Question

a. Calculate the cell potential for the electrolysis of water.

b. Discuss whether energy is created or produced in the electrolysis of water or in redox reactions. How does the energy used to charge the fuel cell compare to the energy available for use?

Answer 1

node (oxidation): 2 H₂O(l) → O₂(g) + 4 H⁺(aq) + 4e⁻    E^oox = -1.23 V (E^ored = 1.23 ))

Cathode (reduction): 2 H⁺(aq) + 2e⁻ → H₂(g)    E^ored = 0.00 V

Thus, the standard potential of the water electrolysis cell is -1.23 V at 25 °C at pH 0 ([H⁺] = 1.0 M). At 25 °C with pH 7 ([H⁺] = 1.0×10⁻⁷ M), the potential is unchanged based on the Nernst equation.

b. Electrolysis is used to drive an oxidation-reduction reaction in a direction in which it does not occur spontaneously by driving an electric current through the system while doing work on the chemical system itself, and therefore is non-spontaneous.

A fuel cell is a lot like a battery. It has two electrodes where the reactions take place and an electrolyte which carries the charged particles from one electrode to the other. In order for a fuel cell to work, it needs hydrogen (H₂) and oxygen (O₂). The hydrogen enters the fuel cell at the anode. A chemical reaction strips the hydrogen molecules of their electrons and the atoms become ionized to form H⁺. The electrons travel through wires to provide a current to do work. The oxygen enters at the cathode, usually from the air. The oxygen picks up the electrons that have completed their circuit. The oxygen then combines with the ionized hydrogen atoms (H⁺), and water (H₂O) is formed as the waste product which exits the fuel cell. The electrolyte plays an essential role as well. It only allows the appropriate ions to pass between the anode and cathode. If other ions were allowed to flow between the anode and cathode, the chemical reactions within the cell would be disrupted.

The reaction in a single fuel cell typically produces only about 0.7 volts. Therefore, fuel cells are usually stacked or connected in some way to form a fuel cell system that can be used in cars, generators, or other products that require power.

The reactions involved in a fuel cell are as follows:

Anode side (an oxidation reaction):
2H₂ => 4H⁺+ 4e^-

Cathode side (a reduction reaction):
O₂ + 4H⁺ + 4e^- => 2H₂O

Net reaction (the "redox" reaction):
2H₂ + O₂ => 2H₂O