Video Transcript
The standard electrode potential of a copper half-cell, where a copper metal
electrode is placed in a solution of copper two plus ions, is known to be positive
0.337 volts. What is the standard cell potential when this copper half-cell is connected to a
similar half-cell of zinc metal and zinc two plus ions that has a standard electrode
potential of negative 0.7618 volts?
Whenever we want to find the standard cell potential for a galvanic cell, we need to
examine the two half equations which combine to form a full redox reaction for the
cell. The two half reactions for this cell can be found on the electromotive series, which
is a list of many half reactions with their associated standard electrode
potentials.
We are told that one half-cell contains a copper electrode in a solution of copper
two plus ions. The corresponding half reaction on the electromotive series is Cu2+ aqueous plus two
electrons reacts to form Cu solid. And the standard electrode potential, also called the standard reduction potential,
for this half reaction is positive 0.337 volts. And we are told that the other half-cell is a zinc, zinc two plus ion half-cell. So the other half reaction is Zn2+ aqueous plus two electrons reacts to form Zn
solid. This half reaction has a standard electrode potential of negative 0.7618 volts.
Note that typically reactions are usually written as reductions from left to right on
the electromotive series, otherwise known as the table of standard reduction
potentials. And that is why standard electrode potentials are also called standard reduction
potentials. Also, the reactions are often written with equilibrium arrows, although here we have
shown single-headed arrows.
We now need to determine which one of these equations shows the oxidation reaction
for this cell and which shows the reduction reaction. Going up the list, the potential of the ion to be reduced increases. Thus, the half reaction with the higher, or more positive, standard reduction
potential will be the reduction half reaction. The half reaction with the lower, or more negative, standard reduction potential will
be the oxidation half reaction. Oxidation occurs at the anode and reduction at the cathode.
From the question, we see that the standard reduction potential of Cu2+ is greater or
more positive than for Zn2+. This means that zinc atoms will be oxidized at the anode and copper ions will be
reduced at the cathode. Since zinc is more likely to be oxidized when it is connected to copper in a galvanic
cell, this arrow needs to be flipped left to right if we want to write the half
reaction as it actually occurs, like this. This is the correct oxidation half reaction.
We now know that copper is the cathode and zinc is the anode. Now we can use the following equation to determine the standard cell potential. 𝐸 cell equals 𝐸 cathode minus 𝐸 anode. The superscript ⦵ signs indicate standard conditions of 25 degrees Celsius,
one-atmosphere pressure, and one-molar solution concentrations. 𝐸 cathode is the standard reduction potential of copper, the cathode, with a value
of positive 0.337 volts. And 𝐸 anode is the standard reduction potential of zinc, the anode, with a value of
negative 0.7618 volts.
We can substitute these values into the equation, being careful that there are two
minus signs. Then, solving, we get positive 1.0988 volts. A positive value for 𝐸 cell indicates that a spontaneous redox reaction will occur
when copper is the cathode and zinc is the anode.
In conclusion, what is the standard cell potential of this cell? The answer is positive 1.0988 volts.
