Expt 018 -- Halide Ions

Solutions of halide ions are mixed with other chemicals to study systematically the properties of the halide ions and to see periodic trends.

Safety

Several of the chemicals are toxic (silver nitrate, sodium fluoride, ammonia, bleach, sodium thiosulfate). Bleach is corrosive. Fluoride solutions can cause serious chemical burns. Silver solutions cause black stains on skin. Wear goggles and apron. Wash spilled NaF immediately. Wash spilled NaF immediately from skin using soap. Do not mix bleach and ammonia. Use adequate ventilation when working with either bleach or ammonia. Wash hands after the experiment.

Procedure

1. Place an acetate sheet (or equivalent plastic covering) on top of Data Table (the large dark dot in the center of each box provides contrast against which to view light colored reaction products).
2. Prepare a data table for recording observations that is similar to the one on which the reactions are being studied.

   !!!Click here to See Picture.

3. Add one drop of NaF to each square in the first row or the data sheet. Add NaCl to the each square in the second row. Add KBr to each square in the third row and KI to each square in the fourth row.
4. To each column, add one drop of the reagent at the top of the grid. Stir with a toothpick if precipitates are present. Do not use the same end of the toothpick to stir more than one reaction.

   !!!Click here to See Movie.

5. Repeat the procedure for each column.

   !!!Click here to See Movie. Movie uses time lapse to show at 6 times actual rate.

   !!!Click here to See Picture.

6. Add one drop of aqueous Ca(NO₃)₂ to each square so that it mixes with the drop already placed in that square. Observe and record any evidence of change.

   !!!Click here to See Picture.

7. Using the third column of the data table as a backdrop, add one drop of AgNO₃ to one drop of NaF, NaCl, KBr, and KI. Observe and record any evidence of change.

   !!!Click here to See Picture.

8. Using the fourth column of the data table as a backdrop, add one drop of AgNO₃ to one drop of NaF, NaCl, KBr, and KI, as indicated. Add three drops of NH₃ to each puddle formed, and mix thoroughly with a toothpick. Observe and record any evidence of change.

   !!!Click here to See Picture. Enlarged

9. Using the fifth column, add one drop of AgNO₃ to one drop of NaF, NaCl, KBr, and KI, as indicated. Add three drops of Na₂S₂O₃ to each puddle formed, and mix thoroughly with a toothpick. Observe and record any evidence of change.

   !!!Click here to See Picture.

10. Use the sixth column, add one drop of starch solution to one drop of NaF, NaCl, KBr, and KI. Observe and record any evidence of change. Add one drop of NaOCl to each of these puddles. Observe and record any evidence of change.

    !!!Click here to See Picture.

11. Rinse the plastic sheet into a disposal jar provided by the instructor and located under the hood. This jar should contain a large amount of water.
12. The instructor may want to provide unknown solutions. These will be solutions of NaF, NaCl, KBr, and KI, but they will be labeled A, B, C, and D (or in some similar way.) Use the information obtained from the experiment above to perform experiments that permit relabeling each of the solutions.
13. Once the experiments are complete, rinse the plastic sheet into a disposal jar provided by the instructor and located under the hood. This jar should contain a large amount of water.
14. Clean off equipment. Wipe the lab table thoroughly with a wet sponge or paper towels. Wash a sponge at the sink. Discard soiled towels carefully with ordinary solid waste.
15. Wash hands using soap.

Questions

1. List the halides that form insoluble silver salts, and write a chemical equation to describe the formation of any of these.
2. List the halides that form insoluble calcium salts, and write a chemical equation to describe the formation of any of these.
3. List the silver halides that dissolve in ammonia, and write a chemical equation to describe the reaction.
4. List the silver halides that dissolve in sodium thiosulfate, and write a chemical equation to describe the reaction.
5. List the halides that react with bleach, and write a chemical equation to describe the reaction.

Handout

Handout Makeup

Name ___________________________ Class ________

Teacher__________________________

SmallScale 018 Halide Ions

Watch the movies.

• Identify UnknownA
  • Unknown number ______ Halide ______________
• Identify UnknownB
  • Unknown number ______ Halide ______________

Curriculum-

This activity can be used when halogen elements are studied. It also fits with complex ions, and can be used to demonstrate principles underlying photography. When used as a photography-oriented activity, this is an excellent applied chemistry activity.

Safety-

• Several of the chemicals are toxic (silver nitrate, sodium fluoride, ammonia, bleach, sodium thiosulfate). Bleach is corrosive. Fluoride solutions can cause serious chemical burns. Silver solutions cause black stains on skin. Wear goggles and apron. Wash spilled NaF immediately. Wash spilled NaF immediately from skin using soap. Do not mix bleach and ammonia. Use adequate ventilation when working with either bleach or ammonia. Wash hands after the experiment.
• The fluoride concentrations are low, and fluoride burns are not expected. The household bleach presents a greater hazard.

Time-

Teacher Preparation: 20 minutes

Class Time: 30 minutes

Materials-

• 2 kits of plastic transfer pipets with pulled tips containing:
  • Reagent Kit:
    • 0.5 mL of 0.5 M Ca(NO₃)₂ -- (Dissolve 11.807 g calcium nitrate tetrahydrate (Ca(NO₃)₂¥4H₂O) in enough water to make 100 mL solution.)
    • 1.0 mL of 0.1 M AgNO₃ -- (Dissolve 1.7 g silver nitrate (AgNO₃) in enough water to make 100 mL solution.)
    • 1.0 mL of 3 M NH₃ -- (Use the stock solution as provided.)
    • 1.0 mL of 0.2 M Na₂S₂O₃ -- (Dissolve 5.0 g sodium thiosulfate pentahydrate (Na₂S₂O₃¥5H₂O) in enough water to make 100 mL solution.)
    • 0.5 mL of 5% NaOCl -- (Use commercial household laundry bleach.)
    • 0.5 mL of starch solution -- (heat 100 mL of distilled water; spray in starch spray from a laundry spray can, in bursts, with continuous stirring, until the solution takes on a bluish gray hue.)
  • Halide Kit:(0.5 mL of each salt required.)
    • 0.1 M NaF -- (Dissolve 0.420 g sodium fluoride (NaF) in enough water to make 100 mL solution.)
    • 0.1 M NaCl -- (Dissolve 0.584 g sodium chloride (NaCl) in enough water to make 100 mL solution.)
    • 0.2 M KBr -- (Dissolve 2.38 g potassium bromide (KBr) in enough water to make 100 mL solution.)
    • 0.2 M KI -- (Dissolve 3.32 g potassium iodide (KI) in enough water to make 100 mL solution.)
• 1 acetate sheet
• 1 data table (See Handout above)

Disposal-

Add 200 mL of water to the liquid in the disposal jar. Treat the contents of the disposal jar with 10 mL of 0.1 M Na₂S. Wait one day. Filter the silver sulfide, and save this for silver recovery. Discard the filtrate at the sink with large amounts of running water.

Lab Hints-

The acetate sheet is an inexpensive spot plate. Surface tension keeps aqueous solution well puddled on the sheet's surface. There are three alternatives to using grids. The simplest is to place the acetate on the grid directly. Next is to use plastic paper holders, and slide the grid into the holder. Finally, the grid can be laminated.

Data Table-

Answers-

Q1. List the halides that form insoluble silver salts, and write a chemical equation to describe the formation of any of these.

A1. Cl^-, Br^-, and I^- form insoluble halides. Ag⁺ + Cl^- --> AgCl.

Q2. List the halides that form insoluble calcium salts, and write a chemical equation to describe the formation of any of these. (The precipitate, although present, is difficult to see on the movies.)

A2. Only fluoride forms an insoluble calcium salt:

Ca²⁺ + 2 F^- --> CaF₂

Q3. List the silver halides that dissolve in ammonia, and write a chemical equation to describe the reaction.

A3. Only the silver chloride dissolves in 3 M NH₃:

AgCl + 2 NH₃ --> Ag(NH₃)₂⁺ + Cl^-

Q4. List the silver halides that dissolve in sodium thiosulfate, and write a chemical equation to describe the reaction.

A4. Both AgCl and AgBr dissolve in sodium thiosulfate:

AgCl + 2 S₃O₃^2- --> Ag(S₂O₃)₂^3- + Cl^-

Q5. List the halides that react with bleach, and write a chemical equation to describe the reaction.

A5. KI and KBr are oxidized. The iodide solution gives a dark blue with starch. The bromine is hard too see.

Handout Ans.-

Use the second digit of the unknown number as the key.

1. Br^-
2. Cl^-
3. I^-
4. F^-

Key Words 1-

halide, halogen, precipitation, solubility, complex ion

Elements-

Cl Br F I Ag S N