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Combined or Mixed Equilibria of Cu2+

Chemical Concept Demonstrated

  • Complex-formation equilibrium

Demonstration

Each of the dishes contain Cu 2+ ion solution.
  • Aqueous ammonia is added dropwise into the first dish.
  • Excess ammonia is added into the first dish.
cu2.gif

Observations

A precipitate forms as the ammonia is added dropwise into the dish.    Additional ammonia causes the precipitate to dissolve and a dark-blue solution is formed.

Explanations (including important chemical equations)

A light-blue colored precipitate of Cu(OH)2 is formed when aqueous ammonia is added to Cu 2+  ion solution. 

Cu 2+  (aq) + 2 (OH)- (aq) <=> Cu(OH)2 (s)             Ksp = 2.2 x 10-20

In the presence of excess NH3, this equilibrium shifts to the left as the free Cu 2+ ion is sequestered from the solution as the Cu(NH3)4 2+ complex ion.

Cu 2+  (aq) + 4 NH3 (aq) <=> Cu(NH3)4 2+ (aq)    Ksp = 2.1 x 1013

The intensity of the absorption of light by the Cu(NH3)4 2+ complex appears to be orders of magnitude greater than the corresponding Cu(H2O)6 2+ complex.  The molar absorbances of these complexes, however, differ, roughly, only by a factor of four.  The difference between the intensities of the colors of these complexes can be explained by noting that the maximum wavelength for the Cu(H2O)6 2+ complex is about 800 nm, whereas the maximum wavelength for the Cu(NH3)42+ complex occurs at about 600 nm.  Since a larger portion of the absorption in the Cu(NH3)4 2+ complex is in the visible portion of the spectrum, the color appears considerably more intense.

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