Video Transcript
Using the standard electrode
potentials in the table below, calculate the standard cell potential for a galvanic
cell consisting of Au3+/Au and Ni2+/Ni half-cells. The table gives two half-equations
with their standard electrode potentials. Au3+ aqueous plus three electrons
giving Au solid, and its standard electrode potential is 1.498 volts. The other half-equation is Ni2+
aqueous plus two electrons giving Ni solid with its standard electrode potential of
negative 0.257 volts. The answer options are (A) 1.241
volts, (B) 1.755 volts, (C) negative 1.241 volts, or (D) negative 1.755 volts.
We are told that we have a galvanic
cell with gold and nickel half-cells. The setup of the cell would look
like this. In a galvanic cell, a redox
reaction occurs spontaneously, generating a current in the wire connecting the
electrodes. We don’t know which direction the
electrical current is moving in this galvanic cell. We are asked to calculate the
standard cell potential or 𝐸 cell. Standard conditions are one molar
concentrations for the electrolytes, one-atmosphere air pressure, and 25 degrees
Celsius. 𝐸 cell is the potential difference
between the electrodes at the beginning of the reaction. It is the maximum potential
difference between these electrodes and 𝐸 cell is equal to 𝐸 cathode minus 𝐸
anode.
We are given the standard electrode
potentials for gold and nickel, but which is the cathode and which is the anode? These standard electrode potentials
are actually standard reduction potentials. That is why both half-equations are
written as reductions with the electrons on the left-hand side of the arrows. The larger or more positive
standard reduction potential, the more easily that electrode is reduced. Gold’s value of 1.498 volts is
bigger than and more positive than nickel’s value of negative 0.257 volts, telling
us gold is more easily reduced than nickel. Therefore, gold will be the cathode
and nickel the anode.
Now, let’s put our values into the
equation. 𝐸 cell is equal to 1.498 volts
minus negative 0.257 volts. Be aware of these two negative
signs. This gives an answer of positive
1.755 volts. The positive sign tells us that the
reaction with gold as the cathode and nickel as the anode is feasible and will occur
spontaneously. But what reaction is that? It is the overall reaction where
gold is reduced at the cathode and where nickel is oxidized at the anode. So, the direction of electron flow
would be from the nickel electrode to the gold electrode. We were asked to calculate the
standard cell potential for the gold-nickel galvanic cell, and the answer is 1.755
volts.
