Using the Latimer diagrams (pH = 0): 1.401 V 1.692 V [Au(H2O)6]3+ → [Au(H2O)6]+ → Au 0.926 V 1.154 V — Analytical Chemistry Chemistry Question
Redox equilibria in aqueous solutions
Using the Latimer diagrams (pH = 0):
1.401 V 1.692 V [Au(H2O)6]3+ → [Au(H2O)6]+ → Au
0.926 V 1.154 V [AuCl4]¯ →[AuCl2]¯ → Au
0.810 V 0.960 V [AuBr4]¯ → [AuBr2]¯ → Au
answer the following questions:
Is Au(I) stable to disproportionation in aqueous solutions in the absence and in the presence of chloride and bromide ions? Support your answer by calculations.
Model Answer
Ion [Au(H2O)6]+ is unstable towards disproportionation, because E° ([Au(H2O)6]+ / Au) > E° ([Au(H2O)6]3+ / [Au(H2O)6]+). For the reaction 3 [Au(H2O)6]+ → 2 Au + [Au(H2O)6]3+ + 12 H2O, E° = 1.692 – 1.401 = 0.291 V. In the presence of chloride- and bromide-ions Au(I) remains unstable for the same reason: 3 [AuCl2]– → 2 Au + [AuCl4]– + 2 Cl–, E° = 1.154 – 0.926 = 0.228 V. 3 [AuBr2]– → 2 Au + [AuBr4]– + 2 Br–, E° = 0.960 – 0.810 = 0.150 V.
Is it possible to oxidize gold powder by pure oxygen in the aqueous solution in the presence of chloride ions if (р(О2) = 1 atm and E0 (O2, H+ / H2O) = 1.229 V?
Model Answer
In the presence of chloride ions gold powder can be oxidized by pure oxygen, because Е°(O2/H2O) exceeds E°([AuCl2]–/Au). 4 Au + O2 + 8 Cl– + 4 H+ → 4 [AuCl2]– + 2 H2O, E° = 1.229 – 1.154 = 0,075 V.
At what pH interval is it possible to oxidize gold powder by hydrogen peroxide (E0 (H2O2, H+ / H2O) = 1.763 V) in the presence of chloride-ions? Assume that the activities of all ions in the solution except H+ are 1.
Model Answer
The redox potential Е(Н2O2,H+/H2O) depends on pH (the Nernst equation): Н2O2 + 2 H+ + 2 e– = 2 H2O. Е(Н2O2,H+ / H2O) = Е° – (0.059 / 2) log(1 / [H+]2) = 1.763 – 0.059 pH. The potential E([AuCl2]– / Au) doesn’t change its value in acidic medium: E([AuCl2]– / Au) = E°([AuCl2]–/Au) = 1.154 V. So, the reaction 2 Au + H2O2 + 4 Cl– + 2 H+ → 2 [AuCl2] – + 2 H2O is possible if Е(Н2O2,H+ / H2O) > E([AuCl2]– / Au): 1.763 – 0.059 pH > 1.154, pH < 10.3. In fact, this pH interval is restricted to acidic medium as in basic solutions dichloroaurate(+1) decomposes to gold(+1) oxide.