The insoluble Chlorides AgCl, Hg2Cl2, PbCl2 Type I Cations The insoluble Chlorides AgCl, Hg2Cl2, PbCl2

The only metals to make insoluble chlorides Formula Ksp AgCl 1.8 x 10-10 Hg2Cl2 1.1 x 10-19 PbCl2 1.7 x 10-5 Make the chlorides by reacting with Ag+, Hg22+ and Pb2+ with HCl If you don’t add too much HCl they all precipitate out of solution they are all white solids If you add too much HCl then the PbCl2 and AgCl dissolve PbCl2(s) + 2Cl-(aq)  [PbCl42-](aq) AgCl(s) + Cl-(aq)  [AgCl2-](aq) Once we have these as the metal chlorides we need a way to identify which of the metals are present, is it just one or all of them?

Separate the Pb and Confirm Of the chlorides PbCl2 has the largest Ksp – in hot water some of the PbCl2 dissolves (PbCl2 has a 3-fold increase in solubility from 20—100oC while the AgCl and the Hg2Cl2 stay as solids) PbCl2(s) → Pb2+(aq) + 2 Cl-(aq) Separate by centrifuging. To the supernatant add Lead(II)Chromate Pb2+(aq) + CrO42-(aq) → PbCrO4(s, yellow) A yellow PPT confirms Pb2+

Separate Ag+ from Hg22+ To the PPT from the first step, add 10 drops of 4M or 6M NH4OH AgCl(s) will dissolve in NH3(aq) AgCl(s) + 2 NH3(aq) → [Ag(NH3)2+](aq) + Cl-(aq) meanwhile the mercurous chloride, disproportionates Hg2Cl2 (s) (white) + 2NH3(aq) → Hg(s, black) + HgNH2Cl (s, white) + NH4Cl(aq) A black/white/grey PPT confirms the presence of the mercurous cation

Confirm Ag To the solution (supernatant) add 6M HNO3 till acidic Ag(NH3)2+(aq) + 2H+(aq) → Ag+(aq) + 2 NH4+(aq) Ag+(aq) + Cl-(aq) → AgCl(s) __________________________________________________ Ag(NH3)2+(aq) + 2 H+(aq) + Cl-(aq) → AgCl(s, white) + 2 NH4+(aq) White PPT of AgCl(s) confirms Ag+

Flow Diagram Ag+(aq), Hg22+(aq), Pb2+(aq) HCl AgCl(s,white), Hg2Cl2(s, white), PbCl2(s, white) Hot water Pb2+(aq) AgCl(s,white), Hg2Cl2(s, white) NH3(aq) K2CrO4(aq) PbCrO4(s, yellow) Ag(NH3)2+(aq) Hg(s, black), HgNH2Cl (s, white) Yellow ppt confirms Pb2+ Grey ppt confirms Hg22+ HNO3(aq) AgCl (s, white) White ppt confirms Ag+

Technique Tips Cleanliness is VITAL use clean test tubes, stirring rods etc Dispense from clean droppers, or the droppers that are reserved for that particular reagent Do not place the tip of the dropper into the solution in the test tube Always screw the cap back on the reagent bottle after use Mixing of a reaction mixture: Small amounts in the bottom of the test tube can be mixed by flicking the bottom of the test tube Larger amounts stir with a clean glass stirring rod Use small test tubes for centrifuging and always ensure there is a test tube in the slot opposite the one you are using otherwise the centrifuge is unbalanced After centrifuging carefully pour off the liquid (decantate or supernatant) into a clean test tube Wash the precipitate with deionized water so that subsequent chemical tests are not interfered with by contaminants Heat test tubes in a water bath Test pH by placing the wet stirring rod on the pH paper (do not put the pH paper in the test tube)

Today and Monday Work individually Run the known along with your unknown Record observations of each step Use your observations to determine what you have in the unknown Check with me Bottle beer?