Chem 1B Saddleback College Dr. White 1. Experiment 5: Separation and Identification of Group I Cations (The Chloride Group: Ag +, Pb 2+, and Hg 2

Transcription

1 Chem 1B Saddleback College Dr. White 1 Experiment 5: Separation and Identification of Group I Cations (The Chloride Group: Ag +, Pb 2+, and Hg 2 2+) Objective To understand the chemical reactions involved in the separation and identification of the Group I cations, silver, lead (II), and mercury (I) To successfully identify the Group I cation(s) in an unknown solution. To complete a flow diagram summarizing the behavior of your unknown. Introduction Of the cations encountered in our qualitative analysis scheme, only three form insoluble chlorides, Ag +, Pb 2+, and Hg These can be separated from a mixture of the other cations by adding a slight excess of hydrochloric acid. A slight excess helps to insure complete precipitation of all ions. On the other hand, a large excess of hydrochloric acid should be avoided, since both silver ion and lead (II) ion form chloro complex ions, PbCl 4 2 (aq) and AgCl 2 (aq), to some extent in excess HCl. After precipitating the Group I cations as chlorides, they are physically separated from the remaining mixture by centrifuging and decanting the supernatant liquid into another test tube for further analysis. As you perform the experiment, collect all waste solutions in a waste beaker. This mixture should then be discarded in the appropriate waste container. DO NOT POUR ANY OF THE SOLUTIONS DOWN THE DRAIN. Unknowns and Knowns A known solution containing all 3 group I cations is provided for your use. Testing a known sample is helpful in this analysis since doing so will allow you to observe what a positive test looks like. Note that the experimental conditions, such as ph, for the known test must be the same as that for the unknown. First, you will perform the procedures on the known solution and then you will perform the procedures on the unknown solution. Both silver chloride and lead (II) chloride are white solids. Therefore, the color of the precipitate formed when HCl is added is not indicative of its identity and further analysis is needed for identification. The solubility of PbCl 2 increases approximately threefold as the temperature of the solution increases from 20 C to 100 C. The solubility of AgCl and Hg 2Cl 2 ncreases very little over this temperature range. Thus, in hot water appreciable amounts of PbCl 2 will dissolve while AgCl and Hg 2Cl 2 remain insoluble. Addition of K 2CrO 4(aq) to a solution that contains Pb 2+ will yield PbCrO 4(s), a golden yellow precipitate. Of the three Group I ions, only the silver ion is soluble in aqueous ammonia due to the formation a complex ion. From this information a separation and identification scheme for the Group I cations is developed as seen in the flowchart on page 4 of this lab. Reagents Available 6 M HCl 12 M HCl 1 M K 2CrO 4 6 M NH3 6 M HNO3 Dilute, Known Solution of Ag +, Pb 2+, and Hg 2 2+ Safety All of the Group I cations are toxic and 6M HCl, NH3 and HNO3 are irritants. Avoid contact and wash immediately if any is spilled or splashed on you. Wear eye protection at all times. Also, wear gloves when using 12 M HCl.

2 Chem 1B Saddleback College Dr. White 2 Outline of Procedure: Record all observations and make a flow chart as you progress through the procedure. Experimental Procedure: 1. Use centrifuge tubes and be sure to label everything very carefully. (a) Put 3 ml of a solution to be tested for the Group I cations into a test tube. Add 0.5 ml 6 M HCl, dropwise, with shaking just until no more precipitate appears to form. Stir thoroughly; then centrifuge. Do not decant yet. Chemistry and Relevant Background Information: 1. Precipitation of Group I cations: The Group I cations (Ag +, Pb 2+, and Hg 2 2+) are the only cations in our qualitative analysis scheme that form insoluble chlorides. Addition of HCl separates the Group I cations from Groups II- IV. (b) Test for complete precipitation by adding 1 drop of 6M HCl to the clear supernatant. If cloudiness is observed or a precipitate forms, add another drop, stir and centrifuge. Repeat this process until no new precipitate forms upon addition of the 1 drop of 6M HCl. (c) Decant the supernatant liquid into a clean test tube. If an unknown that also contains Groups II through V is being tested, save the supernatant for further analysis. DO NOT FORGET TO LABEL THE TEST TUBE! (d) Wash the precipitate twice by adding 10 drops of cold, deionized water then stir, centrifuge and decant. The washes can be discarded. 2. (a) Prepare a boiling water bath. Add about 4 ml of deionized water to the precipitate from step 1d and heat the mixture in the boiling water bath for at least 3 minutes, stirring constantly. (b) Centrifuge quickly and decant the hot supernatant into a clean test tube. Set aside any remaining precipitate for further analysis. DO NOT FORGET TO LABEL IT! (c) Add three drops of 6 M HC 2H 3O 2 and 3-4 drops of 1 M K 2CrO 4 solution to the test tube containing the supernatant liquid. A golden yellow solid indicates the presence of Pb 2+. Centrifuging out the solid may help with the identification since the supernatant is yellow- orange. Carefully observe and record your observations. You may discard this ppt. 3. (a) If Pb 2+ is present, wash the precipitate from step 2b with 4 ml DI water (if Pb 2+ is not present, skip to step 3b). Heat the solution for 3-4 minutes. Stir the solution several times every minute to dissolve any remaining PbCl 2. Promptly centrifuge and decant the supernatant into the waste. Add 4 more ml of 2. Separation and Identification of Pb 2+ ion: The solubility of PbCl 2 increases considerably in hot water compared to cold water while the solubility of AgCl and Hg 2Cl 2 remain relatively constant. Hot water can thus be used to separate PbCl 2(s) from AgCl(s) and and Hg 2Cl 2. The precipitate from step 1 is treated with hot water. After centrifuging and decanting, the resulting supernatant is tested for the presence of Pb 2+ by adding K 2CrO 4(aq). The formation a golden yellow solid confirms the presence of Pb (a) Ridding the Precipitate of Pb 2+ If Pb 2+ is present, it must be completely removed from the precipitate before continuing.

3 Chem 1B Saddleback College Dr. White 3 water and heat with stirring again for 3-4 minutes. Decant the supernatant into a centrifuge tube and test for lead ions by following the procedure in step 2c. If a positive test results, repeat the washing and heating until a negative test for lead results. Set aside the precipitate. (b) To the precipitate that contains no Pb 2+ (either from step 2b or 3a) add 2 ml 6 M NH 3 and stir well. Carefully observe and record your observations. If there is a precipitate, this indicates that Hg 2 2+ is present. Centrifuge and decant the supernatant into a clean test tube for further analysis. Also, keep the precipitate for possible further analysis. DO NOT FORGET TO LABEL THEM! 4. Add 6 M HNO3 drop wise with stirring to the supernatant from step 3b until the solution just tests acidic to litmus paper. Carefully observe and record your observations as the solution is acidified. (b) Identification of Hg 2 2+ ion: If Hg 2Cl 2 (s) is present, it reacts with NH 3 to form black Hg (s) and white HgNH 2Cl (s). The resulting precipitate mixture is black or dark grey. The balanced net ionic chemical equation for this reaction is: Hg 2Cl 2(s) + 2NH 3(aq) Hg(s) + HgNH 2Cl(s) + NH 4 +(aq) + Cl - (aq) AgCl that is present in the precipitate dissolves upon addition of the NH 3 as it forms the Ag(NH 3) 2 + complex ion.* *If both silver ion and mercury (I) ion is present, a side redox reaction of Ag(NH 3) 2 +and Hg(s) occurs. (see step 5) 4. Identification of Ag + ion: To verify the presence of Ag +, the resulting solution is treated with nitric acid. If Ag(NH 3) 2 + is present, it is decomposed by the acid and AgCl(s) is again formed (Cl - is present in the solution). The H + from the nitric acid reacts with the NH 3 in the complex ion to form NH 4 +. Reappearance of a white precipitate confirms the presence of Ag + in the original solution. 5. (a) If the test for silver ion was inconclusive (i.e. Hg 2 2+ is present and the Ag + test came out negative), add 1 ml 12 M HCl and 0.5 ml 6 M HNO 3 to the precipitate from step 3b. (b) Heat in the water bath until all the solid dissolves. Pour the liquid into a 50- ml beaker and boil it gently for a minute. Add 5 ml water. If a white precipitate forms, it is probably AgCl. Put the contents of the beaker back into a tube and centrifuge. Discard the supernatant. (c) To the precipitate, add 0.5 ml 6 M NH 3 with stirring and the precipitate should dissolve if it is AgCl. 5. Alternative confirmation of the presence of Ag + ion: If both Hg 2 2+ and Ag + are present, the Ag(NH 3) 2 + and Hg(s) both present in all of step 3 tend to undergo an oxidation- reduction reaction: 2 Ag(NH 3) 2 +(aq) + Hg(s) + Cl - (aq) 2Ag(s) + HgNH 2Cl(s) + NH 4 +(aq) + 2NH 3(aq) If enough Hg(s) is present, nearly all the Ag + may be reduced to Ag(s) and then the confirmation test for silver ion in step 4 may give a false negative. Addition of 12 M HCl and 6 M HNO 3 (known as aqua regia) will cause the solids to dissolve into the complex ions HgCl 4 2- and AgCl 2 -. Adding water to the strongly acidic solution re- precipitates AgCl which can then be separated from the HgCl 4 2- by centrifuging. The dissolving of AgCl (s) in NH 3 (see reaction in step 3) and the re- precipitation in HNO 3 (see reaction in step 4) provide definite results for the presence of Ag +. (d) Add 1 ml 6 M HNO 3 and if the silver ion is present a precipitate of AgCl reforms.

4 Chem 1B Saddleback College Dr. White 4 Outline of Procedure for Group I Cations (Group I Flow Chart) (3b) 6M NH 3

5 Chem 1B Saddleback College Dr. White 5 Name: Lab Day/Time: Exp. 5: Separation and Identification of Group I Cations Data and Results Draw a flow chart indicating the behavior of your unknown: Conclusion: The cation(s) present in unknown number is/are.

6 Chem 1B Saddleback College Dr. White 6 Post-Lab Questions 1. Write the balanced net ionic chemical equation, including phase labels, for the precipitation reaction that occurs when HCl(aq) is added to a solution containing silver ions: 2. Write the balanced net ionic chemical equations, including phase labels, for the precipitation reaction that occurs when HCl(aq) is added to a solution containing lead (II) ions: 3. Write the balanced net ionic chemical equations, including phase labels, for the precipitation reaction that occurs when HCl(aq) is added to a solution containing mercury (I) ions: 4. Write the balanced net ionic chemical equation, including phase labels, for the reaction that occurs when K 2CrO 4(aq) is added to a solution containing Pb 2+ ions: 5. AgCl (s) dissolves upon addition of the NH 3 as it forms the Ag(NH 3) 2 + complex ion Write the balanced net ionic chemical equation, including phase labels, for the reaction that occurs when NH 3(aq) is added to AgCl(s): 6. Write the balanced net ionic chemical equation, including phase labels, for the reaction that occurs when HNO 3(aq) is added to a solution containing Ag(NH 3) 2 + and Cl -. (See step 4 for a description of this reaction). 7. Explain why adding a slight excess of hydrochloric acid insures more complete precipitation of the Group I cations, but a large excess should not be used. 8. What is the purpose of washing the precipitate with hot water in step 3(a) of the procedure? Be as specific as possible in your answer.

7 Chem 1B Saddleback College Dr. White 7 9. For an unknown solution that contains at least one of the Group I cations, answer the following questions. (It may be helpful to reference the flow chart.) a. Upon adding 6 M HCl to the unknown solution, a white precipitate forms. What cation(s) may be present in the unknown? Explain. b. When the white precipitate from (a) is treated with hot water, a white precipitate and colorless supernatant result. Adding K 2CrO 4 (aq) to the supernatant results in no reaction. What conclusion can be made about the presence or absence of the cations present in the unknown? Explain. c. When the precipitate from (b) is treated with 6 M NH 3(aq), it completely dissolves. Then HNO 3 is added and a white precipitate is formed. What conclusion can be made about the presence or absence of the cations present in the unknown? Explain.