In the copper cycle, copper was oxidized by nitric acid making a green solution,

Question

In the copper cycle, copper was oxidized by nitric acid making a green solution, then the addition of water turned the solution blie. Please explain with reference to spectrochemical series. What is happening to the metal ion for the color change to occur? In the copper cycle, copper was oxidized by nitric acid making a green solution, then the addition of water turned the solution blie. Please explain with reference to spectrochemical series. What is happening to the metal ion for the color change to occur?

Explanation / Answer

Consider the reaction ->

4 HNO3(l) + Cu(s) ==>Cu(NO3)2(s and aq) + 2 NO2(g) + 2 H2O(l)

The green soultuin is of Cu(NO3)2

Now again--------------

Water, due to its high dielectric constant, causes the compound to dissociate into its constituent ions

Cu(NO_3)_2 ightarrow Cu^{+2} + {NO_3}^{–2}

Cu^{+2} being a transition metal, would form an octahedral aqua complex in water

Cu^{+2} + 6H_2 0 ightarrow [Cu(H_2 0)_6]^{+2}

So, the final equation is

Cu(NO_3)_2 + 6H_2 0 ightarrow [Cu(H_2 0)_6]^{+2} + {NO_3}^{–2}

Also note that the nitrate ion itself would be solvated, this is different from what happens to Cu^{+2}, for in Cu^{+2} it is the oxygen atoms of the water molecules that form a co-ordinate bond with the Cu^{+2} ion, while in case of the nitrate anion the hydrogen atoms of the water molecules would orient themselves towards the negatively charged oxygen atoms of the nitrate anion, but that is usually not shown in a chemical equation.

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