1. The problem statement, all variables and given/known data Which two of the following could be used to produce crystals of nickel? A Addition of a piece of copper to a NiSO4 B Addition of a piece of zinc to a NiSO4 solution. C Addition of a piece of potassium to a NiSO4 solution. D Electrolysis of molten NiSO4. 2. Relevant equations 3. The attempt at a solution The reaction should change (Ni)2+ to Ni so (Ni)2+ must be reduced by accepting electrons from the added metal. So the piece of metal should act as a reductant. From the tables (Cu)2+ + 2e --> Cu (+0.34) (Ni)2+ + 2e --> Ni (-0.23) (Zn)2+ + 2e --> Zn (-0.76) (K)+ + 2e --> K (-2.93) It looks like both Zn and K are worthy candidates as the reductant and produce nickel crystals. If we look at D, we can see that out of the three species in the electrolyte, (Ni)2+, (SO4)2- and H2O the strongest oxidant is (Ni)2+ and the strongest reductant is H2O because O2 + 4H+ + 4e --> 2H2O (+1.23) (S2O8)2- + 2e --> 2(SO4)2- (+2.01) (SO4)2- and H2O as oxidants are weaker than (Ni)2+ because 2H2O + 2e --> H2 + 2(OH)- (-0.83) (SO4)2- + H2O + 2e --> (SO3)2- + 2OH- (-0.93) So it is possible to produce nickel crystals via electrolysis. Hence B,C,D are all possible solutions but the solutions only had B,D as the correct solution. Why isn’t C correct? Although I notice the question did specify that there are two correct answers, there seems to be an extra one.