Using the provided solutions, determine experimentally if they form soluble comp
ID: 503921 • Letter: U
Question
Using the provided solutions, determine experimentally if they form soluble complex ions with ammonium hydroxide. Using 1 - 2 mL of each solution, first add NH_4OH dropwise until a precipitate appears. Then attempts to dissolve the precipitate by adding excess NH_4OH. If a your text or an online site (search for 'complex ion formation constants') for help with the complex ion formulas. Write the colors of the hydroxide precipitates which do not dissolve in excess ammonia. In some cases you may note color changes in the solutions as you add the ammonia but not precipitate. In these cases, the complex ions form. Add a few drops of a freshly prepared solution of sodium sulfide to the solution of the copper-ammonia complex from part B. This reaction MUST be run in the HOOD. Before disposing of the black solid, add some concentrated NH_3 (in the hood!).Does the solid dissolve? Place in the waste container. 1. Write the net ionic equation for the reaction which occurs. In net ionic equations complex ions stay in the molecular ion form. 2. Even complex ions will ionize to some extent. Since this ionization is an equilibrium, the laws of equilibrium apply. The reaction is: [Cu(NH_3)_4]^+2 (aq) Cu^+2 (aq) + 4 NH_3 (aq) Write the equilibrium constant expression for this system. 3. The constant written in (2) is called the "instability constant". Tabulated values for equilibrium constants for these systems are usually given as "formation" or "stability" constants and refer to the reverse of the reaction given above. Write the reverse reaction and give the new equilibrium constant expression. What is the mathematical relationship between the stability and instability constants for a given system? If a complex has a small instability constant, is the complex stable or unstable?Explanation / Answer
C. Al+3 forms gelatinous white precipitate when treated with aq. NH4OH solution. Formula is Al(OH)3
Co+2 forms light green ppt with aq NH4OH solution. With excess ammonia it forms a red brown ppt. Formula is [Co(NH3)6]+2
Cr+3 forms green ppt with aq NH4OH solution. With excess NH3 it forms pink solution Formula is [Cr(NH3)6]3+
Fe+3 forms brown ppt with aq. NH4OH solution. Formula [Fe(OH)3(H2O)3]
Mn+2 forms dark brown ppt with excess NH3 solution. Formula [Mn(H2O)4(OH)2]
Ni+2 forms a green ppt with less ammonia solution but it forms blue solution with excess NH3 solution. Formula [Ni(NH3)6]2+
Zn+2 first forms a white ppt with less NH3 which on excess of NH3 forms a complex which is water soluble. Formula [Zn(NH3)4]2+
D. [Cu(NH3)4]2++ Na2S=CuS+2NaNH2
2. Equilibrium constant Keq={[Cu]2+[(NH3)4]}/[Cu(NH3)4]2+
3. Reverse reaction is [Cu+2] + 4NH3=[Cu(NH3)4]+ Keq=[Cu(NH3)4]2+/{[Cu+2][NH3)4
The stability constant and instability constant is reciprocal of one another ie their multiplication is 1
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