A hydrogen-oxygen fuel cell operates by continuously feeding H2(g) and O2(g) into…
CHEMISTRY · Multi-part · AP · Chemistry · Chemical Reactions · Chemical Reactions
Problem
A hydrogen-oxygen fuel cell operates by continuously feeding H2(g) and O2(g) into separate compartments that are separated by an ion-conducting membrane. Within the cell, the following half-reactions occur at 25 °C:
At the anode: H2(g) → 2 H+(aq) + 2 e−
At the cathode: O2(g) + 4 H+(aq) + 4 e− → 2 H2O(l)
Engineers propose the following two-step method for storing electrical energy produced by the cell for later use in an isolated research station.
Step 1 – Electrolyze liquid water produced by the cell to regenerate H2(g) and O2(g), which are captured in pressurized tanks.
Step 2 – Cool the gases until they liquefy, allowing compact storage of LH2 and LO2 in cryogenic tanks.
Assume ideal behavior of gases and negligible heat exchanges other than those described.
(1) For each physical or chemical event listed below, state whether it involves breaking intramolecular bonds, breaking intermolecular interactions, forming intramolecular bonds, and/or forming intermolecular interactions. Briefly justify each choice.
(i) Oxidation of H2 at the fuel-cell anode.
(ii) Electrolysis of liquid water in Step 1.
(iii) Liquefaction of H2(g) in Step 2.
(2) Classify each event in part (a) as a physical change or a chemical change. Provide particulate-level justification for each classification.
(3) Using your answers from part (b), determine whether the overall two-step storage method can be described as reversing the chemical change that occurs during operation of the fuel cell. Support your conclusion by referring explicitly to the identity of species present and the nature of the bonds broken and formed across the full cycle.