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.
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.