Tests for Iron(II) and Iron(III)

Transcription

1 Name Class Date Microscale Tests for Iron(II) and Iron(III) In this experiment, the complex hexacyanoferrate(ii) ion (ferrocyanide), Fe(CN) 4 6, and the hexacyanoferrate(iii) ion (ferricyanide), Fe(CN) 3 6, will be used in identification tests for Fe 2 and Fe 3 ions. The charges on the two complex ions clearly indicate the difference in the oxidation state of the iron present in each. The (CN) group in each complex ion has a charge of 1. Thus, iron(ii) is present in the ferrocyanide ion, [Fe 2 (CN ) 6 ] 4. Iron(III) is present in the ferricyanide ion group, [Fe 3 (CN ) 6 ] 3. A deep blue precipitate results when either complex ion combines with iron in a different oxidation state from that present in the complex. The deep blue color of the precipitate is caused by the presence of iron in both oxidation states. The color provides a means of identifying either iron ion. If the deep blue precipitate is formed on addition of the [Fe 2 (CN ) 6 ] 4 complex, the iron ion responsible must be the iron(iii) ion. Similarly, a deep blue precipitate formed with the [Fe 3 (CN ) 6 ] 3 complex indicates the presence of the iron(ii) ion. Both of the deep blue precipitates are known to have the same composition. The potassium salt of the complex ion has the formula KFeFe(CN) 6 H 2 O. The thiocyanate ion, SCN, provides a test for confirming the presence of Fe 3 ion. The soluble FeSCN 2 complex imparts a blood red color to the solution. OBJECTIVES Observe tests of known solutions containing iron(ii) or iron(iii) ions. Compare results for the two ions and infer conclusions. Design a procedure for identifying the two ions in one solution. MATERIALS FeCl 3, 0.1 M Fe(NH 4 ) 2 (SO 4 ) 2, 0.2 M gloves K 3 Fe(CN) 6, 0.1 M K 4 Fe(CN) 6, 0.1 M KSCN, 0.2 M lab apron plastic wrap, 8 cm 30 cm safety goggles sheet of paper, white thin-stemmed pipets (5) Always wear safety goggles, gloves, and a lab apron to protect your eyes and clothing. If you get a chemical in your eyes, immediately flush the chemical out at the eyewash station while calling to your teacher. Know the location of the emergency lab shower and eyewash station and the procedures for using them. Holt ChemFile B 23 Microscale Experiment

2 Name Class Date Tests for Iron(II) and Iron(III) continued Do not touch any chemicals. If you get a chemical on your skin or clothing, wash the chemical off at the sink while calling to your teacher. Make sure you carefully read the labels and follow the precautions on all containers of chemicals that you use. If there are no precautions stated on the label, ask your teacher what precautions to follow. Do not taste any chemicals or items used in the laboratory. Never return leftover chemicals to their original containers; take only small amounts to avoid wasting supplies. Call your teacher in the event of a spill. Spills should be cleaned up promptly, according to your teacher s directions. Acids and bases are corrosive. If an acid or base spills onto your skin or clothing, wash the area immediately with running water. Call your teacher in the event of an acid spill. Acid or base spills should be cleaned up promptly. Procedure 1. Put on safety goggles, gloves, and a lab apron. 2. Check that you have a labeled pipet for each solution listed in the materials. 3. Place the plastic-wrap rectangle on a white sheet of paper. 4. Along the top of the plastic wrap, place 5 drops of a freshly prepared iron(ii) ammonium sulfate solution in each of the three locations shown in Figure Along the bottom of the plastic wrap, place 5 drops of a freshly prepared iron(iii) chloride solution, as shown in Figure 1. Iron(II) ammonium sulfate Iron(III) chloride 6. Add 1 drop of 0.1 M K 4 Fe(CN) 6 solution to the first sample of iron(ii) ions at the top of the plastic wrap and l drop to the first sample of iron(iii) ions at the bottom. Record your observations in Table Add 1 drop of 0.1 M KSCN solution to the second sample of iron(ii) ions and 1 drop to the second sample of iron(iii) ions. Record your observations in Table 1. Holt ChemFile B 24 Microscale Experiment

3 Name Class Date Tests for Iron(II) and Iron(III) continued 8. Add 1 drop of 0.1 M K 3 Fe(CN) 6 solution to the third sample of iron(ii) ions and 1 drop to the third sample of iron(iii) ions. Record your observations. 9. Clean all apparatus and your lab station. Return equipment to its proper place. Dispose of chemicals and solutions in the containers designated by your teacher. Do not pour any chemicals down the drain or in the trash unless your teacher directs you to do so. Wash your hands thoroughly before you leave the lab and after all work is finished. TABLE 1: RESULTS OF TESTS FOR IRON(II) AND IRON(III) Iron ion Hexacyanoferrate(II) Hexacyanoferrate(III) Thiocyanate ion ion [Fe 2 (CN ) 6 ] 4 ion [Fe 3 (CN ) 6 ] 3 SCN Fe 2 Fe 3 Observations in step 6 Observations in step 7 Observations in step 8 Analysis 1. Organizing Ideas Explain specifically how you would make a conclusive test for an iron(iii) salt. 2. Organizing Ideas Which test for iron(ii) ions is conclusive? 3. Relating Ideas When iron(ii) ammonium sulfate was mixed with the [Fe 2 (CN ) 6 ] 4 ion, the precipitate was initially white but turned blue upon exposure to air. What happened to the iron(ii) ion when the precipitate turned blue? Holt ChemFile B 25 Microscale Experiment

4 Name Class Date Tests for Iron(II) and Iron(III) continued Conclusions 1. Designing Experiments Suppose you have a solution containing both an iron(ii) salt and an iron(iii) salt. How would you proceed to identify both Fe 2 and Fe 3 in this solution? 2. Relating Ideas Blueprint paper can be made by soaking paper in a brown solution of [Fe 3 (CN ) 6 ] 3 and iron(iii) ammonium citrate. Wherever the paper is exposed to bright light, the paper turns blue. Explain why this happens. Holt ChemFile B 26 Microscale Experiment

5 Microscale Testing Reaction Combinations of H 2 and O 2 Teacher Notes RECOMMENDED TIME: 50 min, including rocket launching and other suggested activities SKILLS ACQUIRED Constructing models Collecting data Communicating Identifying/recognizing patterns Inferring Organizing and analyzing data Predicting RATING Teacher Prep 4 Student Set-Up 3 Concept Level 2 Clean Up 2 Easy Hard THE SCIENTIFIC METHOD Make Observations The students determine the oxygen gas/hydrogen gas ratio that produces the loudest explosion. Analyze the Results Analysis questions 1 to 4 and Conclusions questions 1 to 3 Draw Conclusions Conclusion question 1 to 8 Communicate the Results Analysis question 4 and Conclusion questions 1, 3, and 6 MATERIALS To make 250 ml of 1.0 M hydrochloric acid, HCl, observe the required safety precautions. Stir 10.6 ml of concentrated HCl into enough deionized water to make 250 ml of solution. The 3% hydrogen peroxide, H 2 O 2, may be store-bought, provided it is fresh. To prepare 25 ml of 1.0 M potassium iodide, KI, solution, dissolve 0.42 g of KI in enough water to make 25 ml of solution. This will be far more than a class will use, and it can be kept for next year. Distribute the KI in dropper bottles. Make collection bulbs by cutting off the bulbs of graduated or thin-stemmed pipets, as in Figure 1. To calibrate the bulbs, fill one bulb completely with water. (If you have trouble filling the bulb completely, or students do later, consider using a thin-stemmed pipet to deliver the water into the bulb instead of having the water squeezed up from the petri dish.) Once the bulb is filled, squeeze all the water out into a 10 ml graduated cylinder, and record the volume. Empty the graduated cylinder and refill the bulb with water. Squeeze out, into the graduated cylinder, one-sixth of the volume of water you recorded earlier. With the bulb Holt ChemFile B 47A Teacher s Guide: Microscale Experiment

6 Testing Reaction Combinations of H 2 and O 2 continued held mouth-downward, mark the Graduated pipet water level on the outside surface of Cut Cut the bulb. Squeeze out a second sixth of the volume, and mark the level again. Repeat for the remainder of the Nozzle Collection bulb water. When one bulb has been calibrated, hold a wood splint across it, with the splint s end even with the end Thin-stemmed pipet of the bulb. Mark off the splint at the Cut Stretch same places that the bulb is marked. Use the splint as a template for calibrating the rest of the bulbs. Once calibrated, the bulbs can be reused from Cut here for nozzle Collection bulb class to class and year to year. To make a micro hydrogen gas, H 2, Figure 1 or oxygen gas, O 2, generator, cut the tapered tip off a graduated pipet to form a nozzle, or with a steady pull, stretch a thin-stemmed pipet to a much narrower diameter and cut it as shown in Figure 1. (Each stem provides two nozzles.) Punch or drill a hole (or melt a hole with a hot nail) in the center of the vial cap. Slip the tapered nozzle into the hole. If the nozzle segment is inserted until the flared-end section wedges into the hole, a relatively airtight junction is formed. You may wish to use a hot-glue gun to ensure a good seal. For the H 2 generator, break the zinc, Zn, into pieces. Put about 0.15 g of zinc into each vial. The vials should be about one-sixth full. The concentration of the HCl solution may be decreased to moderate the reaction rate. For the O 2 generator, the concentrations given for the H 2 O 2 solution (3%) and the KI solution (1.0 M) are meant only as guidelines. You may wish to adjust them as needed to moderate the reaction rate. Having the solution at (or slightly above) room temperature will also help speed the reaction. Construct a piezoelectric sparker from a charcoal/fireplace lighter found in many stores. Once wired as described in the following instructions, it can be used in place of a Tesla coil for a variety of chemical demonstrations. If the wire and tape are removed, the sparker can be used as a Bunsen burner lighter or for any demonstration requiring a quick and safe light. The materials are listed for making one piezoelectric sparker. Four or five sparkers are sufficient for an entire class. See Figure 2. Using scissors, split the end of a 10 cm length of speaker wire down the center for a distance of 3.5 cm. Strip the last 2.3 cm of insulation from both sides of the fork, as shown in Figure 2. Drill a 1/8 in. hole (or melt a hole with a hot 6D common nail) through the center of the film canister lid. Slide the wire through, and secure the lid in place about 4 cm from the unforked end of the wire with hot glue or other waterproof adhesive. The film canister lid serves as both a launch support pad and a splash guard. Slide one of the two stripped wire ends down into the small butane nozzle of the charcoal lighter, all the way down, so that the insulation is against the nozzle. Holt ChemFile B 47B Teacher s Guide: Microscale Experiment

7 Testing Reaction Combinations of H 2 and O 2 continued Speaker wire Lid to film canister Spark gap Collection bulb 4 cm 3.5 cm Glue Tape a. c. 2.3 cm e. b. Butane nozzle of charcoal/ fireplace lighter f. d. Figure 2 (Rotate the wire as you insert it.) Lay the other stripped end along the metal barrel of the lighter, and secure it in place with three to four wrappings of electrician s tape. To use the sparker, put the bulb over the wire, aim, and pull the trigger. Rather than arcing at the butane nozzle, the device will arc at the tip of the wire (the path of least resistance). An alternate method for producing a spark is to wire the assembly to an induction coil box. This may be found in the physics equipment storeroom. Another method for igniting the mixture is to bring the open end of the bulb close to a small flame. The loudest sound is produced by this method. ADDITIONAL MATERIALS Materials (not in student s list) micro H 2 generator: graduated pipet or thin-stemmed pipet (as in Figure 1) vial with cap micro O 2 generator: graduated pipet or thin-stemmed pipet (as in Figure 1) vial with cap piezoelectric sparker (as in Figure 2): nozzle from piezoelectric butane charcoal/fireplace lighter (as in Figure 2) 10 cm of 24 AWG speaker hookup wire film canister lid electrician s tape hot-glue gun and glue scissors wire strippers 1/8 in. drill or 6D common nail tongs or forceps (optional) refrigerator for chilling gases Holt ChemFile B 47C Teacher s Guide: Microscale Experiment

8 Testing Reaction Combinations of H 2 and O 2 continued SAFETY CAUTIONS Read all safety precautions, and discuss them with your students. Safety goggles and a lab apron must be worn at all times. In case of a spill, use a dampened cloth or paper towels to mop up the spill. Then rinse the towels in running water at the sink, wring them out until they are only damp, and put them in the trash. If you divide the reactant up into several bottles of each and set up reactant stations in different parts of the lab, students will not have far to walk after obtaining chemicals, and the likelihood of spills and accidents will be decreased. Wear goggles, face shield, impermeable gloves, and a lab apron when you prepare the HCl. Work in a hood known to be in operating condition, with another person nearby to call for help in case of an emergency. Be sure you are within 30 s walking distance of a safety shower and an eyewash station known to be in operating condition. TIPS AND TRICKS This experiment involves a number of concepts that could be discussed: activation energy, exothermic reactions, stoichiometry, rocketry and propulsion, and why the ratio of the volumes of the two gases in the optimum combination turns out to be the same as the mole ratio in the chemical equation. The last concept provides an opportunity to preview Avogadro s principle and the concept of molar volume in gases. TECHNIQUES TO DEMONSTRATE If students are unfamiliar with the O 2 and H 2 generators, demonstrate how they are used and how to replenish them. Show students how to perform the pop test using the piezoelectric sparker. If piezoelectric sparkers are not available, the gases can be pop-tested with a Bunsen burner, but the danger of unexpected explosions and burning bulbs is greatly increased. Therefore, if students are to use Bunsen burners, demonstrate this procedure: Light the burner and adjust it to a cool flame. When the bulb is filled, remove it from the nozzle, and hold it securely in a horizontal position with its mouth 1 cm from the midsection of the flame. Gently squeeze a very small portion of the contents of the bulb into the flame. If burners are used, it would be best to omit the rocket portion of the experiment. Show students how to fill the collection bulb from the petri dish as described in step 4. If this proves difficult, consider using thin-stemmed pipets to deliver the water into the collection bulb. Holt ChemFile B 47D Teacher s Guide: Microscale Experiment

9 Testing Reaction Combinations of H 2 and O 2 continued DISPOSAL Oxygen: After the lab, solutions may be washed down the sink, provided your school drains are connected to a sanitary sewer system with a treatment plant. Hydrogen: Combine all mixtures of zinc and acidic liquid. Filter to separate the zinc, and wash the zinc at least three times with water, collecting the washings with the acidic filtrate. After the washed zinc is thoroughly dry, it may be saved for reuse. Neutralize the combined filtrate and water washes with 0.1 M NaOH. Filter this liquid. The filtrate may be poured down the drain. When the Zn(OH) 2 precipitate is dry, it may be wrapped in newspaper and discarded in the trash. Holt ChemFile B 47E Teacher s Guide: Microscale Experiment