Video Transcript
A hydrogen fuel cell can operate under certain conditions. But the overall reaction of hydrogen combining with oxygen to form water is the same
regardless. Consider the following equations. (A) Half O2 gas plus H2O gas plus two e− forms two OH− aqueous. (B) H2 gas forms two H+ aqueous plus two e−. (C) Half O2 gas plus two H+ aqueous plus two e− forms H2O gas. (D) H2 gas plus two OH− aqueous forms two H2O gas plus two e−. Which of the equations can be paired so that when they are combined they give the
overall reaction for a hydrogen fuel cell? (A) AD and BC, (B) AB and CD, (C) AC and BD.
A hydrogen fuel cell is a type of primary galvanic cell. It uses refillable reactants to run redox reactions, which generate a flow of
electrons that can be used to power electronic devices. To better understand the 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.
A typical hydrogen fuel cell consists of an anode and a cathode. These are often made of hollow containers that are lined with porous carbon. The porous carbon allows for connection between the internal room and the electrolyte
solution inside. Typically, the electrolyte solution is either an aqueous solution of potassium
hydroxide or an acidic solution of sulfuric acid.
As we can see in the diagram, the incoming hydrogen and oxygen are both able to pass
through porous layers within the cell to spontaneously react with the
electrolyte. This forms electrons that can deliver electrical current. These spontaneous redox reactions are what allow galvanic cells to generate their
power.
Redox reactions are chemical reactions where electrons are transferred from one
species to another. In a typical redox reaction, one species will lose electrons and be oxidized and
another species gains electrons and is reduced. In the case of our hydrogen fuel cell, hydrogen gas is oxidized, while oxygen gas is
reduced. The anode and cathode will each facilitate a different type of redox reaction. As with all electrochemical cells, an oxidation half reaction will take place at the
anode, while a reduction half reaction will take place at the cathode.
Let’s clear a little space and see how the flow of electrons can be traced to two
separate half equations occurring in the cell, which combine to give the overall
redox reaction. At the anode, hydrogen gas feeds into the system and loses electrons. This forms hydrogen ions that react with the hydroxide ions in the electrolyte to
form water molecules, which is actually the waste product for this fuel cell. At the cathode, oxygen gas is reduced by the newly formed water and electrons to form
hydroxide ions. Both of these reactions are forms of half equations, meaning that if they are
combined together, they form a complete redox reaction. So we can combine the equations.
We can then remove the terms that are present on both sides of the equation. The overall equation shows that a hydrogen fuel cell takes in both hydrogen and
oxygen and produces water as a product. And during the process, an electrical current is produced.
If we compare the two half equations with the equations labeled from A to D, we can
see that they match with equations D and A. If we bring back the second part of the question, we find that the combination of
equations that we worked out would give the overall reaction for a hydrogen fuel
cell when combined, A and D, is given in option (A). So we can already tell that option (A) is the correct answer to this question. But for completeness, let’s look at the combination BC.
For the equations to combine, one needs to be an oxidation reaction, anode side, and
one reduction reaction, cathode side. They need to balance each other and form a net hydrogen fuel cell equation. It is clear that equations B and C do fit this criteria. Thus, the equations that can be paired so that when they are combined they give the
overall reaction for a hydrogen fuel cell are AD and BC, answer choice (A).
