Producing Nickel Crystals: B & D are Correct Answers

[pivoxa15]

Homework Statement

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.

Homework Equations

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.

 

[chemisttree]

The solution in C contains water. What happens when potassium contacts water?

There is no water in "molten NiSO4". Actually, there is no such thing as molten nickel sulfate since it decomposes at its melting point to NiO and a gas. So, D is actually a wrong answer. (Can you figure out what the gas(es) might be?)

The only reasonable answer is B.

 

[pivoxa15]

When K contacts H2O, the K will act as a reductant and H2O as an oxidant hence react with each other. But K will also react with Ni so Ni crystals will also be formed. Ni is a stronger oxidant than H2O.

The gases would be sulphur trioxide. And sulphur oxide and oxygen.

 

[chemisttree]

Potassium will indeed react with water. The potassium explodes into molten white-hot purlpish flames and dances and skitters over the surface of the water. The result is potassium hydroxide solution and perhaps some small secondary fires here and there. The pH of the solution will afterwards be 14+. What will happen to nickel in solution at this pH? My Merck says that nickel hydroxide is "insol in water".

 

[pivoxa15]

What is 'Merck'? So nickel hyroxide or Ni(OH)2 will precipitate into a solid in water. Hence no nickel solids will form. In this case K has created the exitence of OH- which will react with Ni and form a solid.

 

[chemisttree]

"The Merck Index", 11th ed. (1989), ISBN Number 911910-28-X

From the Editor's preface:

"In the 100 years since the Merck Index was first published in 1889, it has evolved from a 170-page alphabetic catalog of chemicals and drugs sold by Merck, into an authoritative and widely quoted 2300-page multidisciplinary encyclopedia."

Susan Budavari, Editor
Merck Sharp & Dohme Research Laboratories
Rahway, New Jersey 07065