ChemBytes - Chapter 12-13

Answers

CHEMBytes: Additional Problems on electrochemistry
  1. Complete and balance the following reactions which occur in (acid/basic) solution by the ion-electron half-reaction method:
    acidic solution
    (a) CuS + NO3- <=> Cu2+ + NO + SO42-
    (b) ClO3- + As2S3 <=> Cl- + H2AsO4- + SO42-
    (c) MnO42- <=> MnO2 + MnO4-
    		 basic solution
    (d) Al + NO3- + OH- <=> Al(OH)4- + NH3
    (e) ClO- + Fe(OH)3 <=> Cl- + FeO42-
    (f) HOO- + Cr(OH)3- <=> CrO42- + OH-

    (a) 3 CuS + 8 NO3- + 8 H+ <=> 3 Cu2+ + 3 SO42- + 8 NO + 4 H2O
    (b)14 ClO3- + 3 As2S3 + 18 H2O <=> 14 Cl- + 6 H2AsO4- + 9 SO42- + 24 H+
    (c) 3 MnO42- + 4 H+ <=> MnO2 + 2 MnO4- + 2 H2O
    (d) 8 Al + 3 NO3- + 5 OH- + 18 H2O <=> 8 Al(OH)4- + 3 NH3
    (e) 3 ClO- + 2 Fe(OH)3 + 4 OH- <=> 3 Cl- + 2 FeO42- + 5 H2O
    (f) 2 HOO- + Cr(OH)3- <=> CrO42- + OH- + 2 H2O

  2. Consult a table of standard electrode potentials and select an (oxidizing/reducing) agent capable of the following transformations:
    oxidizing agent:
    (a) Cl- <=> Cl2
    (b) Pb <=> Pb2+
    (c) Fe2+ <=> Fe3+     		reducing agent:
    (d) Fe2+ <=> Fe
    (e) Ag+ <=> Ag
    (f) Mn2+ <=> Mn

    (a) any entry with E° > +1.36 V
    (b) any entry with E° > -0.126 V
    (c) any entry with E° > +0.771 V
    (d) any entry with E° < -0.41 V
    (e) any entry with E° < +0.799 V
    (f) any entry with E° < -1.03 V

  3. Which of the following oxidizing agents become stronger as the hydronium ion concentration increases? Which are unchanged? Which become weaker?
    (a) Cl2 (b) Cr2O72- (c) Fe3+ (d) MnO4-

    (b) and (d) become stronger as hydronium ion increases; (a) and (c) are unchanged

  4. By using the appropriate half-cell potentials, calculate the cell potential and the equilibrium constant for the following reaction:
    Fe3+ + I- Fe2+ + 1/2 I2
    State what you expect to happen when equal volumes of 2 M Fe3+ and 2M I- solutions are mixed.
    potential = 0.235 V; Keq = 9.6 X 103
  5. A cell consists of a standard Ag, Ag+ half cell (1M Ag+) combined with another half-cell in which a silver wire dips into a solution of 1M Br- which is saturated and in contact with solid AgBr. The electrode of this latter cell is negative, and the cell generates 0.77 volt. What is the concentration of Ag+ in equilibrium with 1M Br- and solid AgBr? What is the apparent solubility product (Ksp) of AgBr?
    [Ag+] = 9.4 X 10-14; Ksp = 9.09 X 10-14
  6. From the following standard electrode potentials, calculate the equilibrium constant for the indicated reaction:
    Cu2+ + 2e- Cu      E° = 0.34V
    Cu2+ + e- Cu+      E° = 0.15V
    Cu + Cu2+ 2Cu+      Keq = ?
    Keq = 3.6 X 10-7