Exp 8 Qualitative Analysis of Group I Cations PDF

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Inorganic Chem Lab assignments from Dr. Patricia Smith...

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CHEM-102

Experiment #8

Qualitative Analysis of Group I Cations Pre-Lab: 

After reading the lab, complete items a, b, c, and d (title, purpose, chemicals and equipment, and summary of procedure) as described on page 10 of Exp. 1 on an 8 1/2 x 11 sheet of paper.



Answer the following questions on 8 1/2 x 11 sheet of paper or in your laboratory notebook if one is required by your instructor:

1. Which chloride salt is insoluble in cold water but soluble in hot water? 2. Which chloride salt dissolves in aqueous NH3? 3. How could you distinguish: a. BaCl2 from AgCl? b. HNO3 from HCl? 4. Complete and balance the following equations: a. b. c. d.

AgCl(s) + NH3(aq)  Pb2+(aq) + I-(aq)  Hg2Cl2(s) + NH3(aq)  Ag(NH3)2+(aq) + H+(aq) + Cl-(aq) 

5. What can you conclude if no precipitate forms when HCl is added to an unknown solution? Lab Activities: 

Go over the prelab questions with your lab instructor



Complete lab and fill in data sheet.

Objective: To become familiar with the chemical properties of the following dissolved cations: Ag+, Pb2+, Hg22+ 1|Page

Introduction: GROUP I CATIONS: Pb2+, Ag+, Hg22+ Chemistry of Group I Cations Because the chlorides of Pb 2+, Ag+, and Hg22+ are insoluble, they may be precipitated and separated from the cations of groups II, III, and IV by the addition of HCl. The following equations represent the reactions that occur: Pb2+(aq) + 2Cl-(aq)  PbCl2(s) Ag+(aq) + Cl-(aq)  AgCl(s) Hg22+(aq) + 2Cl-(aq)  Hg2Cl2(s)

white [1] white [2] white [3]

A slight excess of HCl is used to ensure complete precipitation of the cations and to reduce the solubility of the chlorides by the common-ion effect. However, a large excess of chloride must be avoided, because both AgCl and PbCl 2 tend to dissolve by forming soluble complex anions: PbCl2(s) + 2Cl-(aq)  PbCl42-(aq) AgCl(s) + Cl-(aq)  AgCl2-(aq)

[4] [5]

PbCl2 is appreciably more soluble than either AgCl or Hg2Cl2. Thus, even when PbCl2 precipitates, a significant amount of Pb2+ remains in solution and is subsequently precipitated with the group II cations as the sulfide PbS. Because of its solubility, Pb 2+ sometimes does not precipitate as the chloride, because either its concentration is too small or the solution is too warm. Lead Lead chloride is much more soluble in hot water than in cold. It is separated from the other two insoluble chlorides by dissolving it in hot water. The presence of Pb 2+ is confirmed by the formation of a yellow/orange colored precipitate, PbI2, upon the addition of KI: Pb2+(aq) + 2I-(aq)  PbI2(s)

yellow [6]

Mercury (I) Silver chloride is separated from Hg2Cl2 by the addition of aqueous NH3. Silver chloride dissolves because Ag+ forms a soluble complex cation with NH3: AgCl(s) + 2NH3(aq)  Ag(NH3)2-(aq) + Cl-(aq)

[7]

Mercury(I) chloride reacts with aqueous ammonia in a disproportionation reaction to form a dark gray precipitate: Hg2Cl2(s) + 2NH3(aq)  HgNH2Cl(s) + Hg(l) + NH4+(aq) + Cl-(aq)

[8]

2|Page

Although Hg2Cl2 is white, the precipitate appears dark gray because of a colloidal dispersion of Hg(l). Silver To verify the presence of Ag+, the supernatant liquid from the last reaction is acidified and AgCl re-precipitates if Ag+ is present. The acid decomposes Ag(NH3)2+ by neutralizing NH3 to form NH4+. It is necessary that the solution be acidic, or else the AgCl will not precipitate and Ag+ could be missed. Ag(NH3)2+(aq) + 2H+(aq) + Cl-(aq)  AgCl(s) + 2NH4+(aq)

[9]

The flow chart in Figure 8 -1 shows how the group I cations are separated and identified. You should become very familiar with it and consult it often as you perform your analysis. Procedure: WEAR YOUR SAFETY GLASSES WHILE PERFORMING THIS EXPERIMENT First you will analyze a known that contains all three cations of group I. Record on your report sheet the reagents used in each step, your observations, and the equation for each precipitation reaction. After completing the analysis of a known, obtain an unknown. Follow the same procedures as with the known. Also record conclusions regarding the presence or absence of all cations. G1-1 Precipitation of Group I Cations Measure out 10 drops (~0.5 mL) of each of the three test solution into the same small (10 mm x 75 mm) centrifuge tube. Add 10 drops of 6 M HCl, stir thoroughly, and then centrifuge. Test for completeness of precipitation by adding 1 drop of 6 M HCl to the clear supernate. If the supernate turns cloudy, add another 2 drops of 6 M HCl, stir, and centrifuge. Repeat this process until no more precipitate forms. Decant the supernate into a test tube and discard it. Wash the precipitate by adding 5 drops of cold distilled water and stirring. Centrifuge and discard the supernatant. G1-2. Separation and Identification of Pb2+ Add 15 drops of distilled water to the precipitate and place the test tube in a hot-water bath. Stir using a stirring rod and heat for 1 minute or longer. Quickly centrifuge and decant the hot supernate into a clean test tube. Repeat this procedure two more times, combining the supernates, which should contain Pb 2+ if it is present. Save the precipitate for procedure G1-3. Add 3 drops of 1 M KI to the supernatant. The formation of a yellow precipitate, PbI2, confirms the presence of Pb2+. G1-3. Separation and Identification of Ag+ and Hg22+ Add 10 drops of 6M NH3 to the precipitate from step G1-2. The formation of a dark gray precipitate indicates the presence of mercury. Centrifuge and decant the clear supernatant into a clean test tube. Add 20 drops of 6M HNO3 to the decantate. Stir the solution and test its acidity. Continue to add HNO3 dropwise until the solution is acidic (Blue litmus paper turns red.). A white cloudiness confirms the presence of Ag+. 3|Page

Testing the unknown Measure out 30 drops (~1.5 mL) of the unknown solution and repeat steps G1-1 through G1-3. Identify the cation or cations present in the known based on your observation.

FIGURE 8-1: Group I Flow Scheme

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CHEM-102

Name: _________________________

Expt. #8

Scheduled Lab Time: ___________ Date: _________________________ Unknown # _____________________

Data and Observations: PART A: Group I Cations Record the reagent used in each step, your observations, and the equations for each precipitation reaction, whether or not you observe precipitation in your unknown. Procedure

Reagent

Observations

Equations

Mark (+) if observed in unknown

G1-1

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Procedure

Reagent

Observations

Equations

Mark (+) if observed in unknown

G1-2

G1-3

Based on the observation, identify cations present in your unknown: ______________________________________________

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