Exploring the Halides

Exploring the Halides Introduction: The elements in Group 17 of the periodic table are nonmetals called the halogens. The word halogen comes from two ancient Greek words that mean "salt' and "former." There are five members of the halogen family. They are fluorine, chlorine, bromine, iodine, and astatine. Each halogen can react with a number of different metals to form compounds called halides. For example, when sodium metal reacts with chlorine, the halide sodium chloride is formed. The first four halogens react with metals to form classes of halides with easily recognizable names as follows: fluorides from fluorine, chlorides from chlorine, bromides from bromine, and iodides from iodine. In this investigation, you will observe chemical reactions of small quantities of four halides with specific solutions, or test reagents. You will use your observations of these reactions to determine which halide is present in a solution containing one or more unknowns. Pre-Lab Questions: 1. What are the five halogens and what is the chemical symbol for each? 2. What structural characteristic do the atoms of all the halogens share? 3. What are fluorides, chlorides, bromides, and iodides? 4. What method will you use to study halide reactions? 5. What safety precautions should you observe when working with bleach or ammonium hydroxide? 6. What are the hazards in handling the silver nitrate solutions and what precautions should you follow? Purpose: To learn how to determine whether a fluoride, chloride, bromide, or iodide is present in a solution. Materials: Well-plate, 24 well dropper bottles of Ca(NO3)2 Beral pipets NaF AgNO3 Toothpicks NaCl Na2S2O3 NaBr NH4OH NaI starch unknown bleach Safety:  Wear your goggles and lab apron at all times.  Silver nitrate will discolor the skin.  Avoid inhaling fumes from the bleach and ammonium hydroxide.  Never mix bleach and ammonium hydroxide.  Handle all chemicals with care; avoid spills and contact with your skin. Procedure: 1. Add five drops of sodium fluoride solution, NaF, to each of the four wells in column 1 of your well plate. 2. Add five drops of the remaining three halides (NaCl, NaBr, NaI) to the wells in columns 2, 3, and 4, respectively, as shown in figure 1. 3. Place two drops of calcium nitrate solution into each of the first four wells in row A. Mix the solutions thoroughly with a clean toothpick. 4. Record your observations in the Data Table. Pay particular attention to the color of any precipitate that forms. It may help to observe the well plate first on top of a piece of white paper. If no observable change takes place, write NR for no reaction. 5. Into each of the row B wells add two drops of silver nitrate. USE CAUTION WHEN HANDLING THE SILVER NITRATE SOLUTION. 6. Mix thoroughly with a clean toothpick. Record your observations as described in Step 4. 7. Into the same row B wells to which you added silver nitrate, add two drops of ammonium hydroxide solution. AVOID INHALING THE AMMONIUM HYDROXIDE FUMES. 8. Mix thoroughly with a clean toothpick. Observe as before and record you observations. 9. Into each of the row C wells add two drops of AgNO3solution, mix thoroughly, then add five drops of sodium thiosulfate solution (Na2S203), mix thoroughly, and record all observations. 10. Into each of the row D wells place two drops of starch solution and then two drops of bleach solution. Thoroughly mix the solutions with a clean toothpick, and record your observations. AVOID INHALING THE BLEACH FUMES.  DO NOT MIX BLEACH AND AMMONIUM HYDROXIDE SOLUTIONS. THE RESULTING VAPORS ARE TOXIC AND EXPLOSIVE.  AVOID SPILLING OR SPLASHING BLEACH ON YOUR SKIN OR CLOTHING. WASH ANY SPLASHES WITH PLENTY OF WATER. 11. Obtain from your teacher a solution containing a mixture of two unknown halides. Record the solution's identification number in the Data Table. Use the test reagents and the procedure you followed above to determine which halides your unknown contains. If time permits, you may repeat the tests to verify your results. 12. Empty the contents of your well plate into the waste jar. Thoroughly wash your well plate and dry it completely. Wash your hands before leaving the room. Data Sheet for Well Plate: Unknown # _____

NaF NaCl NaBr NaI Unknown 1 2 3 4 5 6

 Ca(NO ) 3 2 A

 AgNO + 3 B

 AgNO + 3 C

 Starch + D

Conclusions 1. Describe the reactions that occurred with sodium fluoride. Did they follow the pattern you saw with the other halides? 2. Based on your observations, describe the precipitates formed by the reactions of the halides with silver nitrate. 3. Which of the precipitates dissolved when ammonium hydroxide was added? When sodium thiosulfate was added? 4. How did the halides react to the starch solution? 5. How did the halides react to the bleach solution? 6. Explain your reasoning in identifying the halides present in your unknown. 7. Chlorine is often added to drinking water to kill disease-causing organisms. Design an experiment to determine if chlorine is present in your tap water. 8. How might you determine the difference between iodized salt and noniodized salt? Suggest a reason why the results of this experiment might the inconclusive.