Chapter 19, Problem 112SCQ

The free energy change for a reaction, Δ_rG°, is the maximum energy that can be extracted from the process as work, whereas Δ_rH° is the total chemical potential energy change. The efficiency of a fuel cell is the ratio of these two quantities.

$\text{Efficiency = }\frac{{\Delta }_{\text{r}}G°}{{\Delta }_{\text{r}}H°}\times 100\%$

Consider the hydrogen-oxygen fuel cell, where the net reaction is

${\text{H}}_{\text{2}}\left(\text{g}\right)\text{ + }\frac{1}{2}{\text{ O}}_{\text{2}}\left(\text{g}\right)\to {\text{ H}}_{\text{2}}\text{O}\left(\mathcal{l}\right)$

1. (a) Calculate the efficiency of the fuel cell under standard conditions.
2. (b) Calculate the efficiency of the fuel cell if the product is water vapor instead of liquid water.
3. (c) Does the efficiency depend on the state of the reaction product? Why or why not?