Question Video: Calculating the Standard Cell Potential for a Hydrogen Cell | Nagwa Question Video: Calculating the Standard Cell Potential for a Hydrogen Cell | Nagwa

Question Video: Calculating the Standard Cell Potential for a Hydrogen Cell Chemistry • Third Year of Secondary School

The two reduction half-equations for a hydrogen cell under alkaline conditions are as follows: 2H₂O (l) + 2e⁻ ⇌ H₂ (g) + 2OH⁻ (aq). 𝐸^⦵ = −0.83 V. 1/2O₂ (g) + H₂O (l) + 2e⁻ ⇌ 2OH⁻ (aq). 𝐸^⦵ = +0.40 V. What is the overall cell potential, 𝐸^⦵ cell, of this hydrogen cell?

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Video Transcript

The two reduction half equations for a hydrogen cell under alkaline conditions are as follows. Two H2O liquid plus two e− are in equilibrium with H2 gas plus two OH− aqueous. The standard reduction potential equals negative 0.83 volts. One-half O2 gas plus H2O liquid plus two e− are in equilibrium with two OH− aqueous. The standard reduction potential equals positive 0.40 volts. What is the overall cell potential, 𝐸 plimsoll symbol cell, of this hydrogen cell?

The overall cell potential or the standard cell potential is the difference in electrical potential that exists between two half-cells under standard conditions. The standard cell potential can be calculated by subtracting the standard reduction potential of the anode from the standard reduction potential of the cathode. In this question, we’ve been given the standard reduction potential for each half reaction in a hydrogen cell. But in order to calculate the standard cell potential, we need to know which half reaction occurs at the cathode and which occurs at the anode.

In the hydrogen cell, oxidation occurs at the anode and reduction occurs at the cathode. We can determine which of the half reactions is likely to be the oxidation half reaction and which is likely to be the reduction half reaction by looking at the standard reduction potentials. A more positive reduction potential reflects a greater tendency to accept electrons. So the half reaction with a more positive reduction potential will be the reduction half reaction. As the second reaction has a more positive standard reduction potential, this is the reduction half reaction that occurs at the cathode. This means that the first reaction must be the oxidation half reaction that occurs at the anode.

Now that we know which half reaction occurs at the cathode and which occurs at the anode, we can calculate the overall cell potential. The standard reduction potential of the cathode is positive 0.40 volts, and the standard reduction potential of the anode is negative 0.83 volts. Performing the calculation gives us a value of positive 1.23 volts. So the overall cell potential of this hydrogen cell is positive 1.23 volts.

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