### 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.