Solution Manual for Laboratory Manual for Principles of General Chemistry, 10th Edition

Experiment 2 Identification of a Compound: Chemical Properties INTRODUCTION This experiment focuses on the chemical properties of a substance for identification. You may recognize this experiment as an open-ended version of an anion qual scheme; it is a good experiment for students to make observations and draw their own conclusions. Work Arrangement: unknown. Partners for the known compounds; individuals for the Time Requirement: 2.5 hours LECTURE OUTLINE 1. Follow the Instruction Routine outlined in “To the Laboratory Instructor”. 2. Provide an overview of the Experimental Procedure. 3. Discuss, with examples, various observations that signify the occurrence of a chemical reaction: • Demonstration. Carbon dioxide gas is evolved when hydrochloric acid is mixed with solid or aqueous sodium carbonate. • Demonstration. A calcium carbonate precipitate appears when aqueous solutions of calcium chloride and sodium carbonate are mixed. • Demonstration. Heat is evolved in an acid-base neutralization reaction…feel the test tube after mixing. • Demonstration. An intensified blue color appears with the addition of 6 M NH3 to a cupric sulfate solution. 4. Note for students that circled superscripts in the Experimental Procedure are stopping points at which students are to make an entry on the Report Sheet. 5. Part A. Discuss the chemistry for each of the four salts—explain how an anion can be identified in the presence of a number of anions. 6. Explain that when an observation of a chemical reaction is made, it should be recorded on the Report Sheet—the purpose of subsequent testing is to characterize the properties of the reaction system. For example, the OH- ion precipitates Mg2+, but Mg2+ is soluble with the addition of HCl(aq). 7. Part B is nearly open-ended. Students must rely entirely on observations for a determination of their unknown. The Next Step. Have students look ahead to Experiments 3, 4, 37, 38, and 39 in order to better appreciate the chemistry that is being performed in this experiment. As an example, test a soil sample to see if it is alkaline…if upon adding HCl(aq) dropwise to a soil sample produces bubbles, it is probably alkaline evolving CO2(g). CAUTIONS & DISPOSAL 10 • Part A. Silver salt solutions produce black stains on the skin. It looks bad but no real danger exists/persists. You (or the student) will find out the next day if any silver nitrate touched the skin! • None of the chemicals in this experiment are considered dangerous, but, if there is any contact with the skin, wash the skin immediately. • Salt solutions are to be discarded in an available “Waste Salts” container. • Ensure that students are using the correct technique for checking the odor from a reaction (Technique 17A). Identification of a Compound: Chemical Properties 1. Students are to use five clean, labeled test tubes or a clean 24-well plate for testing—you should advise students which setup (Figure 2.2a or 2.2b) is to be used. Contamination of glassware can cause the appearance of “weird” precipitates (Technique 2)! 2. Part A.4. We often issue the unknown to students at the beginning of the lab—by doing this, students can perform parallel tests to check the properties of their unknown and make comparisons as they proceed through Part A.1–3. This reduces the time for analysis and eliminates the question “what color is this?” 3. Advise students of the procedure for filling out the reaction matrices for Parts A and B on the Report Sheet. 4. Part A. Discuss the observations and interpretations of the reaction mixtures. Technique, observations, and interpretations are critical to the success of this experiment. Some assistance may be required for writing the formulas of the precipitates on the Report Sheet. 5. Part B. Part B is a repeat (in procedure) of Part A. However, none of the solutions are known…the student must match the unknown solution with one of the originals, strictly by making comparative observations. 6. Encourage students to engage in a habit of thoroughly cleaning all glassware that was used in the experiment and to clean their desk area at the conclusion of the laboratory period. Test NaCl Na2CO3 MgSO4 NH4Cl H2O AgNO3 p p p p nr NaOH nr nr c go nr HCl nr g nr nr nr REPORT SHEET INFORMATION Place the following in dropper bottles to minimize the waste of solutions. Part A Known Test Solutions Reagents 0.2 M NaCl 1 mL 0.2 M AgNO3 2 mL 0.2 M Na2CO3 1 mL 0.2 M NaOH 2 mL 0.2 M MgSO4 1 mL 0.2 M HCl 2 mL 0.2 M NH4Cl 1 mL Part B A large number of test solutions and compatible test reagents can be selected to provide the analyses in Part B. Suggested sets of solutions are: Set 1 Known Test Solutions 0.2 M HCl 0.2 M HNO3 0.2 M NaOH sat’d Ca(OH)2 0.2 M KNO3 Set 2 Known Test Solutions 0.2 M Na3PO4 0.2 M Na2CO3 0.2 M Na2SO4 0.2 M Na2S 0.2 M NaCl 1 mL 1 mL 1 mL 1 mL 1 mL 1 mL 1 mL 1 mL 1 mL 1 mL Reagents phenolphthalein 0.2 M Na2CO3 0.2 M AgNO3 TEACHING HINTS CHEMICALS REQUIRED 2 mL 2 mL 2 mL Reagents 0.2 M Ba(NO3)2 2 mL (Caution…for disposal of barium salts) 0.2 M HNO3 2 mL 0.2 M CuSO4 2 mL Experiment 2 11 Set 3 Known Test Solutions 0.2 M HCl 0.2 M NaOH 0.2 M Na2SO4 0.2 M NH3 0.2 M H3PO4 or sat’d Ca(OH)2 0.2 M CuSO4 0.2 M HNO3 0.2 M NH4Cl Reagents phenolphthalein 2 mL 0.2 M Na2CO3 2 mL 0.2 M Ba(NO3)2 2 mL (Caution…for disposal of barium salts) 1 mL 1 mL 1 mL 1 mL test tubes , small 24-well plate (optional) permanent marker dropper and/or Beral pipets “Waste Salts” container SPECIAL EQUIPMENT PRELABORATORY ASSIGNMENT 1 mL 1 mL 1 mL 1 mL 1 mL 1. a. b. 15 1 ≈5–10 Glassware is clean when no water droplets adhere to the clean part of the glassware. ~ 75 mm and ~3 mL 2. See Technique 17a…fan vapors toward the nose. 3. The formation of a precipitate (or cloudiness), the appearance of a color change, the evolution of a gas, the detection of an odor, a change in temperature. 4. 1.0 mL 5 drops x 20 drops = 0.25 mL 1.0 mL 10 drops x 20 drops = 0.50 mL 5. For NaCl, Na+(aq) and Cl-(aq) For Na2CO3, Na+(aq) and CO32-(aq) For MgSO4, Mg2+(aq) and SO42-(aq) For NH4Cl, NH4+(aq) + Cl-(aq) 6. a. b. c. LABORATORY QUESTIONS 1. Addition of HCl(aq): HCl reacts with CaCO3 to produce CO2; no reaction occurs with CaCl2. 2. Addition of AgNO3(aq). Ag+ precipitates Cl-, but has no effect on SO42- (unless large amounts of Ag+ are added). 3. a. b. c. d. 4. test tube 1 0.1 M HCl test tube 2 0.1 M KOH test tube 3 0.1 M Na2CO3 5. a. b. 12 test tube 1 silver nitrate test tube 2 sodium sulfide test tube 3 potassium iodide AgNO3(aq) + KI(aq) → AgI(s) + KNO3(aq) 2 AgNO3(aq) + Na2S(aq) → Ag2S(s) + 2 NaNO3(aq) Gas is evolved. CO2 White precipitate forms. AgCl White precipitate forms. Mg(OH)2 Pungent gas is evolved. NH3 test tube 1 hydrochloric acid test tube 2 sodium carbonate test tube 3 silver nitrate A white precipitate of silver chloride would form. See the solubility rules in the manual, Appendix E. Identification of a Compound: Chemical Properties NH3 (NH3 precipitates Mg2+ as Mg(OH)2(s) but not Ag+) or HCl (Clprecipitates Ag+ as AgCl(s) but not Mg2+) b. Ag+ (Ag+ forms a precipitate with HCl as AgCl(s) but not H2SO4) or Ba2+ (Ba2+ forms a precipitate with H2SO4 as BaSO4(s) but not HCl) c. NH3 (NH3 forms a deep blue solution with Cu2+ as [Cu(NH3)42+] but Ba2+ remains colorless) or H2SO4 (Ba2+ precipitates with H2SO4 as BaSO4(s) but Cu2+ does not) d. NH3 (NH3 forms a deep-blue solution with Cu2+ as [Cu(NH3)42+] but forms a white precipitate with Mg2+ as Mg(OH)2(s)) See the solubility rules in the manual, Appendix E. 6. a. 1. Cite two observations that indicate the occurrence of a chemical reaction. [Answer: see Introduction to Experiment 2 in the laboratory manual] 2. 3. 4. 5. 6. A mixture of potassium carbonate and hydrochloric acid results in the evolution of a gas. What is the gas? [Answer: CO2] A mixture of zinc chloride and silver nitrate produces a white precipitate. What is the formula of the precipitate? [Answer: AgCl] A mixture of sodium sulfate and ammonium chloride produces no observable result; however, the mixture of sodium hydroxide and ammonium chloride produces a detectable odor. What substance causes the odor? [Answer: NH3 gas] A mixture of lead nitrate and ammonium acetate produces no observable result; however, the mixture of lead nitrate and ammonium sulfate produces a white precipitate. What is the precipitate? [Answer: lead sulfate, Pb(SO4)2] A mixture of sodium carbonate and sodium hydroxide produces no observable result; however, the mixture of sodium carbonate and barium hydroxide produces a white precipitate. What is the precipitate? 7. 8. Describe the technique for detecting an odor. Identify two silver salt precipitates. 9. Identify a hydroxide precipitate. LABORATORY QUIZ [Answer: barium carbonate, BaCO3] [Answer: See Technique 17A] [Answer: silver chloride, AgCl, silver carbonate, Ag2CO3] [Answer: magnesium hydroxide, Mg(OH)2] Experiment 2 13