Video Transcript
Which of the following equations
shows the reaction at the cathode of a hydrogen fuel cell under alkaline
conditions? (A) Two H2 gas forms four H+
aqueous plus four e−. (B) O2 gas plus four H+ aqueous
plus four e− forms two H2O gas. (C) Two H2 gas plus four OH−
aqueous forms four H2O gas plus four e−. (D) O2 gas plus two H2O gas plus
four e− forms four OH− aqueous.
The focus of our question is the
hydrogen fuel cell. To better understand the series of
reactions that take place within a hydrogen fuel cell, it is helpful to understand
the structure of the cell and how electrons flow within it. Within our hydrogen fuel cell, both
the anode and the cathode are hollow containers that are lined with a porous
carbon.
The electrolyte solution can be an
aqueous alkaline solution of potassium hydroxide or an acidic solution of sulfuric
acid. As we can see, the incoming
hydrogen and oxygen are both able to pass through the porous anodes and cathodes,
respectively, and react with the electrolyte to form electrons. These reactions are what give
galvanic cells their power. Galvanic cells rely on redox
reactions and the flow of electrons they produce to power devices. Electrons will leave the anode,
flow through the circuit, and arrive at the cathode.
In the case of our hydrogen fuel
cell, hydrogen gas is oxidized and oxygen gas is reduced. At the anode, with a solution of
potassium hydroxide, KOH, hydrogen gas feeds into the system. It loses electrons forming hydrogen
ions that react with the hydroxide ions in the electrolyte. This, in turn, forms water
molecules. At the cathode, oxygen gas is
reduced by the water and electrons produced by the previous reaction to form
hydroxide ions. The combination of the two
half-equations can be represented as two H2 gas plus O2 gas forms two H2O gas.
Our question asks about the
reaction that occurs at the cathode of a hydrogen fuel cell. As we can see, the reaction at the
cathode can best be represented by O2 gas plus two H2O gas plus four e− forms four
OH− aqueous. And therefore, answer choice (D) is
correct.
