020 Halogens and Halogen Reactions

Expt 020 -- Halogens and Halogen Reactions

Small amounts of three halogens (chlorine, bromine, and iodine) are prepared and studied.

Compounds of the halogen elements are widespread in our society: water is fluoridated and pools are chlorinated; bromides and iodides find they way into many applications. The halogens share one property in common: they are oxidizing agents. Fluorine is one of the strongest oxidizing agents known; among the elements, it is the strongest.

Safety

Contact with chlorine, bromine, and/or iodine will cause burns and stains. They are toxic. Cyclohexane is flammable and toxic. Use the small amounts as directed. Make sure ventilation is adequate; work under a hood if it is not. Wash hands frequently during and after the experiment to avoid ingesting these substance. If contact with these substances occurs, wash hands thoroughly and then tell the teacher. Know the location of the fire equipment; review use of that equipment before beginning work.

Procedure

Wear goggles and an apron. Do not remove the goggles for any reason while any students are working with these materials.
Prepare three reaction vessels from wide-stem transfer pipets, or use the vessels prepared by the teacher. Cut the wide stem transfer pipets with a scissors. The stem should be about 6-7 cm long so that the tip can be held while the contents of the bulb are agitated vigorously. The inside diameter of the stem must be large enough so that a standard plastic transfer pipet can be inserted through the stem into the bulb.

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Label the vessels as 1, 2, and 3. Using a transfer pipet, add 10 drops of 0.1 M KI to vessels 1 and 3.

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Using a different transfer pipet, add 10 drops of 0.1 M KBr to vessels 2 and 3.

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Using a third transfer pipet, add 10 drops of distilled water to vessels 1 and 2. At this point, 1 and 3 contain I^-, 2 and 3 contain Br^-, and all three vessels have the same total volume.
Using a fourth plastic transfer pipet, add 30 drops of cyclohexane to each of the three vessels.

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Add 50 drops of water to one well of a 12-well plate (or to a 50-mL beaker). Add 50 drops of household bleach to the well (beaker). Carefully add 1 M HCl dropwise to the well until many bubbles can be seen forming (much less than boiling water; like a cool glass of soda). The solution will take on a slight yellow tinge.

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Cover the well (beaker) with a small watch glass or parafilm. Be cautious; these bubbles contain chlorine, a toxic gas. Record observations about the bubbles.

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Using a fresh plastic transfer pipet, add 5 drops of the chlorine-producing mixture to each vessel. Hold the vessel at the top of the stem; clamp the vessel shut with the first two fingers of one hand; and stroke the bulb vigorously with the other hand.

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Note and record the observations as the agitation is accomplished. After about one minute, set the vessels into the wells of a 12- or 24-well plate, and let the layers separate. Note and record the observations.

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Add a few drops of 1 M HCl to the chlorine-producing mixture. Add 15 drops of this mixture to vessel 1, clamp with fingers, agitate vigorously, and record the observations. Repeat until a dramatic change is observed.

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Note the number of drops required.
Carefully note and record the appearance of vessel 2. Add the same total number of drops of the chlorine-producing mixture to vessel 2 that were added to vessel 1; agitate; wait; note and record the observations.
Repeat the same sequence of additions to vessel 3. Again, clamp, agitate, wait, and note and record observations. Add 5 more drops of the chlorine-producing mixture to vessel 3 than were added to vessel 1.
Compare the three reactions, and carefully record your observations.
Follow exactly the teacher's instructions about discarding the contents of these vessels and discarding any unused chlorine-producing mixture. (These probably include pouring the reaction vessels' contents into one beaker or jar, and the excess chlorine water into a different beaker or jar.)
Wash hands.

Questions

The chlorine-producing species are OCl^-, Cl^-, and H⁺. Write a balanced net-ionic equation to describe the production of chlorine from these reactants.
Describe aqueous chlorine.
If chlorine reacts with iodide ion to form iodine, write a balanced chemical reaction for this process.
Describe a cyclohexane solution of iodine.
If chlorine reacts with bromide ion to form bromine, write a balanced chemical reaction for this process.
Describe a cyclohexane solution of bromine.
Chlorine reacts with excess iodine to form iodate ion according to the reaction:

6 H₂O + I₂ + 5 Cl₂ --> 2 IO₃^- + 10 Cl^- + 12 H⁺
Iodate ion is colorless. Based upon the observations and these facts, provide an explanation for the observations made regarding vessel 3.
Consult a periodic table. If fluorine had been available, predict how it would have fit into the series.

Handout Makeup

Name ___________________________ Class ________

Teacher__________________________

SmallScale 020 Halogens and Halogen Reactions

Watch the movies. Use the movies and pictures to answer the questions.

Curriculum-

Use this experiment when discussing halogens, oxidation reduction, net ionic equations, or activity series. The solubility of the halogen in nonpolar cyclohexane can be discussed under solution formation.

Safety-

Enforce rules about goggles and aprons strictly.
Cyclohexane: flammable, toxic by inhalation and contact. "This chemical has potential associated risk, but its use by students may be justified, providing the smallest amounts possible are used and the students have been informed as to the correct means of handling it." Make sure that students do not have flames or spark sources in use. Place the cyclohexane in limited amount (no more than 50 mL per class of 30) in a covered vessel under the hood or in a well ventilated area.
Household laundry bleach is very corrosive, caustic, and toxic.
1 M HCl is corrosive. Handle these chemicals with caution.
Chlorine water can present a serious problem. Control the amount to less than 1.5 mL quantities in toto for each working group, and have the students generate their own chlorine water as per the instructions in the procedure.
The cyclohexane solutions of halogens should not come in contact with skin. In this procedure, amounts are controlled and, so long as goggles and aprons are used, problems from these solutions should be minimal.

Time-

Teacher Preparation: 20 minutes

Class Time: 30 minutes

Materials-

5 mL of cyclohexane directly as purchased. (Set out no more that 50 mL of cyclohexane at a time.)
1.5 mL of 0.1 M KI -- A 1.66 g KI to 20 mL of distilled water. Add enough distilled water to bring the total volume to 100 mL.
1.5 mL of 0.1 M KBr -- A 1.19 g KBr to 20 mL of distilled water. Add enough distilled water to bring the total volume to 100 mL.
3 mL of household laundry bleach (Set out no more that 50 mL at a time.)
1 mL of 1 M HCl -- Measure 33 mL of 3 M HCl stock solution. Dilute with distilled water to a total volume of 100 mL.
2 mL 5% NaHSO₃ -- Add 10 g NaHSO₃ to 200 mL of water and stir until dissolved.
6 Ordinary plastic transfer pipets
1 12- or 24-well plate per group (to support reaction vessels)
1 50-mL beaker (optional; can use one well of 12-well plate)
1 small watch glass of suitable size to cover the chlorine-producing mixture
2 disposal beakers or jars.
3 Reaction vessels (wide bore plastic transfer pipets cut with stems 6-8 cm in length)

Disposal-

Allow unused chlorine-producing mixture to stand under the hood for a week. Discard this at the sink using very large amounts of water (50 mL water for each mL of bleach.)
Add 5% NaHSO₃ dropwise to the cyclohexane/water mixtures under the hood. Continue until the mixture is colorless or only slightly colored. Allow the cyclohexane to evaporate under the hood by pouring into a low, flat glass tray. Once the cyclohexane has evaporated, pour the aqueous layer slowly into a stream of running water at the sink.

Observations-

HCl reacts with OCl^- to produce chlorine:

2 H⁺ + Cl^- + OCl^- --> Cl₂ + H₂O
Chlorine oxidizes both iodide and bromide ions:

2 I^- + Cl₂ --> I₂ + 2 Cl^-
2 Br^- + Cl₂ --> Br₂ + 2 Cl^-
Iodine is violet in cyclohexane solution; bromine is orange, deep yellow when dilute. Iodide is more easily oxidized than bromide so that a mixture of these produces I₂ when less than one mole of chlorine is added per mole of iodide.
Adding chlorine to I₂ produces further oxidation:

6 H₂O + I₂ + 5 Cl₂ --> 2 IO₃^- + 10 Cl^- + 12 H⁺
Because iodate ion is colorless, this reaction causes the characteristic violet color of iodine to disappear. Once this color is discharged, adding more chlorine causes bromine to form from bromide.

Answers-

Q1. The chlorine-producing species are OCl^-, Cl^-, and H⁺. Write a balanced net-ionic equation to describe the production of chlorine from these reactants. A1. 2 H⁺ + Cl^- + OCl^- --> Cl₂ + H₂O
Q2. Describe aqueous chlorine. A2. Chlorine produces a yellow-green solution. The bubbles of gas produced appear yellow-green. If an odor is noted, it is the same as a school swimming pool -- and for the same reason.
Q3. If chlorine reacts with iodide ion to form iodine, write a balanced chemical reaction for this process. A3. 2 I^- + Cl₂ --> I₂ + 2 Cl^-
Q4. Describe a cyclohexane solution of iodine. A4. Cyclohexane solutions of iodine are less dense than water and are violet in color.
Q5. If chlorine reacts with bromide ion to form bromine, write a balanced chemical reaction for this process. A5. 2 Br^- + Cl₂ --> Br₂ + 2 Cl^-
Q6. Describe a cyclohexane solution of bromine. A6. Cyclohexane solutions of bromine are less dense than water and are orange (or deep yellow) in color.
Q7. Chlorine reacts with excess iodine to form iodate ion according to the reaction:
6 H₂O + I₂ + 5 Cl₂ --> 2 IO₃^- + 10 Cl^- + 12 H⁺
Iodate ion is colorless. Based upon the observations and these facts, provide an explanation for the observations made regarding vessel 3. A7. Because iodate ion is colorless, this reaction causes the characteristic violet color of iodine to disappear. Once this color is discharged, adding more chlorine causes bromine to form from bromide.
Q8. Consult a periodic table. If fluorine had been available, predict how it would have fit into the series. A8. Fluorine is above chlorine in the periodic table. If all other things were the same, fluorine would be expected to be the most reactive of the halogens. [In fact, fluorine is the strongest oxidizer among elements.]

Reference-

This procedure was adapted from a procedure developed by John McNeilly of Nebraska City, Nebraska.

Key Words 1-

oxidation, reduction, redox, activity series, solvent extraction, nonpolar solvent, halogen

Elements-

Cl Br I