ΔHf° of 2H2O(g) to represent the amount of heat put into the system since we had to expend 2×ΔHf°(H2O(g)) to from the 2H2(g) and O2(g) that are used for fuel. Since we are burning our fuel electrochemically, we extract the maximum work for each electron, given by ΔG° = -nFE° . (efficiency = work out/heat in) -­‐457/-­‐484 = 94.4% (5 points) *if you used incorrect values from previous sub-­‐ questions, but had the correct setup, you were given full credit. If you multiplied 2 to the enthalpy -­‐> -­‐3 points. If you used nFE to calculate free energy -­‐> -­‐3 points. You were not given values for nFE equation so should use the answer from (d). f) SOFCs operate in the temperature range from 500-1000°C. This avoids the use of expensive catalysts like Pt used in lower temperature fuel cells. Calculate the modified efficiency at 1000 K (727°C) using ΔG(T) = ΔH° – TΔS° (and constant values of enthalpy a...
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