General Chemistry Lab Experiment 3 Part 1. Ionic Bonds

Transcription

1 General Chemistry Lab Experiment 3 Part 1 Ionic Bonds INTRODUCTION Ionic compounds are collections of ions held together by the force of electrostatic attraction between its positive and negative components, the cation and anion respectively. When dissolved in water, the ionic compound exists as charged positive and negative ions. Qualitative analysis is a systematic procedure for the separation and identification of ions and compounds present in a sample. Cation analysis involves the separation and identification of a positively charged cation present in a sample. If you have a solution containing several cations, it is usually possible to select a reagent that will form a precipitate with one of the cations but not with the others. If you place the test tube and precipitate into a centrifuge, the solid particles in solution are forced to the bottom of the test tube. Thus, the precipitated cations are separated from the remaining cations in solution. The solution is then separated and the analysis continues for the remaining cations. For example, a solution containing Barium, Ba 2+, Calcium, Ca 2+, and Magnesium, Mg 2+, ions can be separated using ammonium sulfate. The sulfate ion, SO 4-2, will precipitate Ba 2+ but does not react with either Ca 2+ or Mg 2+ ions (see Figure 1). There is no reaction between Ca 2+ and SO 4 2-, or Mg 2+ and SO 4 2-, because CaSO 4 and MgSO 4 are soluble. Figure 1 Precipitation of Ba 2+ as BaSO

2 When a barium cation and a sulfate ion are together in the same solution, a precipitate forms because barium sulfate, BaSO 4, is insoluble. The reaction is shown in the equations below. Molecular Equation: The (aq), aqueous, indicates an ionic solution. BaCl 2 (aq) + Na 2 SO 4 (aq) BaSO 4 (solid) + 2NaCl (aq) Ionic Equation: The ions are shown as they exist in the reaction. The charged ions are in solution. Ba Cl - + 2Na + + SO 4 2- BaSO 4 (solid) + 2Na + + 2Cl - Net Ionic Equation: The spectator ions are cancelled out. Ba 2+ + SO 4 2- BaSO 4 (solid) If calcium or magnesium ions are in the solution, no precipitate forms because calcium sulfate, CaSO 4, and magnesium sulfate, MgSO 4, are soluble in water. Ca 2+ is separated from Mg 2+ by precipitating the cation with ammonium oxalate, (NH 4 ) 2 C 2 O 4 to form CaC 2 O 4 as shown in the net ionic equation below. Ca 2+ + C CaC 2 O 4 (solid) The remaining Mg +2 is precipitated with NaOH. 3Mg HPO OH - Mg 3 (PO 4 ) (solid) + 2H 2 0 In this experiment you will separate and identify Ba 2+, Ca 2+, and Mg 2+. First, a known solution containing all three cations will be analyzed to develop the necessary techniques. Second, an unknown solution with one or more of the three cations will be analyzed to determine the cations present. Flame testing is a technique you will use to confirm the presence of an ion. A flame-test is performed by dipping a wire into a solution and then holding the wire in a hot flame while observing the color produced (Figure 2). Many elements produce colored flames. For example, sodium is yellow, potassium is violet, and copper is green. Since sodium is always present as an impurity, the yellow sodium flame invariably contaminates flametests. 2-11

3 Figure 2 Flame-test Technique A drop of solution is placed on the tip of a wire, which is held above a hot flame. The color of the flame is characteristic for the presence of a given element. This will not be done in today's experiment, but a demonstration will be performed on pure solutions of barium and calcium. Litmus paper can be used to determine whether a solution is acidic or basic. A stirring rod is placed into the solution and touched to the litmus paper. Acidic solutions turn blue litmus paper red. Basic solutions turn red litmus paper blue (Figure 3). Figure 3 Litmus-paper Technique If the solution is neutral, there is no change to red or blue litmus papers. The qualitative cation analysis begins with a known solution containing Ba 2+, Ca 2+, and Mg 2+. Figure 4 presents an overview of the analysis. 3-11

4 Figure 4 Cation Analysis The systematic separation and identification of Ba 2+, Ca 2+, and Mg 2+ cations. 4-11

5 PROCEDURE General Directions Clean three test tubes and a stirring rod with distilled water. Label the test tubes #1, #2, and #3. As a solution is analyzed, record the color of each precipitate in the Data Table. Use a plastic or Pasteur pipet to transfer supernates from one test tube to another. A. Analysis of a Known Solution 1. Separation and Identification of Ba 2+ in a Known Solution Place 10 drops of the known solution into test tube #1. Add 10 drops of ammonium sulfate, (NH 4 ) 2 SO 4, and mix with a glass-stirring rod. Note: A white precipitate suggests Ba 2+ is present. and then test for completeness of precipitation by adding another drop of ammonium sulfate. If the solution precipitated after adding the drop of ammonium sulfate, add another 5 drops, mix, and recentrifuge. If no precipitate is observed, all the Ba +2 is completely precipitated. Proceed to the next step. Pipet only the supernate into test tube #2 and save for Step Separation and Identification of Ca 2+ in a Known Solution Add 10 drops of ammonium oxalate, (NH 4 ) 2 C 2 O 4, to the solution in test tube #2. Note: A white precipitate suggests Ca2+ is present. and then test for completeness of precipitation by adding another drop of ammonium oxalate. If precipitation occurs add another 5 drops and recentrifuge. Pipet the supernate into test tube #3 and save for Step Identification of Mg 2+ in a Known Solution Add 10 drops of sodium monohydrogen phosphate, Na 2 HPO 4, to the solution in test tube #3. Add 1 drop of sodium hydroxide, NaOH, and stir with a glass rod. Note: A white precipitate suggests Mg 2+ is present. 5-11

6 B. Analysis of an Unknown Solution 1. Separation and Identification of Ba 2+ in an Unknown Solution Place 10 drops of unknown solution into test tube #1. Add 10 drops of ammonium sulfate, (NH 4 ) 2 SO 4, and mix with a glass stirring rod. Note: If there is no precipitate, Ba 2+ is absent. Go directly to Step 2. and then test for completeness of precipitation by adding another drop of ammonium sulfate. Pipet the supernate into test tube #2 and save for Step Separation and Identification of Ca 2+ in an Unknown Solution Add 10 drops of ammonium oxalate, (NH 4 ) 2 C 2 O 4, to the solution in test tube #2. Note: If there is no precipitate, Ca 2+ is absent. Go directly to Step 3. If a precipitate forms, centrifuge and then test for completeness of precipitation by adding another drop of ammonium oxalate. Pipet the supernate into test tube #3 and save for Step Identification of Mg 2+ in an Unknown Solution Add 10 drops of sodium monohydrogen phosphate, Na 2 HPO 4, to the solution in test tube #3. Add 1 drop of sodium hydroxide, NaOH, and stir with a glass rod. Note: If there is no precipitate, Mg 2+ is absent. 4. Based upon the observations in steps 1-3, identify the cation(s) present in the unknown solution. 6-11

7 Name Class DATA Part A Known Analysis Test Tube #1 Mixture of Ba +2, Ca +2, Mg +2 (NH 4 ) 2 SO 4 (NH 4 )C 2 O 4 Supernate Test Tube #2 Precipitate: Yes No Color Cation Present? Supernate Test Tube #3 Precipitate: Yes No Color Cation Present? Na 2 HPO 4 NaOH Precipitate Yes No Color Cation Present? 7-11

8 Part B Unknown Analysis Test Tube #1 Mixture of Ba +2, Ca +2, Mg +2 (NH 4 ) 2 SO 4 (NH 4 )C 2 O 4 Supernate Test Tube #2 Precipitate: Yes No Color Cation Present? Supernate Test Tube #3 Precipitate: Yes No Color Cation Present? Na 2 HPO 4 NaOH Precipitate Yes No Color Cation Present? 8-11

9 Name Class POSTLABORATORY ASSIGNMENT 1. An unknown solution of the barium, calcium, and magnesium group did not give a precipitate with (NH 4 ) 2 SO 4 solution. Upon addition of (NH 4 ) 2 C 2 O 4 solution to the unknown, a white precipitate formed. To the supernatant liquid was added a solution of Na 2 HPO 4 and NaOH, but no precipitate resulted. State the ion(s) present in the unknown solution. 2. Write the formula for each of the following cations. (a) aluminum (b) stannic (c) calcium (d) copper (II) (e) hydrogen (f) ferric (g) magnesium (h) mercury (I) (i) potassium (j) zinc 3. Complete the table below as shown by the example. Combine the ions into a correct formula and name the compound. 9-11

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11 Name: Class: PRELABORATORY ASSIGNMENT 1. In your own words, define the following terms: cation centrifuge precipitate (ppt) supernate 2. Why is it necessary to use distilled water throughout this experiment? 3. How do you test for the completeness of precipitation? 4. An unknown cation solution is analyzed for barium, calcium, and magnesium ions. The unknown solution plus (NH 4 ) 2 SO 4 gives a white precipitate. The supernate is poured into test tube #2. Test tube #2 plus (NH 4 ) 2 C 2 O 4 remained a solution. The supernate is transferred into test tube #3. Test tube #3 plus Na 2 HPO 4 and NaOH gives a white precipitate. Refer to Figure 4 and determine which of the following cations are present in the unknown solution: Ba 2+, Ca 2+, and Mg