Co2+(aq) + 6NH3(aq) ->-< Co(NH3)6 2+(aq) red yellow the color of the mixture can
ID: 795367 • Letter: C
Question
Co2+(aq) + 6NH3(aq) ->-< Co(NH3)6 2+(aq)
red yellow
the color of the mixture can indicate the relative position of equilibrium.
A student measured the absorbance of the above solution at lambda max of the product the student found that the absorbance at lamda max of the product is 0.502.
He then added a little bit of solid cobalt (II) Nitrate and waited for a new equilibrium to be achieved. The new absorbance at lambda max of the product after equilibrium was achieved at 0.610.
Fill in the blanks: when the absorption increases it indicates that the concentration of -------------------------------- and that means the reaction shifted to the ______________________.
2)Why did the reaction shift in this direction?
3)When solid sodium sulfide was added the absorbance decreased to 0.381 explain why the absorbance decreased according to lechateliers number.
Explanation / Answer
The cobalt(II) ions in water are actually a complex ions .. the hexaaquacobalt(II) ion [Co(H2O)6]2+
The reaction of hexaaquacobalt(II) ions with hydroxide ions
Hydroxide ions (from, say, sodium hydroxide solution) remove hydrogen ions from the water ligands attached to the cobalt ion.
Once a hydrogen ion has been removed from two of the water molecules, you are left with a complex with no charge - a neutral complex. This is insoluble in water and a pinky blue precipitate is formed.[this can change colour on standing]
[Co(H2O)6]2+ + 2OH- ---> [Co(H2O)4(OH)2]2+ + 2H2O
in simple chemistry you would write this as Co2+(aq) + 2OH-(aq) ---> Co(OH)2(s)
The ammonia acts as both a base and a ligand to form a coordinate bonded complex. With a small amount of ammonia, hydrogen ions are pulled off the hexaaqua ion exactly as in the hydroxide ion case to give the same neutral complex.
[Co(H2O)6]2+ + 2NH3 ---> [Co(H2O)4(OH)2]2+ + 2NH4+
in simple terms
NH3 + H2O ---> NH4OH --> NH4+ + OH-
Co2+(aq) + 2OH-(aq) ---> Co(OH)2(s)
That precipitate dissolves if you add an excess of ammonia. The ammonia replaces water as a ligand to give hexaamminecobalt(II) ions. Ammonia is a stronger ligand than water, so can replace it.
[Co(H2O)6]2+ + 6NH3 ---> [Co(NH3)6]2+ + 6H2O
The hexaamminecobalt(II) complex is very easily oxidised to the corresponding cobalt(III) complex. In the test-tube this is seen as a rapid darkening to a deep red-brown solution.
In fact the hexaamminecobalt(III) ion is yellow! What you see is a mixture of this ion and various other cobalt(III) ions involving ligand exchange reactions with both water molecules and negative ions present in the solution.
The web site I've given you also has colour reference test tube diagrams as these are quite complicated, as you have realised.
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