E730: Prep Notes

Introduction

A series of precipitations and complex-ion-formations are performed in a single reaction vessel.

  1.       AgCl ( Ag+ + Cl-)                                                Ksp = 1.6 x 10-10
  2.       Ag+ + 2NH3 ( Ag(NH3)2+                                     Kf = 1.7 x 107
  3.       NH3 + H+ ( NH4 )+                                                 K = 1.8 x 109                                                        
  4.       NH4+ + OH- ( NH3 + H2O)                                      K = 1/Kb = 5.6 x 104
  5.       AgBr ( Ag+ + Br-)                                                Ksp = 3.3 x 10-13
  6.       Ag+ + 2S2O32- ( Ag(S2O3)23-                              Kf = 2.0 x 1013
  7.       AgI ( Ag+ + I-)                                                      Ksp = 1.5 x 10-16
  8.       Ag+ + 2CN- ( Ag(CN)2-                                        Kf = 5.6 x 108
  9.       Ag2S ( 2Ag+ + S2-)                                              Ksp = 1.0 x 10-49

Supplies

Short List

  •   Goggle and gloves
  •   1000 mL tall beaker
  •   10 mL graduated cylinder
  •   Stirring rod
  •   pH paper
  •   distilled water in wash bottle
  •   1 L screw top disposal container
  •   Stir bar and stir plate

Chemicals

  •   0.2 M AgNO3
  •   0.2 M NaCl
  •   0.2 M NH4OH
  •   2-6 M HNO3
  •   2-6 M NaOH
  •   2-6 M NH3
  •   0.2 M KBr
  •   0.2 M Na2S2O3
  •   0.2 M KI
  •   0.2 M NaCN
  •   0.2 M Na2S

 Prior to Lecture  (prep time ~ 5 min.)

Solutions ranging in concentration from 0.2 M to 2 M are suitable for this demonstration.  Concentrations of HNO3, NH3, and NaOH should be higher (2-6 M) and all relatively close to each other in concentration.

1.         Prepare 0.2 M AgNO3 (silver nitrate) solution.  ( MW = 169.88 g/mol ; 34 g AgNO3/L)

2.         Immediately before lecture, place 10 mL of 0.2 M AgNO3 in a tall 1000-mL beaker and dilute to approximately 200 mL.  Place a stir bar in the solution.

3.         Assemble the following chemicals:

    1.                  NaCl (sodium chloride) solution                                         MW = 58.44 g/mol
    2.                  NH4OH (ammonium hydroxide) solution                            
    3.                  HNO3 (nitric acid)                                                             
    4.                  NaOH (sodium hydroxide) solution                                   
    5.                  KBr (potassium bromide) solution                                     MW = 119.01 g/mol
    6.                  Na2S2O3 (sodium thiosulfate) solution                              MW = 158.11 g/mol
    7.                  KI (potassium iodide) solution                                           MW = 166.01 g/mol
    8.                  NaCN (sodium cyanide) solution                                       MW = 49.01 g/mol
    9.                  Na2S (sodium sulfide) solution                                          MW = 78.04 g/mol

4.         Assemble the following equipment:

    1.                 1000-mL tall beaker
    2.                 stir bar and stir plate
    3.                 goggles
    4.                 gloves
    5.                 1 L-capacity screw top disposal container
    6.                 pH paper
    7.                 stirring rod
    8.                 distilled water in wash bottle

To Conduct Demonstration:

Add a minimum amount of reagent until precipitation or complexation is complete.

  1.                Place a small amount of AgNO3 in a 1000-mL short form beaker on a stir plate and begin stirring.  (In a  tall form beaker, the solution will not stir well at the top.)
  2.                Add NaCl solution.  AgCl precipitates.
  3.                Add NH3 solution.  Ag(NH3)2+ forms and the precipitate disappears.
  4.                Add HNO3.  AgCl reappears as Ag(NH3)2+ is reacted away.
  5.                Add NaOH solution.  Ag(NH3)2+ is re-formed and the precipitate disappears.
  6.                Add KBr solution.  AgBr precipitates
  7.                Add Na2S2O3 solution.  Ag(S2O3)23- forms as AgBr dissolves.
  8.                Add KI solution.  AgI precipitates.
  9.                Pour solution into disposal container and cap tightly.

Safety and Disposal

Goggles should be worn.  Silver solutions will stain skin black on contact.  Gloves should be worn.  Acidic cyanide solutions can release poisonous HCN gas.  Be sure all solutions containing CN- are kept basic.