WORKSHEET 3 (Electrochemistry)
CHEM 118 – Spring 2014
George A. Papadantonakis
1. a) Balance in acidic solution by half-reaction method:
i) IO3- (aq) + I- (aq) → I3- (aq)
ii) Au (s) + HNO3 (aq) + HCl (aq) → AuCl4- (aq) + NO (g)
b) Balance in basic solution by half-reaction method:
i) Cl2 (g) → Cl- (aq) + ClO- (aq)
ii) Al (s) + MnO4- (aq) → MnO2 (s) + Al(OH)4- (aq)
2. You are told that metal A is a better reducing agent than metal B. What, if anything, can be said about
A+ compared to B-? Explain.
1
3. Hydrogen peroxide can act as an oxidizing or reducing agent as in the unbalanced reaction:
H2O2 ( aq) → O2 (g)
a) Balance the reaction in acidic solution by the half-reaction method
b) Write the anode half-reaction
c) What is the product of H2O2 reduction?
d) For the reaction, ∆Ho = - 189.32 kJ and ∆So = 57.1 J K-1. What is the equilibrium constant for the
reaction at 25 o C?
e) Will an electrochemical cell based on this reaction be galvanic (voltaic) or electrolytic? Explain.
4. The formation constant, Kf, for Ag(NH3)2+ is defined as the equilibrium constant for the reaction:
Ag+ (aq) + 2 NH3 (aq) → Ag(NH3)2+ ( aq)
Calculate the value of Kf, from
Ag+ (aq) + e- → Ag (s) Eo = 0.80 V
Ag(NH3)2+ ( aq) + e- → Ag (s) + 2 NH3 ( aq) Eo = - 0.37 V
5. Electrolysis of a molten metal chloride, MCl3, with a current of 6.50 A for 1397 s deposits 1.41 g of the
metal at the cathode. What is the metal, M?
2
6. Consider the following cell: Al | Al3+(1.00 M) || Pb2+(1.00 M) | Pb
a) Draw a diagram of this cell
b) Given that E˚red(Al3+) = -1.660 V and E˚red(Pb2+) = 0.130 V, calculate the cell potential at 50.0 ˚C after
the reaction has operated long enough for the [Al3+] to have changed by 0.600 M.
7. Given the following two standard reduction potentials,
M3+ + 3e- → M Eo = - 0.10 V
M2+ + 2e- → M Eo = - 0.50 V
Determine for the standard reduction potential of the half-reaction M3+ + e- → M2+
8. Considering the following half-reactions measured at pH 0.0:
Br2 (l) + 2 e- → 2 Br – (aq)
Eo = 1.065 V
BrO3- (aq) + 6 H+ (aq) +5 e- → ½ Br2 (l) + 3 H2O (l) Eo = 1.52 V
3
a) What are the products of bromine disproportionation?
b) Will bromine disproportionate spontaneously at pH 0.0 ?
c) Which is the stronger reducing agent at pH 0.0: Br2 (l) or Br– (aq) ? Why?
9. A voltaic cell is constructed in which the overall reaction is: Pb (s) + 2H+ (aq) → Pb2+ (aq) + H2 (g)
a) Calculate Eo for this cell at 25oC
b) Chloride ions are added until PbCl2 (s) precipitates in the anode compartment and [Cl-] reaches 0.15
M. At this point the cell voltage is 0.22V, the pH is 0.0 and PH 2 = 1.0 atm . Calculate [Pb2+] under these
conditions.
c) Estimate Ksp of PbCl2 (s)
4