E. Cu(NH) Equilibria (Competing Reactions) Copy and expand the table below on a
ID: 564173 • Letter: E
Question
E. Cu(NH) Equilibria (Competing Reactions) Copy and expand the table below on a full sheet of paper so that you have more space for to agitate to mis upon each addition of reagent Contents 2 mL of O.1 M LAh CuSO Not Applicable To the above irst change CV, (aq) + 20H (aq) Test tube #1 add 1 mL of I M CuODs) NHs dropwise (labeled NHOH) Ks-1.76 x 10 To the above add 3 mL of 1.5 M addition of | CV (ag) + 4 NH,(az) TT#1 with addition of HCI | HCl dropwise second chanse From your observations, describe the color of the following. (See your results in the previous table NHs Cu/OH) CuNH) NH Chart continued. Copy and expand the table below on a full sheet of paper so that you have more space for your explanations Contents previously made solution (2 mL of O.1 M CuSO, I mL of NH and 3 mL of 15 M HCD To the above solution add 15 mL of IM NH dropwise Tes Tube #2 extra large Shy Not Applicabl E2 | dark blue Explain from your observations the conditions necessary to create Cu/OH6) verss CNH( (Think about the concentrations of NH, when you observed the different produs)'ar would the equilibrium constant for the formation of CuOHds) be greater or less tan the cquilitrincontatfr the formation of Cu(NHs) (aq)? Explain your reasoning. (Figuring this out is crucial to you ndersianding of what you did in this section .Explanation / Answer
The colors are listed below.
Ion/Complex
Color
Cu2+
Pale blue
Cu(OH)2
Pale or light blue
Cu(NH3)42+
Dark blue
OH-
Colorless
NH3
Colorless
NH4+
Colorless
SO42-
Colorless
Cu(OH)2 is formed from CuSO4 when NH3 is added dropwise to CuSO4. NH3 forms an aqueous solution in water, NH4OH which decomposes into OH-. The OH- reacts with CuSO4 to form pale blue Cu(OH)2. The reaction can be shown as below.
NH3 (aq) + H2O (l) ---------> NH4OH (aq) --------> NH4+ (aq) + OH- (aq)
CuSO4 (aq) + 2 OH- (aq) ---------> Cu(OH)2 (s) + SO42- (aq)
When the concentration of NH4+ increases (due to addition of excess NH3 to water), the precipitated Cu(OH)2 dissolves, forming a dark blue solution of tetrammonium copper (II), i.e, Cu(NH3)42+. The reaction is given as
Cu(OH)2 (s) + 4 NH4OH (aq) --------> Cu(NH3)42+ (aq) + 4 H2O (l) + 2 OH- (aq)
The equilibrium constant for the formation of Cu(NH3)42+ (actually, this is the formation constant of the complex ion) is much higher than the equilibrium constant for the formation of Cu(OH)2. This must be true, otherwise, Cu(OH)2 would not have dissolved in excess ammonia, producing Cu(NH3)42+.
Ion/Complex
Color
Cu2+
Pale blue
Cu(OH)2
Pale or light blue
Cu(NH3)42+
Dark blue
OH-
Colorless
NH3
Colorless
NH4+
Colorless
SO42-
Colorless
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