Separate /save some unknown

• DR ROB’S “Get to the point of this lab!”• Worksheet

• Postive cations like Pb2+ , Ag + and Hg+• Can be ptt by OH-, Cr2O72-, S2-• And Cl-

Pb2+ Cr2O72-

.

• The most confusing ion is the S2- ion

• In acidic and in basic solutiopn

• Chemlab sheet• Centiguge sheet

Best website

• http://www.dartmouth.edu/~chemlab/chem3-5/qual_cat/full_text/procedure.html

.

• FROM BOOK RIGHT TO ADEMDUM• MENTION 12 IONS

• YOU WILL DO THREE TODAY

• DO ADDEMDUM FIRST• THEN GO TO BOOK• REVIEW CENTRIFUGE

.

• EASY FOR SOMETHING TO OCCUR DURING THESE LABS

• MUST PLAN BEFORE YOU GET HERE

• USE OR LAB BOOK TO WRITE DOWN REACTIONS-MANY TESTSTUBES ALL AT ONCE!

.

#1 – Qualitative Analysis• Principles of Qualitative Analysis

1.Confirmatory Tests:

These are tests that determine conclusively that a certain ion is present. Interfering ions are removed before a confirmatory test is done.

• .

.

Separations:

These are procedures that separate groups of ions from other groups, or individual ions in a mixture of ions. Separations are followed by confirmatory tests to identify the separated ion.

Precipitation and Separation of Group I Cations:

• Group 1: The silver group: Ag+, Hg22+, Pb2+ -----These ions precipitate (as the chlorides) from 0.3M HCl solution.

• « » »Rationale: Softest acids>>>>>>>>• react strongly enough with a borderline base• to precipitate in acid solution..

You know some of this already

• You have already done lead in CHM121

• And,

• You have just done another ion similar to Ni–which will get to shorty (Cu(NH3)4)

Just add hot waterThese cations are Pb+2, Hg2• +2, and Ag+ are all insoluble in cold water.

Precipitation and Separation of Group I Cations:

• Precipitation and Separation of Group I Cations:

• Pb+2, Hg2+2, and Ag+ are all insoluble in cold water.

• They can be removed as a group from solution by the addition of HCl via simple precipitation:

Ag+ + Cl- → AgCl(s)

chloro-complexes may form !!

• It is important to add enough HCl to ensure complete precipitation, but not too large an excess. In highly concentrated HCl solutions, chloro-complexes may form such as

• AgCl2-1.

Precipitation and Separation of Group I Cations:

• Lead chloride can be separated from AgCl and Hg2Cl2 by heating with water, essentially reversing the above reaction.

• Once Pb+2 is in solution, you can discern its presence by adding chromate ion to produce a yellow solid:

• Pb+2 + CrO4-2 → PbCrO4(s) (yellow)

Begin . . . . . .

• Show the PDF• Show the word files• Show flow chart now

Precipitation and Separation of Group I Cations:

• Hg2Cl2 can be distinguished from AgCl by reaction with ammonia via oxidation reduction to yield finely divided black metallic mercury and a white complex compound HgNH2Cl. As the reaction proceeds, the solid appears to change colors from white to black or gray:

.

• Hg2Cl2(white solid) + 2 NH3(aq) → • NH4+(aq) + Cl-(aq) + Hg(black

liquid) + HgNH2Cl(white solid)

Hg2Cl2(white solid) + 2 NH3(aq) →

NH4+(aq) + Cl-(aq) + Hg(black liquid) +

HgNH2Cl(white solid)

Aminomercuric chlorideand,

• Silver chloride also reacts with aqueous ammonia to form a complex ion that remains in solution. Addition of a

• strong acid will destroy the complex and confirm the presence of silver ion by re-precipitating the white AgCl solid:

• Silver chloride also reacts with aqueous ammonia to form a complex ion that remains in solution. Addition of a

• strong acid will destroy the complex and confirm the presence of silver ion by re-precipitatin the white AgCl solid:

• AgCl(s) + 2 NH3(aq) → Ag(NH3)2 +(aq) + Cl-(aq)

• Ag(NH3)2+(aq) + 2 H+(aq)+ Cl-(aq) → AgCl(s) + 2 NH4aq)