EXPERIMENT #8 The Chemistry and Qualitative Analysis of Cations: Group I

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1 OBJECTIVES: EXPERIMENT #8 The Chemistry and Qualitative Analysis of Cations: Group I To separate the Group I cations from an aqueous solution To separate the Group I cations from each other To observe the confirmation tests for the presence of Ag +, Hg2 2+, and Pb 2+ in a known mixture To analyze an unknown mixture for the presence of Ag +, Hg2 2+, and Pb 2+ To write a balanced chemical equation for each reaction involved in the separation of Group I cations from aqueous solution and from each other To describe an analysis or separation based on the principles used in the analysis of Group I cations BACKGROUND: Cations are divided into several groups based on the selective precipitation of a specific set of compounds or selective solubilities of a specific set of complex ions. One of the many possible schemes is shown on page 94. For example, the cations of Group I are characterized by the insolubility of their chlorides; AgCl, Hg2Cl2, and PbCl2 are insoluble in cold water. This means that from a solution containing a mixture of cations, we can separate Ag +, Hg2 2+, and Pb 2+ from the mixture by adding chloride ion and forming the insoluble chlorides. Only the chlorides of the Group I cations are insoluble in cold water. Ag + (aq) + Cl - (aq) AgCl(s) Pb 2+ (aq) + 2Cl - (aq) PbCl2(s) Hg2 2+ (aq) + 2Cl - (aq) Hg2Cl2(s) Once we have isolated a particular group of cations as a precipitate, the task becomes identifying which ions belonging to the group are in the precipitate. To do this, a carefully worked out procedure is followed in detail. In this procedure a precipitate may form or may go into solution or a specific color may form upon the addition of a certain reagent. Observation of a positive test indicates the presence of a cation; a negative test indicates its absence. The carefully worked out procedure referred to above (and summarized in the GROUP I FLOW CHART on page 93) is based on the chemical principles of precipitate formation, ionic equilibrium, acids and bases, ph, oxidation and reduction, and complex ion formation. In short, qualitative analysis is an exercise in applied chemical equilibrium. An understanding of these principles is necessary for a successful analysis; the procedure should not be thought of as cookbook chemistry. Silver, Ag Silver is a relatively non-reactive metal. It occurs in nature in its elemental form, called native silver, and as silver(i) sulfide (argentite), silver(i) chloride (horn silver), and ruby silver, Ag3SbS3. Commercially, silver is obtained as a by-product in the isolation of copper, lead, and zinc form their ores. Silver is one of the best conductors of heat and electricity and has excellent resistance to corrosion. The major use of silver is in photography. Other important areas of silver use include electronics, sterling silverware (an alloy of silver and copper), and electroplating. Silver is very toxic to lower forms of life, but not very toxic to higher forms. Silver salts are known to 91 P a g e

2 EXPERIMENT #8 GROUP I CATIONS protect drinking water, and silver fibers woven into socks can prevent the growth of bacteria responsible for foot odor. (Emsley, Nature s Building Blocks, Oxford, 2001.) Mercury, Hg Mercury is one of the two elements that is a liquid at room temperature and the only metal that is a liquid at room temperature. It is obtained by heating HgS (cinnabar) in air. Mercury and its salts are toxic to humans (a cumulative poison) and simpler life forms. For this latter reason mercury is used in mildew-proofing paints and has been used to kill intestinal worms. Other uses include the manufacture of electrical apparatus, the cathode material of commercial electrolytic cells -- a use which has resulted in pollution of large bodies of water, such as Onondaga lake -- and in dental amalgams. (An amalgam is an alloy of mercury and some other metal. A dental amalgam is made by adding mercury to a silver-tin alloy. The plastic mass is shaped and then hardens as the mercury displaces the tin, forming a silver-mercury amalgam.) Lead, Pb The Latin name for lead is plumbum, which is the source of the symbol of the element, Pb, and the word plumber. The major mineral source of lead is galena, PbS. Lead is obtained from the ore, first by roasting (heating in the presence of oxygen) to form lead (II) oxide, followed by reduction, with carbon, to the elemental metal. Further purification can be carried out by electrolysis. Lead is a dense, soft, bluish-gray, low melting metal. It is used as the electrode plates in automobile batteries and formerly in the manufacture of tetraethyl lead, an antiknock additive for gasoline and white lead [a mixture of lead(ii) carbonate and lead(ii) hydroxide], a paint pigment. Lead compounds are toxic to humans, particularly children. It is a cumulative poison. For this reason lead is no longer used as a gasoline additive or paint pigment. PROCEDURE: Precipitation of Group I Cations In a small test tube place approximately 1 ml of the known or unknown solution of Group I cations. Add a few drops of 6M hydrochloric acid, HCl [1]. Do not add excess. Centrifuge (see box below) the resulting suspension. Add another drop of 6M HCl to check for complete precipitation of the Group I cations. Carefully pour off the supernatant liquid (the clear liquid above the precipitate). (Store this supernate in a clean test tube if you are analyzing a general unknown.) Add no more than 0.5 ml of cold deionized water to the precipitate, thoroughly mix, centrifuge, and discard the supernate. (Combine with the first supernate when analyzing a general unknown.) Repeat the washing procedure and discard the supernate. Washing must be performed properly in order to remove any cations from the other cation groups. CAUTION When centrifuging, make sure your test tube is perfectly counterbalanced with the same type of test tube filled with the same level of water. If the centrifuge makes funny noises, TURN OFF IMMEDIATELY AND ASK YOUR INSTRUCTOR FOR ASSISTANCE. Allow the centrifuge to stop on its own. Do not attempt to use your hand as a brake 92 P a g e

3 EXPERIMENT #8 GROUP I CATIONS * Materials under the vertical double line are precipitates; materials under the single vertical line are in solution. Contamination by trace amounts of metal ions and other contaminants in test tubes and other glassware leads to unexplained and unpredictable results in qualitative analysis. Thoroughly clean glassware with soap solution and elbow grease, rinse thoroughly with tap water and three times with deionized water before use. Separation of Lead, Pb 2+ The white precipitate from step [1] is treated with less than ml of hot, deionized water [2] and thoroughly mixed with a stirring rod. The suspension is quickly centrifuged, and the supernate is carefully transferred into a clean test tube. The white precipitate is washed twice more with hot water. (The washings must be done carefully and efficiently in order to remove all the lead from the precipitate. Any lead remaining in the precipitate will interfere with the tests for silver and mercury.) These washings may be discarded or combined with the original wash in the test tube. This solution is treated with 1M potassium chromate, K2CrO4 [4]. Formation of a yellow precipitate (PbCrO4) confirms the presence of Pb P a g e

4 EXPERIMENT #8 GROUP I CATIONS Detection of Mercury, Hg2 2+ The washed, white precipitate from step [2] is treated dropwise with 6M aqueous ammonia, NH3 or NH4OH [3]. Formation of a black to gray precipitate confirms the presence of Hg2 2+. Add more NH3 in order to dissolve (mix thoroughly) any white precipitate still present. This mixture is centrifuged, and the supernate is saved for step [5]. Detection of silver, Ag + The supernate from step [3] is acidified with 6M nitric acid, HNO3. Formation of a white precipitate (AgCl) confirms the presence of Ag +. Follow this procedure with a known mixture of all three cations from Group I. When you are satisfied with you work, obtain a Group I unknown solution to analyze. WASTE DISPOSAL Dispose of all solutions and precipitates containing silver, mercury, and lead ions in the appropriately marked containers in the hood; do not pour these materials down the sink. Tubes must be diametrically placed and contain the same amount of liquid. 94 P a g e

5 NAME Section Date Data And Observations: Analysis of Group I Cations: Ag +, Hg2 2+, Pb 2+ I. Known Solution Step Observations [1] Addition of HCl(aq) to solution [2] Addition of hot water to Group I precipitate [4] Addition of K2CrO4(aq) to supernate [3] Addition of conc. NH3(aq) to white precipitate [5] Acidification of supernate with conc. HNO3 95 P a g e

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7 NAME Section Date ADDITIONAL ASSIGNMENT: Analysis of Group I Cations -- Known Answer all of the following questions with reference to the Group I Cation flowchart. 1. What ion or substance will form a precipitate with NH4Cl(aq) and also with K2CrO4(aq)? 2. What ion or substance will form a precipitate with AgNO3(aq), but not with Bi(NO3)3(aq)? 3. What ion or substance will readily dissolve ZnCl2(s), but not PbCl2(s)? 4. What ion or substance will form a precipitate with KCl(aq) and also with CuCl2(aq)? 5. Which chloride salt forms a precipitate upon addition of concentrated aqueous ammonia? 6. Which chloride salts are insoluble in hot water? 97 P a g e

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9 NAME Section Date Data And Observations: Analysis of Group I Cations: Ag +, Hg2 2+, Pb 2+ II. Unknown Solution (Unk No. ) Step Observations [1] Addition of HCl(aq) to solution [2] Addition of hot water to Group I precipitate [4] Addition of K2CrO4(aq) to supernate [3] Addition of conc. NH3(aq) to white precipitate [5] Acidification of supernate with conc. HNO3 Group I Cations found in unknown: 99 P a g e

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11 NAME Section Date ADDDITIONAL ASSIGNMENT I: Analysis of Group I Cations -- Unknown Answer all of the following questions with reference to the Group I Cation flow chart and your accumulated knowledge of chemistry 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 BaCl2(s) from AgCl(s)? 4. How could you distinguish HNO3(aq) from HCl(aq)? Complete and balance the following equations by referring to the Group I Cation flow chart and your accumulated knowledge of chemistry. (You may need to figure out what additional product(s) form in addition to the main products shown on the flow chart.) Indicate the product as (s), (l), (aq), or (g) as appropriate. 5. AgCl(s) + NH3(aq) 6. Pb 2+ (aq) + CrO4 2- (aq) 7. Hg2Cl2(s) + NH3(aq) 8. Ag(NH3)2 + (aq) + H + (aq) + Cl (aq) 101 P a g e

12 9. What can you conclude if no precipitate forms when HCl is added to an unknown solution? 10. What can you conclude if a white precipitate forms when a solution of silver nitrate is added to tap water? 102 P a g e

13 NAME Section Date ADDDITIONAL ASSIGNMENT II: Analysis of Group I Cations -- Unknown Answer the following question with reference to the Group I Cation flowchart. A solution may contain Ag +, Pb 2+, and Hg2 2+. A white precipitate forms on addition of 6M HCl. The precipitate is partially soluble in hot water; the residue dissolved on addition of 6M NH3. Which of the ions are present, which are absent, and which remain undetermined? EXPLAIN!!! (Answers without an explanation will not receive credit.) Present Absent In Doubt 103 P a g e

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