Nickel ion and concentrated ammonia solution reaction | Ni2+ + NH3

Asked by: THOMAS SIMON, 2019-05-16

Answered by: Heshan Nipuna, Department of Chemical and Process Engineering, University of Peradeniya


Question

What will happen when aqueous nickel (Ni2+) ion is treated with concentrated ammonia solution?


Answer:

We may assume that Ni2+ is associated with an anion such as NO3- or Cl-. Nickel nitrate and nickel chloride are soluble compounds in water.


Nickel ion solution and ammonia solution reaction

When concentrated ammonia solution is added to the aqueous green colour Ni2+ solution ( Ni(NO3)2 or NiCl2 ), blue colour hexaamminenickel(ii) ion ( [Ni(NH3)6]2+ ) is given.


If dilute ammonia solution is added instead of concentrated ammonia solution, first a green colour precipitate is given. When ammonia solution is added more, you can see that green colour precipitate dissolve and give blue colour solution.


Aqueous Ni2+ + NH3 reaction

ni2+ + nh3

This is a testing experiment of Ni2+ ion.



What will happen if a NaBr solution is added to hexaamminenickel(ii) ion?

Then saturated NaBr colourless solution is added to the [Ni(NH3)6]2+ solution. Now aqueous solution includes following cations, anions and molecules.

  • Na+
  • Ni2+
  • Br-
  • NO3- or Cl-
  • NH3 molecules

Note that NiBr2 is water soluble compound.


If Ni2+ and Br- concentrations are high, NiBr2 can be precipitated and it is a yellowish-green odorless solid.



Nickel ion and dilute ammonia solution reaction

If aqueous dilute ammonia solution is added to the Ni2+ ion, a green precipitate, nickel hydroxide ( Ni(OH)2) is given.


What will happen if we add aqueous NaOH to nickel hydroxide?

We can see nothing as colour change, dissolving, gas releasing etc. Green precipitate exists without change. So there is no effect (reaction) with NaOH or ecxess NaOH.



Why concentrated ammonia solution is used?

To form the coordination complex, there should be excess NH3 molecules. Therefore, concentrated ammonia solution is required to provide excess NH3.


Can you identify nickel +2 ion and zinc +2 ion from concentrated ammonia solution?

Yes. It is possible.

  • With concentrated ammonia solution, nickel +2 first gives Ni(OH)2 green precipitate. With excess concentrated ammonia solution, precipitate dissolves and gives a blue colour solution.
  • Zinc +2 ion gives a white precipitate ( Zn(OH)2 ). With excess ammonia solution, a colourless solution is given.


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