In part III of the lab (see p. 11), you perform a series of chemical reactions w
ID: 493986 • Letter: I
Question
In part III of the lab (see p. 11), you perform a series of chemical reactions which represent the types of chemical reactions and equilibria that you have studied throughout Chem 123. These reactions are done in the well-plate labeled A and B in the lab instructions. For half cell A when NH_3 (aq) is added to Cu^2+ (aq), a complex ion will form via is a Lewis acid-base reactions. The K_f for Cu(NH_3)_2^2+ is 2.1 times 10^7. a. Write the reaction for the formation of Cu(NH_3)_2^2+. b. Interpret the magnitude of K_f. When NH_3 (aq), what will happen to the unreacted or "free" Cu^2+ concentration? Choose one. c. After the complex ion forms, the next step in the lab has you add H_2 SO_4 (aq). Write the reaction that will occur. d. When the H_2 SO_4 (aq) is added predict, what will happen to the "free" [Cu^2+]? Choose one. For half-cell B, when NaOH(aq) is added to Cu^2+ (aq), a sparingly soluble solid, Cu(OH)_2, will form. a. Write the reaction for the formation of Cu(OH)_2. The D_sp for Cu(OH)_2 is 4.8 times 10^-20. b. Interpret the magnitude of K_sp. When NaOH(aq) is added to Cu^2+ (aq), what will happen to the "free" [Cu^2+]? Choose one. c. After the nearly insoluble solid forms, the next step in the lab you add H_2 SO_4 (aq). Write the reaction that will occur. When the H_2 SO_4 (aq) is added, predict what will happen to the "free" [Cu^2+]? Choose one.Explanation / Answer
1)a)
The reaction for formation of complex ion is
Cu+2(aq) + 4NH3 (aq) ----------------> [Cu(NH3)4]+2
b) The Kf value is very large
When ammonia is added to aquesous Cu+2 solution , the complex is formed and thus the free [Cu+2] decreases.
c) when aqueous H2SO4 is added to the above solution
[ Cu(NH3)4]+2 + 4H+ (aq) + 2SO4-2 -(aq)----------> Cu+2 (aq) + 4NH4+ (aq) +2SO4-2 (aq)
this reaction occurs
d) Hence The free [Cu+2] increases on addition aq. H2SO4.
2)a) The reaaction is
Cu+2(aq) + 2 OH- (aq) ------------------> Cu(OH)2 (s) Kf= 1/Ksp
b) AS the Ksp is very small the Kf value is very large.
When aq.NaOH is addede to Cu+2 solution, more and more precipitation occurs and thus [Cu+2] decreases.
c) After all the solid is formed aq.H2SO4 is added. and the following reaction occurs.
CU(OH)2 (s) + 2H+ (aq) -------------> Cu+2(aq) + 2H2O(l)
d) thus when aq. H2SO4 is added ,free the [Cu+2] increases.
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