Concept #1: Galvanic versus Electrolytic Cells
Concept #2: An electrolytic cell presents an electrochemical cell that is nonspontaneous.
Example #1: A certain electrochemical cell involves a five electron change and has a value of Keq = 3.0 x 1016 at 298 K. The value of ΔHo for the reaction is -68.3 kJ/mol. Calculate the values of ΔGo, ΔEo, for a standard electrochemical cell constructed based on this reaction and also ΔSo for the reaction.
Practice: Given the following standard reduction potentials,
Hg22+(aq) + 2 e– 2 Hg (?) E° = +0.789 V
Hg2Cl2(s) + 2 e– 2 Hg (?) + 2 Cl-(aq) E° = +0.271 V
determine Ksp for Hg2Cl2(s) at 25 °C.
Example #2: The cell notation for a redox reaction is given as the following at (T = 298 K):
Zn (s) Ι Zn2+ (aq, 0.37 M) ΙΙ Ni2+ (aq, 0.059 M) Ι Ni (s)
a) Write the balanced half-reactions occurring at the anode and the cathode.
b) Write out the complete balanced redox reaction.
c) Determine the Eo cell.
d) Calculate the maximum electrical work that can be produced by this cell.
e) Calculate the reactant quotient, Q, for this cell and the cell potential under non-standard conditions.
Example #3: Answer each of the following questions based on the following half reactions:
HALF REACTIONS Eo (V)
Cl2 (g) + 2 e – 2 Cl – (aq) + 1.36
l2 (g) + 2 e – 2 l – (aq) + 0.535
Pb2+ (aq) + 2 e – Pb (s) - 0.126
V2+ (aq) + 2 e – V (s) - 1.18
a) Which is the strongest oxidizing agent?
b) Which is the strongest reducing agent?
c) Will I – (aq) reduce Cl2 (g) to Cl – (g)?
Example #4: Electric current, or flow of electrons is measured in Amperes (A). One Ampere is the delivery of one coulomb (C) of charge per second. What mass of Zinc (in g) is oxidized (to Zn2+) by a dry cell battery that supplies 125 mA of current for two hours (recall that Faraday's constant is the charge in coulombs on a mole of electrons)?
Example #5: Consider the combustion of formaldehyde CH2O and the data below for the following five questions.
I. CH2O (g) + O2 (g) → CO2 (g) + H2O (l)
II. 4H+ + 4 e - + O2 (g) → 2 H2O (l) ΔE° = 1.23V
Which compound is reduced in reaction I?
{C}a. {C}CH2O
{C}b. {C}H2O
{C}c. {C}O2
{C}d. {C}CO2
{C}e. {C}None of these
What is the change in oxidation number of the carbon in reaction I?
{C}a. {C}-4
{C}b. {C}-1
{C}c. {C}0
{C}d. {C}1
{C}e. {C}4
Example #6: The purpose of a galvanic cell is to:
a. Transduce chemical energy to electrical energy.
b. Purify solids.
c. Allow for oxidation without reduction.
d. To consume electricity.
Example #7: During the process for electrolysis of water a current is passed through water and produces hydrogen gas and oxygen gas. Which of the following statements is true?
a. O2 gas is produced at the anode.
b. H2 gas is produced at the cathode.
c. In the reaction, H2 moles are twice the O2 moles.
d. All of the following are correct.
Example #8: Which statement is false?
a. Reduction occurs at the cathode.
b. A reducing agent will lose electrons.
c. Cations migrate to the cathode in both electrolytic and electrochemical cells.
d. Li (s) is the strongest oxidizing agent; F2 is the strongest reducing agent.
Example #9: Which of the following reactions may occur at the anode?
a. Ga3+ (aq) + 3 e – → Ga (s)
b. Cu2+ (aq) + 2 e – → Cu (s)
c. 2 Cl– (aq) → Cl2 (g) + 3 e –
d. Co (s) + e – → Co+ (aq)
Example #10: Define a salt bridge.
A) A pathway, composed of salt water, that ions pass through.
B) A pathway in which no ions flow.
C) A pathway between the cathode and anode in which ions are reduced.
D) A pathway between the cathode and anode in which ions are oxidized.
E) A pathway by which counterions can flow between the half-cells without the solutions in the half-cell totally mixing.
Example #11: What statement is NOT true about standard electrode potentials?
A) E°cell is positive for spontaneous reactions.
B) Electrons will flow from more negative electrode to more positive electrode.
C) The electrode potential of the standard hydrogen electrode is exactly zero.
D) E°cell is the difference in voltage between the anode and the cathode.
E) The electrode in any half-cell with a greater tendency to undergo reduction is positively charged relative to the standard hydrogen electrode and therefore has a positive E°.
Example #12: Use the standard reduction potentials below to determine which element or ion is the best reducing agent.
Pd2+ (aq) + 2 e– → Pd (s) Eo = + 0.90 V
2 H+ (aq) + 2 e– → H2 (g) Eo = 0.00 V
Mn2+ (aq) + 2 e– → Mn (s) Eo = - 1.18 V
a) Pd (s) b) H+ (aq) c) Mn2+ (aq) d) H2 (g)
Example #13: Consider an electrochemical cell where the following reaction takes place:
Na2O (aq) + Ba (s) → 2 Na (s) + BaO (aq)
What is the cell notation for this cell?
What is the ratio of oxidizing agent to reducing agent?