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11 Nov 2019

Write a balanced net ionic equation for the following reactions.

a) The precipitation and confirmation of Hg2^2+

b) The precipitation and confirmation og Ag+

c) The precipitation and confirmation of Pb^2+

tubes organized and carefully following the instructions. Be sure the test tubes chosen will fit in the centri- fuge. PROCEDURE 1. In a small test tube that will fit in the centrifuge, obtain approximately 1mL of a known solution to be analyzed. This known solution contains Ag", Pb2+ and Hg,?â. 2. Add 8 drops of 6M HCl to the solution. Stir with a glass stirring rod. A white precipitate indicates one or more of the ions is present. 3. Centrifuge the mixture making sure there is a blank test tube containing about the same amount of wa- ter in the opposite opening of the centrifuge. 4. After centrifuging, add one more drop of HCl to ensure precipitation is complete. Centrifuge again if nec- essary. 5. Decant the supernatant liquid. This liquid may be discarded in the sink. 6. Wash the precipitate by adding imL of distilled water and stirring. Centrifuge the solution and discard the wash water. Separation and Confirmation of Pb2+ 7. Prepare a boiling water bath by filling a large beaker approximately 2/3 full of distilled water and heat the water on a hot plate. 8. Add 1mL of distilled water to the precipitate from step 6 and heat in a boiling water bath for about three minutes with occasional stirring. 72 - General Chemistry Il Lab Manual

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To test for Ag⁺

1. add 2 drops 6M HCl to 1 mL of unknown sample. Centrifuge (3-4 minutes). Add 1 more drop of 6M HCl and centrifuge (if necessary). A precipitate will form; decant (pour off) solution to another test tube (save solution)

2. wash precipitate from step 1 with 1 or 2 mL of distilled water. Stir, centrifuge, and decant the liquid (can be discarded). Then add 1 mL of distilled water to precipitate and place the test tube in boiling water. Let the test tube boil, and stir occasionally. Centrifuge the hot test tube (solution) and then decant after centrifuging (save the decant solution). Save the precipitate.

3. Add 10 drops of 6M HNO₃ to the precipitation from step 2 and stir it. Centrifuge and decant the solution (save it).

4. Add 6M HNO₃ to the solution from step 3 until it is acidic toward litmus paper. If Ag⁺ is present in the acidic solution, a white precipitate of AgCl will form.

To test for Cu²⁺ and Sb³⁺

1. Add 15M NH₃ to 1 mL of unknown sample until the solution is basic to litmus paper. A precipitate may form at this point due to the formation of insoluble oxychlorides of bismuth, tin, or antimony. Add 1 drop 6M HCL for each mL of solution. Verify the pH level by using methyl violet paper, the paper should turn blue-green. Adjustment may be necessary to achieve the blue-green color. Add HCl or NH₃ until it is almost the same blue-green color.

2. Add 15-20 drops of 1M thioacetamide to solution and heat in boiling water for 5 minutes. Centrifuge and decant to another test tube (may contain Fe and Cr). Test solution for completeness of precipitation by adding 2 drops of thioacetamide and letting it sit for 1 minute. If precipitation forms, add a few more drops of thioacetamide solution and heat again. Combine the two batches of precipitate. Wash precipitate with 1 M NH₄Cl solution, stir, and warm in bath water; centrifuge, and discard the wash solution.

3. To sulfide precipitate from step 2 add 15 drops of 6M KOH, 2mL of water, and 3 drops 1M thioacetamide. Heat solution in boiling water for 5 minutes and stir occasionally. Centrifuge and decant the solution. Wash any precipitate remaining twice with 5 drops of water and combine the washings with the solution. Tin and antimony will be present in the solution as complex ions, and copper and bismuth will remain as undissolved sulfide precipitate. Save precipitate for later.

4. Add 12 M HCL to the solution from step 3 until it is acidic toward litmus paper. When acidified, some tin and antimony will precipitate as orange sulfides. Add 5 drops of 1M thioacetamide and heat. Centrifuge and decant solution (this solution can be discarded)

5. Add 10-15 drops of 12M HCl to the precipitate from step 4 and heat for 5 minutes to dissolve precipitate. Continue heating until H₂S is gone. Centrifuge out any insoluble sulfur residue.

6. Add 0.5 grams of oxalate acid and 2 mL of water to remainder of solution from step 5. Heat the solution, if necessary, to dissolve the oxalate acid. This reagent forms a stable complex ion with Sn⁴⁺. Add 10 drops of 1M thioacetamide and heat. Formation of an orange precipitate indicates the presence of antimony.

6. Add 30 drops of 6M HNO₃ to remaining sulfide precipitate from step 3. Heat solution gently in boiling water to complete dissolution of the sulfides. Centrifuge the solution to remove sulfur and decant it into a small beaker. Place beaker on hot plate and evaporate the liquid to a volume of about one-half mL. DO NOT LET SAMPLE DRY OUT.

7. Add 15M NH₃ dropwise to the solution obtained in step 8 until it is basic toward litmus paper. Add a few more drops. The formation of a deep blue solution indicates the presence of copper.

Testing for Fe³⁺ and Cr³⁺

1. [If a solution is being used from which group II cations have been precipitated, remove the H₂S and excess acid by boiling the solution in a small beaker until a volume of about 1 mL. Remove any sulfur residue by centrifuging the solution.] [if a solution being used contains only group II ion, then use 1 mL of this solution.]

2. Add 3 drops of 3% H₂O₂ to 1 mL solution from step 1. Add 15 drops of 6M NaOH to solution to make it strongly alkaline (basic). Stir for 1 minute and boil to remove all excess H₂O₂. After H₂O₂ is decomposed, solution will âbumbâ. If oxidation of chromium (II) has occurred, the solution will be yellow. If not, add 3 more drops of H₂O₂ and boil again. Centrifuge and decant. Save precipitate (contains Fe³⁺) and solution (contains Cr³⁺).

3. Acidify solution from step 2 by adding drops of 15 M HNO₃(about 5 drops) and test with litmus paper. Add 6M NH₃ drop by drop with mixing until solution is basic. This treatment will precipitate Al³⁺, will be gelatinous white. May appear yellow because of the presence of CrO₄^2-. Centrifuge and decant the solution, it contains CrO₄²^-. Add 1 M BaCl₂ to solution from step 3 to precipitate yellow BaCrO₄. If precipitate is slow to form, heat in boiling water. Centrifuge the solution and decant. Dissolve the precipitate in 3 drops of 6M HNO₃; heat and stir. Add 4 drops of water and cool it down. Add 6 drops of diethyl ether followed by 1 drop of 3% H₂O₂. Shake gently, and allow layers to settle. A blue color in ether (top) later confirm test for chromium.

4. Add 8 drops of 15M HNO₃ to precipitate from step 2. Heat until precipitate is dissolved. Allow to cool and add 10 drops of 1M NH₄Cl and then add 15 M NH₃ until solution is alkaline to litmus paper. This will precipitate iron as brown. Then add 4-5 drops of NH₃ solution and stir. Centrifuge and decant solution (this solution may be discarded).

5. Dissolve the precipitate from step 5 by adding 10 drops of 6M HCl. Dilute with 2mL of distilled water and add 2 drops of 0.5M KSCN. A blood-red color indicates presence of Fe³⁺.

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I will be given 3 mL of an unknown sample and I will have 75 minutes to do a qualitative analysis for Ag, Cu, Sb, Fe, Cr.

I need help coming up with a flow chart based on the above steps so that it is makes this class assignment smoother since I will only be given 75 minutes to complete it.

Thank you!

Separation and Detection of Silver

Place 10 drops of the original solution to be analyzed in a small test tube and add five drops of distilled water and two drops of 6M HCl. Stire well, centrifuge. Wash the preciptate with 10 drops of distilled water, centrifuge, and add the washing to the supernatant. Dissolve the precipitate in four drops of 6M NH₃, then add 6M HNO₃ to the ammoniacal solution until it becomes acidic to litmus. A curdy white precipitate of AgCl confims the presence of Ag⁺ ions.

In the above procedure, when listing the reagent, are all the various solutions added to the original solutions reagents or is just the original 6M HCl the only reagent? What exactly are/is the reagent(s) for the above procedure?

Procedure Reaction 1: Dissolving the Copper 1. Obtain a clean, dry, glass centrifuge tube. 2. Place a piece of copper wire in a weighing paper, determine the mass of the wire and place it in the centrifuge tube. The copper wire should weigh less than 0.0200 grams. 3. In a fume hood, add seven drops of concentrated nitric acid to the reaction tube so that the copper metal dissolves completely. Describe your observations in the lab report. (Caution, Concentrated nitric acid and nitrogen dioxide are very corrosive. Either will turn your skin yellow on contact. Do not leave any spills on the lab bench or in the fume hood.) 4. When the copper has dissolved, add seven drops of distilled water to the tube. Reaction 2: Preparation of Copper(II) Hydroxide 1. Add 15 drops of 3.0 M aqueous sodium hydroxide to the tube. Make sure that the reactants are well mixed. Shake the tube carefully or gently flick the bottom of the tube with your finger. Remember that the contents of the tube may still be corrosive. 2. Add a second 15 drops of NaOH(aq), mix well, and record your observations. If you have two layers at this point it means that you have not mixed the solution well enough. 3. Centrifuge the reaction mixture. 4. The liquid at the top of the centrifuged mixture is called the supernatant while the solid is called a precipitate. Before separating the supernatant from the precipitate it is necessary to ensure that all of the copper(II) hydroxide has been precipitated. The supernatant should be clear and colorless indicating the absence of any Cu+2 ions in the solution. It should also be basic due to an excess of OH- ions. Using a clean glass stirring rod, transfer a drop of the supernatant onto a piece of red litmus paper. If the litmus paper turns blue then the solution is basic and enough NaOH has been added. If the paper does not turn blue, add more NaOH, mix well, recentrifuge, and repeat the litmus paper test until the paper does turn blue. 5. An efficient separation of supernatants and precipitates is key to obtaining a good final yield of copper. The supernatant liquid can be separated from the precipitate by expelling the air from the bulb of a Pasteur pipet, inserting the tip of the pipet into the supernatant, then gently sucking the supernatant into the pipet. If you expel air or liquid into the precipitate with the pipet, you will stir up the precipitate and will have to repeat the centrifugation step. Remove as much liquid as possible and discard it in the waste container provided on the instructorâs cart. It is better to leave a small amount of supernatant liquid than to remove some of the copper(II) hydroxide precipitate. Reaction 3: Formation of Copper(II) Oxide 1. Set up a hot water bath by placing a beaker of water on a hotplate, placing an iron ring around the beaker, and heating the water to boiling. 2. Place the centrifuge tube containing the copper(II) hydroxide into the boiling water. Carefully hold the tube with a test tube clamp so that it doesnât get water into it. Record your observations. Reaction 4: Formation of Copper(II) Sulfate 1. Add 20 drops of 3.0 M H2SO4 to the solid in the centrifuge tube. Stir carefully to ensure that the copper(II) oxide dissolves completely. Complete dissolution of the mixture will require thorough mixing and possibly heating of the solution. 2. Obtain the mass of a small, clean, glass test tube as accurately as possible. 3. Transfer the liquid from the centrifuge tube into the test tube. Rinse the centrifuge carefully with 1.0 mL of distilled water and transfer the rinse into the test tube containing your sample. 4. Record your observations on the data sheet. Reaction 5: Formation of Copper Metal 1. Add a small quantity of zinc powder to the sample solution. Continue adding zinc in small quantities until the solution loses the blue copper(II) color. Any excess zinc added will need to be removed so donât add it too quickly or in large quantities. When the solution has turned colorless, add several drops of 3.0 M H2SO4 to the tube to dissolve any left over zinc. You can tell that the zinc has dissolved when addition of sulfuric acid does not generate bubbles. 2. Allow the copper metal to sink to the bottom of the tube and carefully remove the supernatant liquid using a Pasteur pipet. 3. Wash the red-brown copper metal in the tube with 1.0 mL of water. Allow the copper metal to settle to the bottom and remove the excess water. Repeat this rinsing process two more times. 4. Describe your observations on the data sheet. Drying the copper metal 1. After removing as much of the third rinse water as possible you are ready to dry the metal. This must be done carefully in a cool Bunsen Burner flame. If the tube is heated too quickly there is a risk of ejecting the contents of the tube as the water boils. Also, if the flame is too hot you may convert the copper metal back into black copper(II) oxide. The objective is to drive the water from the tube as steam. Make sure that as water condenses on the walls of the tube that you continue to heat until all of the water if gone from the tube. 2. Once all of the water is removed from the tube, cool the tube and its contents then determine the mass of copper by weighing the tube and subtracting the tube + copper weight from the weight of the empty tube (Reaction 4 step 2). If the mass of copper is higher than the original mass of the copper wire it either contains water or zinc or has been converted to copper(II) oxide. Excess water can be removed by reheating the tube and reweighing to constant mass. Excess zinc requires addition of sulfuric acid followed by re-rinsing with water and re-drying. Chemistry 1215 Experiment 9 Lab Report Name ______________________________ Data Sheet Mass of copper wire _______________ Mass of clean, dry test tube _______________ Mass of test tube plus copper _______________ Mass of final copper sample _______________ Percent recovery of copper. Show all calculations. Observations 1. Describe your observations for Reaction 1 including colors, gases formed, etc. 2. Describe your observations for Reaction 2 including colors, gases formed, etc. 3. Describe your observations for Reaction 3 including colors, gases formed, etc. Estimate the temperature of the decomposition of Copper(II) hydroxide. 4. Describe your observations for Reaction 4 including colors, gases formed, etc. 5. Describe your observations for Reaction 5 including colors, gases formed, etc. Write a brief discussion of your results including a statement of the final percent recovery of copper and a discussion of reasons why the recovery differs from 100%. Chemistry 1215, Experiment #9; Copper and its compounds, Pre-lab Name ____________________________________ 1. Write a balanced chemical equation including phase labels for the reaction between aqueous copper (II) nitrate and aqueous sodium hydroxide. 2. Nitrogen monoxide (NO) and nitrogen dioxide (NO2) are toxic, corrosive gases that significantly lower blood pressure when inhaled. How are these gases produced in todayâs experiment? What should you do to protect yourself against their toxicity? 3. Iron reacts with oxygen from the atmosphere to produce iron (III) oxide, also known as rust (Fe2O3). What chemical species is oxidized in this reaction? What is the reducing agent? Jaffrey Zagnut couldnât find any nitric acid so he tried to dissolve his copper sample in hydrochloric acid instead. Unfortunately his copper wouldnât dissolve in HCl. Why will copper dissolve in nitric acid but not in hydrochloric acid (after all, HCl is a stronger acid than HNO3). Chemistry 1215, Experiment #9; Copper and its compounds, Post-lab Name ____________________________________ 1. Copper (II) hydroxide is converted into copper (II) oxide by heating the test tube containing Cu(OH)2 in a hot water bath. Is it necessary to use distilled water in this water bath? Why or why not? 2. Copper metal doesnât ârustâ in the presence of oxygen at room temperature. However, it will react with O2 at elevated temperatures. Write a balanced chemical equation describing the formation of copper (II) oxide when copper metal is heated in air. 3. When zinc is dissolved in sulfuric acid a gas is produced. What is the chemical identity of this gas? How is it produced? 4. Jaffrey Zagnut started with a 0.032 g sample of copper which he took through the series of reactions described in this experiment. At the end of the experiment he obtained 0.038 g of a black product. What was his percent yield? What is the most likely source of the error in his experiment? (Hint: consider question 2 above)

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