LECTURE 25 1. Determine the oxidation number of the underlined element in each of the following compounds or ions: (a) HCl +1 (b) NH4+ +1 (c) NaH -1 (d) NaAlH4 -1 (e) FeCl42- +2 (f) ICl3 +3 (g) SO42- +6 (h) Cu(OH)2 +2 2. Hydrogen peroxide (H2O2) is a harmful and reactive byproduct of metabolism. To prevent H2O2 from causing oxidative damage to cells, the enzyme catalase catalyzes the conversion of H2O2 to much less reactive molecules, oxygen and water. 2H2O2 → 2H2O + O2 Using oxidation numbers, determine if H2O2 is reduced and/or oxidized in this reaction. H2O2 is both reduced and oxidized 3. Using half-reactions, balance the following equations in acidic solution. Determine which atom or compound is the oxidizing agent and which is the reducing agent in each reaction. (a) Cr2O7-2 (aq) + C2H6O (aq) à Cr+3 (aq) + C2H4O (aq) Cr2O7-2 (aq) + 6 e- + 14 H3O+ (aq) à 2 Cr+3 (aq) + 21 H2O (aq) C2H6O (aq) + 2 H2O (l) à C2H4O (aq) + 2 e- + 2 H3O+ (aq) Cr2O7-2 (aq) + 3 C2H6O (aq) + 8 H3O+ (aq) à 2 Cr+3 (aq) + 3 C2H4O (aq) + 15 H2O (l) Cr2O7-2 (aq) is the oxidizing agent, C2H6O (l) is the reducing agent (b) MnO4- (aq) + H2O2 (aq) à Mn2+ (aq) + O2 (g) MnO4- (aq) + 5 e- + 8 H3O+ (aq) à Mn2+ (aq) + 12 H2O (l) H2O2 (aq) + 2 H2O (l) à O2 (g) + 2 e- + 2 H3O+ (aq) 2 MnO4- (aq) + 5 H2O2 (aq) + 6 H3O+ (aq) à 2 Mn2+ (aq) + 5 O2 (g) + 14 H2O (l) MnO4- (aq) is the oxidizing agent, H2O2 (aq) is the reducing agent 4. Using half-reactions, balance the following equations in basic solution. Determine which atom or compound is the oxidizing agent and which is the reducing agent in each reaction. (a) CO2 (g) + F2 (g) à FO3- (aq) + C2O4-2 (aq) 2 CO2 (g) + 2 e- à C2O4-2 (aq) F2 (g) + 12 OH- (aq) à 2 FO3- (aq) + 10 e- + 6 H2O (l) 10 CO2 (g) + F2 (g) + 12 OH- (aq) à 5 C2O4-2 (aq) + 2 FO3- (aq) + 6 H2O (l) CO2 (g) is the oxidizing agent, F2 (g) is the reducing agent (b) Cr(OH)3 (aq) + Br2 (l) à CrO4-2 (aq) + Br- (aq) Cr(OH)3 (aq) + 5 OH- (aq) à CrO4-2 (aq) + 3 e- + 4 H2O (l) Br2 (l) + 2 e- à 2 Br- (aq) 2 Cr(OH)3 (aq) + 3 Br2 (l) + 10 OH- (aq) à 2 CrO4-2 (aq) + 6 Br- (aq) + 8 H2O (l) Br2 (l) is the oxidizing agent, Cr(OH)3 (aq) is the reducing agent 5. In some alkaline batteries, a solid zinc electrode in a basic solution is oxidized to ZnO while solid manganese (IV) oxide is reduced to solid manganese (III) oxide. (a) Write the half-reactions for both the anode and cathode of the cell, as well as the overall reaction. (b) Calculate the mass in kg of ZnO(s) formed if 1.0 x 104 A are passed through the cell for 12 hours. (a) Anode: Zn (s) + 2OH- (aq) à ZnO (s) + H2O (l) + 2eCathode: 2MnO2 (s) + H2O (l) + 2e- à Mn2O3 (s) + 2OH- (aq) Overall: Zn (s) + 2MnO2 (s) à ZnO (s) + Mn2O3 (s) (b) 180 kg 6. A jeweler is investigating a novel method for electroplating tungsten onto base metal. The jeweler passes a 30.0 A current through a solution for 1.00 hours and 100. g of tungsten is deposited on the ring. What is the oxidation number of tungsten in the solution? 2, W2+