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Complex ion stability (see question below):Copper ii nitrate, cu(No3)2, forms a light...

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virginia1231 | Student | eNotes Newbie

Posted November 19, 2011 at 11:35 PM via web

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Complex ion stability (see question below):

Copper ii nitrate, cu(No3)2, forms a light blue precipitate when added to a solution of sodium phosphate (na3po4). When copper ii nitrate is added to a solution of sodium nitrite, nano2, a green solution results. If equal concentrations of sodium phosphate and sodium nitrite are added to a solution of copper ii nitrate, a light blue precipitate forms. Write the formula for each complex ion or precipitate formed and determine which complex ion/precipitate has the higher relative stability.

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ncchemist | eNotes Employee

Posted November 20, 2011 at 3:56 AM (Answer #1)

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The equation you had written out was not correct so I deleted it.  Just to clear up any confusion, the nitrate ion is NO3(1-) and the nitrite ion is NO2(1-).  Copper nitrate is a blue powder that readily dissolves in water.  When sodium phosphate is added to the solution, copper phosphate is formed which is a light blue solid and precipitates from water:

3Cu(NO3)2 + 2Na3PO4 --> Cu3(PO4)2(s) + 6Na(1+)(aq) + 6NO3(1-)(aq)

This is the fully balanced equation that shows the copper phosphate as a solid precipitate and the inactive sodium and nitrate ions (or spectator ions) in solution.  Since the copper phosphate is a solid precipitate there are no complex ions involved with this process.

When sodium nitrite is added to an aqueous solution of copper nitrate, no precipitate forms.  This means that there are complex copper ions, nitrate ions, and nitrite ions in solution.  I'll admit that I don't know the exact complex ion that the nitrite group makes with copper (and I cannot find any info online) so I'll write a very general equation:

Cu(NO3)2 + NaNO2 + 6H2O <--> Cu(H20)6(2+)(aq) + 2NO3(1-)(aq) + Na(1+)(aq) + NO2(1-)(aq)

The hexaaquacopper complex ion is a common complex ion for copper (II) to make.  The excess nitro ions might make a unique complex with the copper ion.  They key with complex ion equations is to make sure that the charges all balance out.  Note that the charges on the right hand side of the equation all add up to 0, which is the same as the left hand side.

Regarless of this, the fact that added equimolar amounts of both sodium nitrite and sodium phosphate added to copper nitrate makes a light blue precipitate appear means that the copper phosphate precipitate has a higher stability than the complex ion formed in the second case.

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