Look up the reduction potential for each cell in question on a table of standard electrode potential like this one from Chemistry LibreTexts. [1]
Mg
2
+
(
a
q
)
+
2
l
e
−
→
Mg
(
s
)
−
E
θ
=
−
2.372
l
V
Ni
2
+
(
a
q
)
+
2
l
e
−
→
Ni
(
s
)
−
E
θ
=
−
0.257
l
V
The standard reduction potential
E
θ
resembles the electrode's strength as an oxidizing agent and equivalently its tendency to get reduced. The reduction potential of a Platinum-Hydrogen Electrode under standard conditions (
298
l
K
,
1.00
l
kPa
) is defined as
0
l
V
for reference. [2]
A cell with a high reduction potential indicates a strong oxidizing agent- vice versa for a cell with low reduction potentials.
Two half cells connected with an external circuit and a salt bridge make a galvanic cell; the half-cell with the higher
E
θ
and thus higher likelihood to be reduced will experience reduction and act as the cathode, whereas the half-cell with a lower
E
θ
will experience oxidation and act the anode.
E
θ
(
Ni
2
+
/
Ni
)
>
E
θ
(
Mg
2
+
/
Mg
)
Therefore in this galvanic cell, the
Ni
2
+
/
Ni
half-cell will experience reduction and act as the cathode and the
Mg
2
+
/
Mg
the anode.
The standard cell potential of a galvanic cell equals the standard reduction potential of the cathode minus that of the anode. That is:
E
θ
cell
=
E
θ
(
Cathode
)
−
E
θ
(
Anode
)
E
θ
cell
=
−
0.257
−
(
−
2.372
)
E
θ
cell
=
+
2.115
Indicating that connecting the two cells will generate a potential difference of
The standard electrode potential for a galvanic cell constructed in the appropriate way from these two half-cells is,
EθCell=+2.115lV
Cathode Mg2+/Mg
Anode Ni2+/Ni
What is a galvanic cell ?
A galvanic cell or voltaic cell is an electrochemical device that transforms the chemical energy of spontaneous redox reactions into electrical energy.
electrical cell A voltaic cell is an electrochemical device that produces electricity through chemical processes.
Use a table of standard electrode potential, such as this one from Chemistry LibreTexts, to look up the reduction potential for each of the questioned cells.
Mg2+(aq)+2le−→Mg(s)−Eθ=−2.372lV
Ni2+(aq)+2le−→Ni(s)−Eθ=−0.257lV
A galvanic cell's standard cell potential is equal to the cathode's standard reduction potential less the anode's standard reduction potential. Which is:
Eθcell=Eθ(Cathode)−Eθ(Anode)
=−0.257−(−2.372)
=+2.115
The standard electrode potential for a galvanic cell constructed in the appropriate way from these two half-cells is,
The pink color in the solution fades. Some of the colored indicator ion converts to the colorless indicator molecule.
<h3>Explanation</h3>
What's the initial color of the solution?
is a salt soluble in water. dissociates into ions completely when dissolved.
.
The first test tube used to contain . is a weak base that dissociates partially in water.
.
There's also an equilibrium between and ions.
.
ions from will shift the equilibrium between and to the right and reduce the amount of in the solution.
The indicator equilibrium will shift to the right to produce more ions along with the colored indicator ions. The solution will show a pink color.
What's the color of the solution after adding NH₄Cl?
Adding will add to the concentration of ions in the solution. Some of the ions will combine with ions to produce .
The equilibrium between and ions will shift to the left to produce more of both ions.
The indicator equilibrium will shift to the left as the concentration of increases. There will be less colored ions and more colorless molecules in the test tube. The pink color will fade.