How Can Eleven Colorless Solutions be Chemically Identified?

Transcription

1 EXPERIMENT 1 How Can Eleven Colorless Solutions be Chemically Identified? INTRODUCTION Developed by Department of Chemistry, Brown University In this experiment, the following eleven colorless solutions will be supplied and labeled only with numbers. The numbered solutions will be identified by mixing samples of the solutions with each other, analyzing ph, and conducting flame tests. No other chemicals need to be used. 1.0 M sodium thiosulfate (Na2S2O3) 1.0 M hydrochloric acid (HCl) 1.0 M sodium carbonate (Na2CO3) 0.1 M sodium oxalate (Na2C2O4) 0.1 M barium nitrate (Ba(NO3)2) 0.1 M manganese (II) nitrate (Mn(NO3)2) 1.0 M ammonium chloride (NH4Cl) 0.1 M zinc chloride (ZnCl2) 0.1 M silver nitrate (AgNO3) 0.1 M aluminum nitrate (Al(NO3)3) 3% hydrogen peroxide (H2O2) To prepare for this experiment, draw on the following information to develop a systematic and logical flow chart that will be used to identify the solutions. The most efficient way to start the analysis is by using a single unknown solution. When some unknowns have been identified, they can be used as reagents to identify others. A similar problem using a different set of unknowns is shown at the end of the Introduction. Once all solutions have been tentatively identified, you are required to perform addional tests for verifying the solutions. Flame Tests When a drop of some of the unknown solutions is placed on a wire loop and vaporized in a flame, the flame becomes brightly colored. The intense heat of the flame releases free (not bonded) atoms from molecules. The electrons of these free atoms are raised to high-energy states by the hot flame. When the electron returns to the ground state, light with a discrete frequency (color) characteristic of the element is emitted. Among the ions used in this lab, there are only two strong light emitters. Metal Sodium Barium Flame color yellow/orange green The sodium flame is so intense that only traces of sodium (e.g., from finger prints on the wire) give somewhat yellow flames. Therefore it is important to inspect the flame from the wire before adding each chemical. Remember how yellow the flame is just from the wire. When there is sodium present in the sample the change in intensity of the orange flame color will be significant. PH Tests ph Paper Color (See label on vial) Interpretation strong acid weak acid strong base Solution HCl Al(NO3)3 Na2CO3 While Al(NO3)3 and Na2CO3 solutions do not obviously contain excess H + (H3O + in water) or OH -, they do 1

2 react with water to change the ph: Al 3+ (aq) + 6 H2O(l) [Al(H2O)6] 3+ (aq) (complex ion formation) (Eq. 1) [Al(H2O)6] 3+ (aq) + H2O(l) [Al(OH)(H2O)5] 2+ (aq) + H3O + (aq) (Eq. 2) CO3 2- (aq) + H2O(l) HCO - 3 (aq) + OH - (aq) (Eq. 3) All the other unknown solutions are nearly neutral in ph and produce only ambiguous results with ph paper. Precipitation Reactions When two of the solutions are mixed, a compound may form from the cation in one solution and the anion in the other (double displacement reaction). If the compound is present at a concentration higher than its solubility, it will precipitate. For example, the Ba 2+ from Ba(NO3)2 and the SO4 2- from Na2SO4 react to form insoluble BaSO4. Ba(NO3)2(aq) + Na2SO4(aq) BaSO4(s) + 2 NaNO3(aq) (Eq. 4) Most of the anions and cations in the list form insoluble compounds with at least some of the other ions. Those that form only soluble salts are Na +, NH4 +, and NO3 -. Since some precipitates form slowly, a general rule is to wait for about 2 to 5 minutes before discarding the test mixture. Some precipitates take even longer to form and are noted in the accompanying table. Since the dissolution and precipitation of salts is an equilibrium reaction, the concentrations of the ions will affect the equilibrium position. Adding one more drop of a solution can produce a precipitate where there was none before. A few precipitates formed from Ag + ions will darken and finally turn black after several minutes due to the formation of free metallic silver. Examples are Ag2CO3 and Ag2S2O3. This blackening, which requires light, is essentially the same process that occurs in photographic film. Also note that the OH - ion precipitates silver oxide. 2 Ag + (aq) + 2 OH - (aq) Ag2O(s) + H2O(l) (Eq. 5) The precipitate is dark brown or black when formed, and may become blacker after a few minutes. This reaction might be expected when Na2CO3 is added to AgNO3, since the basic Na2CO3 solution contains OH - ions. However, white Ag2CO3 is obtained instead of Ag2O because [OH - ] in the Na2CO3 solution is too low to produce Ag2O in the presence of CO3 2-. After awhile this white Ag2CO3 precipitate may turn slightly tan/gray. Gas-Producing Reactions Reaction Odor Bubbles NH + 4 (aq) + OH - (aq) NH3(g) + H2O(l) (Eq. 6) ammonia no 8 S2O3 2- (aq) + 16 H + (aq) S8(s) + 8 SO2(g) + 8 H2O(l) (Eq. 7) SO2 no CO3 2- (aq) + 2 H + (aq) CO2(g) + H2O(l) (Eq. 8) none yes Oxidation-Reduction Reactions Reaction Observation Mn 2+ (aq) + H2O2(aq) + 2 OH - (aq) MnO2(s) + 2 H2O(l) (Eq. 9) dark brown ppt 2 Ag + (aq) + H2O2(aq) + 2 OH - (aq) 2 AgO(s) + 2 H2O(l) (Eq. 10) black ppt 2

3 These precipitates (above) catalyze the breakdown of additional H2O2, producing O2 bubbles that may or may not be visible. The reaction of S2O3 2- in acid, discussed in the preceding section, is also an oxidation-reduction reaction. Complex Ion Formation Reaction AgCl(s) + 2 S2O3 2- (aq) Ag(S2O3) 3-2 (aq) + Cl - (aq) (Eq. 11) Observation ppt. dissolves Solubility of ionic compounds in water at 20 O C Chloride ion Cl - Nitrate ion NO3 - - Sodium NaCl NaNO3 ion Na + Carbonate ion CO Na2CO3 Oxalate ion C2O4 2- Na2C2O4 Thiosulfate ion S2O3 2- Na2S2O3 Ammoniu m ion NH 4+ Silver ion Ag + NH4Cl AgCl NH4NO3 AgNO3 (NH4)2CO3 * Ag2CO3 (NH4)2C2O4 Ag2C2O4 (NH4)2S2O3 Ag2S2O3 ** Barium ion Ba 2+ Manganes e ion Mn 2+ BaCl2 MnCl2 Ba(NO3)2 Mn(NO3)2 BaCO3 MnCO3 **** BaC2O4 MnC2O4 ***** BaS2O3 Slightly *** MnS2O3 Zinc ion Zn 2+ ZnCl2 Zn(NO3)2 ZnCO3 ZnC2O4 *** ZnS2O3 Aluminum ion Al 3+ AlCl3 Al(NO3)3 Al2(CO3)3 ***# Al2(C2O4)3 # Al2(S2O3)3 *** * (NH 4 ) 2 CO 3 is unstable, and releases NH 3 gas. ** An equal number of drops of Ag + and S 2 O 3 2- solutions give a white precipitate that vanishes rapidly. In that case, add one drop of S 2 O 3 2- to 10 drops of Ag +. This should produce a yellow precipitate that turns brown and then black in one to two minutes. *** Precipitate takes several minutes to form, and often the solution becomes only slightly turbid. **** Precipitate slowly turns tan as it oxidizes in air to dark brown MnO 2. ***** Precipitation may not be observed. # These tests are sometimes inconclusive. Al 3+ can be identified more clearly by another test. A SOLVED ELEVEN SOLUTION PROBLEM This problem is given as an example to show a logical approach to identify unknown solutions. These are not the solutions that will be used in the actual experiment. Because the solutions are not the same, there are a few more reactions involved. SO3 2- (aq) + 2 H + (aq) SO2(g) + H2O(l) I - is oxidized to dark brown I2 in acid solution by H2O2 or IO3 - In acid solution, I2 is reduced to I - by SO3 2- or S2O3 2- (thiosulfate) 3

4 Solutions of NaOH, H 2 SO 4, Na 2 CO 3, HCl, BaCl 2, H 2 O, Zn(NO 3 ) 2, NaI, NaIO 3, Na 2 SO 3 and Zn(NO3)2 can be distinguished by the following tests: 1. ph tests. Two strongly basic solutions (NaOH and Na2CO3) and two strongly acidic solutions (H 2 SO 4 and HCl) and seven nearly neutral solutions (BaCl 2, H 2 O, H 2 O 2, Zn(NO 3 ) 2, NaI, NaIO 3 and Na 2 SO 3 ). 2. Flame test. Find BaCl 2 from the green flame and the group of five sodium compounds from the bright yellow flame. 3. Mix a drop of BaCl 2 with drops of each of the acidic solutions. A precipitate of BaSO4 identifies H2SO4; the other acid must be HCl. 4. Mix a drop of BaCl 2 with drops of each of the basic solutions. A precipitate of BaCO3 identifies Na2CO3; the other base must be NaOH. 5. Mix a drop of NaOH with a drop of each of the remaining neutral solutions. A precipitate of Zn(OH) 2 picks out Zn(NO 3 ) Mix a drop of Zn(NO 3 ) 2.with a drop of each of the rest of the remaining neutral solutions and precipitates of Zn(NO 3 ) 2 and ZnSO 3 form. 7. One of the neutral solutions (H 2 O, H 2 O 2, NaI) that did not react with Zn(NO 3 ) 2 is identified as NaI because it gives a positive Na flame test. 8. Mix a drop of NaI with a drop of acid and add a drop of each of the two solutions that reacted with Zn(NO 3 ) 2. NaIO 3 gives a brown solution of I 2. Since two out of the three sodium containing solutions have now been identified by precipitation-reactions, the remaining solution with no reaction must be Na 2 SO Mix a drop of NaI with a drop of acid, and add a drop of the solutions left over from step 7 H 2 O or H 2 O 2. H 2 O 2 produces a brown solution of I 2 (which can be turned clear by adding a drop of Na 2 SO3). The other solution must be water, since it is the only solution left at this point that has not been identified. A sample flow chart illustrating this identification process is available in the procedure section below. PROCEDURE (WORK IN PAIRS) Safety Precautions The compounds in this experiment are not toxic, but promptly wash off any spills. Although it is harmless, silver nitrate will stain skin for a few days. The Bunsen burner flame can burn skin or start a fire if exposed to flammable materials. As always, wear goggles. Materials General Equipment ( mm) test tubes Test tube rack Watch glasses Special Equipment Transfer pipets Nichrome wire ph paper Bunsen burner Striker Reagents 1.0 M sodium thiosulfate (Na2S2O3) 1.0 M hydrochloric acid (HCl) 1.0 M sodium carbonate (Na2CO3) 0.1 M sodium oxalate (Na2C2O4) 0.1 M barium nitrate (Ba(NO3)2) 0.1 M manganese (II) nitrate (Mn(NO3)2) 1.0 M ammonium chloride (NH4Cl) 0.1 M zinc chloride (ZnCl2) 0.1 M silver nitrate (AgNO3) 0.1 M aluminum nitrate (Al(NO3)3) 3% hydrogen peroxide (H2O 4

5 Flow Chart for Sample Problem Compounds in bold indicate when they are identified. 1. Use a plastic transfer pipet twice to obtain about 2 ml of each unknown solution in test tubes labeled 1 11(the solution height in the tube should be about 1 inch). Store the tubes in a test tube rack, with a transfer pipet in each. 2. Using the flow chart prepared in the pre-laboratory questions, carry out the necessary procedures to identify all eleven solutions. Write down each test that is performed and the accompanying experimental observations in the laboratory notebook. Any modifications to the planned procedure should be recorded. The results of negative tests, in which nothing appears to happen, must also be recorded. Below is a list of suggestions to ensure reliable results. a. You should begin with a ph test, followed by a flame test and then other relevant reactions (Hint: You may set up a flow chart in which H 2 O 2 never undergoes a chemical reaction. It is then identified simply by being the only one left after all other solutions have been identified ). b. To test an unknown in the flame test, use the supplied needle-nose pliers to twist a small loop in the end of the nichrome wire. Avoid touching the loop, as your hands are contaminated with sodium. 5

6 Clean the nichrome wire loop before testing each sample by heating the loop red-hot until it no longer colors the flame. (Hint: The orange flame color might never dissapear completely. Make sure you remember the coloring of the flame from the pure wire so you can compare it to the flame color after adding sample) Allow the loop to cool so that it no longer glows, and use a transfer pipet to add a drop of the solution to the wire. Heat the loop in the Bunsen flame and note the color. The sodium flame is very intense and persistent. After a positive sodium flame test, cut the old loop off. Make a new loop farther down the wire before testing the next solution. To save time and material, make sure to do this only after a positive sodium flame, not after each test. If you are unsure about the sodium content of a solution, make a note of this in your lab notebook and move on to the next test. Most likely you will be able to figure out the missing solution later on. However, you need to have a positive identification of BaCl 2 (Green flame) c. To make a ph measurement, use the pipet to moisten a strip of ph test paper with a single drop of the solution being tested. With conscientious labeling of the spots, two or three tests can be made on a single strip of ph paper. Do not label the ph paper directly, but lay the ph paper on a paper towel and label the towel. Then compare the color of the paper to the color key given on the ph-paper vial. Compare the ph of all eleven solutions by laying the ph strips side by side. d. The majority of the tests should be carried out on a watch glass. First rinse the watch glass under tap water, and then rinse it with a little distilled water. Dry it with a paper towel before proceeding with the reactions. There should be no visible precipitate present on the watch glass before beginning a new test. Add equal volumes of the solutions to the center of the glass and mix by gently swirling. A couple of drops of each solution will suffice for most tests. A microspatula or a glass stirring rod can also be used for mixing. As each locker is stocked with several watch glasses, a few mixtures may be set aside for further observation after a period of time. e. For the reactions involving gases, begin cautiously by waving vapors towards the nose. Odors may not be noticeable unless the mixture is removed from the hood. Moist ph paper held over the solution containing NH3 gives a basic indication (ph >> 7). The whitish precipitate of sulfur (S8) is a diagnostic of the reaction of thiosulfate with acid, as is the choking stench of the sulfur dioxide product. f. Avoiding contamination is very important. Be sure to wash the microspatula or glass stirring rod with destilled water between mixtures. The watch glass should be cleaned between tests. g. Please keep in mind that you can use Na2CO3 to provide OH - for reactions carried out under basic conditions, such as Reactions 9 and 10 on page When all the solutions have been tentatively identified, perform one additional reaction for each solution to double check the identity. An inconsistent verification test may indicate that the original identification was incorrect and should be repeated. The verification test is to run additional reactions for 10 of 11 solutions. You can use all 11 solutions for each of your tests. This reaction SHOULD be DIFFERENT than the one used in the FINAL step of identifying that solution proposed in your flow chart. Absence of a reaction or no reaction is NOT a suitable verification method. Your verification reactions should be chosen to give a positive result such as precipitate or a color change (excluding PH or flame tests). Therefore you need to come up with a total of 10 reactions. Although a verification reaction involves at least two unknowns, count it as verifying only one of the ions and use another test to verify the second. Sometimes the original identification can be verified by identifying the counterion. For instance, if previously, NH 4 Cl was identified by ammonia smell, the identification can be verified by a reaction that finds Cl - (e.g. silver nitrate). 4. Wash all watch glasses and test tubes thoroughly. Return the nichrome wire and ph-test vials to the supplies table in the back of the room. 6

7 Disposal Collect all used solutions in a waste beaker in your hood. After completing the experiment, dispose all used and unused solutions in the waste container in the hood in the back of the room, next to the supplies bench. Transfer pipets go into the black medical waste containers on the supplies table in the back of the room. Paper towels and ph strips go in the regular wastebasket. DO NOT dispose of these in the red sharps containers next to the waste baskets! Doing so puts at risk the health of the lab managers who will have to remove inappropriatly disposed waste from the sharps containers. 7