Lab 7: Voltaic Cells
Fall 2022
Introductory Questions
Refer to Lab Background and to Chapter 4 (4.4) and Chapter 20 (20.3-20.5) in your book
1. Reduction-potential tables, such as Appendix E in the Backmatter, or the one on this , webpage
provide standard reduction potential values, for reduction half-reactions. E°red,
(a) The activity series (used in Chapter 4) shown to the right lists the most easily
oxidized metal elements at the top. Would you expect the cations (positive
ions) of the most easily oxidized metals to have positive or negative reduction
potential values? Explain your answer.
I would expect the cations of the most easily oxidized metals to have a
negative reduction potential value.
(b) Copper, silver, platinum, and gold, are very low on the activity series. Would
you expect to be when combined with zinc, lead,
copper ions (Cu2+) reduced
or manganese metals in aqueous solution? Explain your answer.
Yes I would expect the copper ions to be reduced when combined with
zinc, lead, or manganese metals in a liquid solution because all of these
compounds produce a negative Ered value.
2. A voltaic cell is composed of an anode compartment and a cathode compartment connected
by a salt bridge. The anode and cathode electrodes are connected by an external circuit.
(a) Which half-reaction occurs at the anode electrode?
Oxidation occurs at the anode electrode.
(b) What is the purpose of the salt bridge? Can the cell run without it?
The salt bridge essentially completes the circuit. The same in a corrosion cell. You need
a pathway for the electrons to travel in order for the reaction to take place.
Part 1: Voltaic Cells with Copper Half-Reaction
3. The voltaic cell notation in the table is explained in the Background information. In the right-hand
column:
Write the for the spontaneous reaction occurring in each complete balanced equation
voltaic cell you prepared. Include physical states.
Then write the i that does not include the nitrate spectator ions. net onic equation Include
physical states.
Both equations for Cell#1 are done for you.
Cell # Voltaic Cell Notation Equations for Spontaneous Reaction
#1
Zn(s) Zn , 1 ) Cu , 1 )
2+(aq M
2+(aq M
Cu( )s
Zn( )s) + Cu(NO3)2(aq) Cu( s) + Zn(NO3)2(aq
Net ionic: Zn( ) + Cu ) Cu( ) + Zns 2+(aq s 2+(aq)
#2
Pb(s) Pb , 1 ) Cu , 1 )
2+(aq M
2+(aq M
Cu( )s
Pb(s) + Cu(NO3)2(aq) Pb(NO3)2 (aq) + Cu(s)
Net Ionic: Pb(s) + Cu2+(aq) Pb2+(aq) + Cu(s)
#3
Mn(s) Mn , 1 ) Cu , 1
2+(aq M
2+(aq
M s) Cu(
)
Mn(s) + Cu(NO3)2(aq) Mn(NO3)2(aq) + Cu(s)
Net Ionic: Mn(s) + Cu2+(aq) Mn2+(aq) + Cu(s)
4. Record the standard cell potential you observed for the spontaneous reaction experimental
occurring in each voltaic cell. Record the value you observed right after you connected the
salt bridge.
Cell # Voltaic Cell Notation E°cell (experimental), V
#1 Zn(s) Zn , 1 ) Cu , 1 )
2+(aq M
2+(aq M
Cu( )s
1.100v
#2 Pb(s) Pb , 1 ) Cu
2+(aq M
2+(aq, 1 M)
Cu( )s
0.470v
#3 Mn(s) Mn , 1 ) Cu , 1 )
2+(aq M
2+(aq M
Cu( )s
1.525v
5. In an experiment using a real voltaic cell, the cell potential will change as time goes on as reactants
convert to products and the system is no longer in standard state conditions.
(a) Would you expect the cell potential for each reaction to as the voltaic cellincrease or decrease
continues to run?
Decrease
(b) Use the equation Nernst equation to explain why the cell Ecell = – (0.05916/ E°cell n Q)log
potential (voltage reading) should change in the direction you predict?
The system is trying to reach equilibrium and once it is reached the cell potential will start
to decrease.
6. (a) Look up and record the (at 25°C) for the following half-standard reduction potential values
reactions. A table of standard reduction potentials is in Appendix E of your ebook.
Remember to include units.
(b) What did you use to obtain the values in 6(a)? source
Pearson E-text appendix E
7. Use the standard reduction potential values you recorded in the last question to calculate
theoretical values for the standard cell potential for the spontaneous reaction occurring in each
voltaic cell. The equation for the calculation is: E°cell = E°red(cathode)
E°red(anode)
Remember that is positive for a spontaneous reaction under standard state conditions. Show E°cell
your work. (In the first column, copy your net ionic equations from Table 3.)
Cell # Equation for Spontaneous reaction
(Net Ionic Equation)
E°cell (Theoretical)
Calculation (work)
E°cell (Theoretical)
Value with units
#1
Zn(s) + Cu ) Cu( ) + Zn
2+(aq
s 2+(aq)
0.34 – (-0.76) 1.1v
#2
Pb(s) + Cu2+(aq) Pb2+(aq) + Cu(s)
0.34 – (-0.13) 0.47v
#3
Mn(s) + Cu2+(aq) Mn2+(aq) + Cu(s)
0.34 – (-1.18) 1.52v
Cu2+(aq) + 2e Cu(
s) E°red = 0.34v
Zn2+(aq) + 2e Zn(
s) E°red = -0.76v
Pb2+(aq) + 2e Pb(
s) E°red = -0.13v
Mn2+(aq) + 2e Mn(
s) E°red = -1.18v
8. Compare your experimental and theoretical values from Tables 4 and 7 by recording them in E°cell
the following table. They should differ in number of significant figures.
Remember to include units.
Cell # Voltaic Cell Notation E°cell cell
(experimental) E° (theoretical)
#1
Zn(s) Zn , 1 ) Cu , 1 )
2+(aq M
2+(aq M
Cu( )s
1.100v 1.1v
#2
Pb(s) Pb , 1 ) Cu , 1 )
2+(aq M
2+(aq M
Cu( )s
0.470v 0.47v
#3
Mn(s) Mn , 1 ) Cu , 1 )
2+(aq M
2+(aq M
Cu( )s
1.525v 1.52v
Part 2: Voltaic Cell with Manganese and Lead
9. In your voltaic cell involving manganese and lead, identify which alligator clip was attached to each
electrode and identify electrode as part of the anode (oxidation) or cathode (reduction).
10. Complete the table:
Write the Voltaic Cell Notation manganese and leadfor the reaction involving . The metal
and ion on the left should be the components in the anode.
Write the for the spontaneous reaction occurring in each complete balanced equation
cell. Include the nitrate ions that are present to balance charge. Include physical states.
Then write the that does not include the nitrate spectator ions. net ionic equation Include
physical states.
Voltaic Cell Notation Equations for Spontaneous Reaction
Mn | Mn2+ || Pb2+ | Pb Complete: Mn(s) + Pb(NO3)2(aq) Mn(NO3)2(aq)
+ Pb(s)
Net Ionic: Pb2+(aq) + Mn(s) Pb(s) + Mn2+(aq)
11. Record the standard cell potential you observed for the spontaneous reaction experimental
occurring in the voltaic cell. Record the first value you observe before the value starts to change.
Include units.
Alligator Clip Electrode
(Lead or Manganese)
Anode or Cathode
Black Manganese Anode
Red Lead Cathode
Observed experimental E°cell = 1.055v
12. Use your value to calculate the for the reaction. The experimental E°cell equilibrium constant, Keq,
relationship between and is provided on the page.E°cell Keq Lab Background
Calculation of Value (work)Keq Keq Value
(3 sig figs)
Ecell = Ecell – (0.05916/n)logKeq
Ecell = (0.05916/2)
1.055 = (0.05916/2) log Keq
(1.055 * 2)/0.05916 = log Keq
35.66599 = log Keq
Keq = 4.63 X 10^35
13. Is your calculated value consistent with a highly spontaneous reaction starting with standard Keq
state conditions? Explain.
Yes. Since Keq is > 1 this makes the reaction highly spontaneous.
14. Gibbs free energy, , is a thermodynamic state function that indicates the spontaneity of a
G
chemical reaction or physical change. A negative value of a ° value indicates a spontaneous
G
reaction when starting with , which are the starting conditions for the standard state conditions
voltaic cells in this lab. Use your value for the manganese/lead reaction to experimental E°cell
calculate the , for that reaction, standard free energy change,
G° in units of kJ.
G°=
nFE°cell where F = 96,500 J/(V·mol e )
-
Calculation of Value (work)
G°
G° Value (kJ)
(4 sig figs)
G°=
nFE°cell
n=2
F= 96,500
Ecell = 1.055v
-203.6kJ
15. Is the sign of your calculated ° value consistent with a spontaneous reaction starting with G
standard state conditions? Explain.
Yes, since the result was negative it indicates a highly spontaneous reaction. The reaction has a high
negative value.
