Laboratory Activities

Activity 1: Production and Chemistry of Chlorine (student version)

Introduction

Halogens represent a very reactive family of elements. One of the most common uses for halogens is as oxidizing agents.

Purpose

To prepare chlorine gas from common household bleach and use it to oxidize various materials.

Safety

1. Wear protective goggles throughout the laboratory activity.

2. Chlorine gas is highly corrosive and thus must not be allowed to enter the laboratory atmosphere. For this reason all work should be done in a properly operating fume hood. It is possible to detect the odor of chlorine at 3.5 ppm; chlorine is toxic at 1000 ppm.

3. Work toward the rear of the hood to avoid a backdraft of fumes from the front of the hood.

4. Be sure to empty carefully all three chlorine gas generator bottles into the NaOH wash solution when you complete the laboratory activity.

5. 12 M HCl is very corrosive and must be handled with care and dispensed in the hood.

6. When heating the steel wool in Step 6b be careful not to heat the test-tube and steel wool too strongly to avoid either melting the test-tube or igniting the steel wool.

7. Any substances spilled on your skin or clothing should be immediately washed off with large amounts of water and the incident should be reported to your teacher.

8. Dispose of all as your teacher directs.

Procedure

Part I. Preparation of Chlorine Gas

1. Obtain four wide-mouth bottles and one stopper fitted with a gas delivery tube.  Place 70 mL of 2 M NaOH washing solution into one bottle (see Figure 1).

2. In all subsequent steps be sure to avoid splashing the sides of the bottles while adding the bleach and acid. Place 20 mL of commercial bleach into one bottle. Add 5 mL of concentrated HCl to the bleach and immediately insert the stopper fitted with gas delivery tube. Submerge the free end of the glass tube in the NaOH washing solution.

3. After a few seconds the bottle fitted with the stopper will be filled with chlorine gas, which you can then use in the tests.

Part II. The Oxidizing Power of Chlorine

Each of the following steps requires a fresh bottle of chlorine. Use the first bottle in Step 4, prepare another bottle in the manner described in Steps 1 through 3 for Step 5, and then yet another for Step 6. Cover the bottles with watch glasses or flat glass plates.

4. Obtain two strips of denim or other dyed cloth. Wet one strip of cloth with tap water; leave the other one dry. Remove the stopper and suspend each strip over the lip of the bottle filled with chlorine gas, making sure the strips do
not touch each other. Cover the bottle. Record your observations.

5. a.) After you have prepared a new bottle of chlorine gas, obtain a strip of white paper. Write on it with pen, ordinary pencil, and various colored felt-tipped markers. Remove the stopper and suspend this paper in the bottle, once again covering the bottle with a watch glass or glass plate.  Try other samples such as a strip of newsprint or a flower petal. Record your observations.
    b.) Repeat Step 5a with the same items moistened with tap water. Record your observations.

6. a.) Place a small amount of steel wool into a new bottle of chlorine gas; quickly cover the bottle. Record your observations.
    b.) Place a small (1-cm diameter) ball of steel wool into a clean, dry 15 x 150-mm test-tube and, holding the test-tube with a test-tube holder, heat the tube and steel wool in a flame. Once the steel wool is hot, remove the cover from the bottle of chlorine gas used in Step 6a and quickly “pour” the steel wool into the bottle; replace the cover. Record your observations.

7. To clean up, remove the solids from the bottles (steel wool, cloth, paper, and other tested items) with forceps and      dispose of as directed by your teacher.  Carefully and slowly pour the entire contents of the three chlorine gas
generator bottles into the bottle containing 70 mL of NaOH wash solution. Pour the entire contents down the drain in the fume hood, while flushing with plenty of water. Then carefully rinse all bottles with tap water three times.

8. Thoroughly wash your hands before leaving the laboratory.

Data Analysis

1. Which strip of cloth—wet or dry—reacted more with chlorine? On what observation did you base your answer?

2. Which marks written on the paper reacted with chlorine? On what observation did you base your answer?

3. Which piece of steel wool—hot or room temperature—reacted to a greater extent with chlorine? On what observation did you base your answer?

Implications and Applications

1. Write a balanced equation representing the reaction of bleach solution with acid. Assume the bleach solution is NaClO(aq).

2. Why is it important to keep the sides of the bottles dry?

3. Provide reasons for your observation regarding the reactivity of wet vs. dry materials.

4. Was there any pattern to the reactivity of the inks? Can you make any generalization about their reactivity toward chlorine?  How do you account for the behavior of the pencil mark on the paper?

5. a.) Why did one piece of steel wool react more vigorously than the other? Did you expect this?
    b.) Why did you not just heat the steel wool in the flame directly?

6. Write a balanced equation for the reaction of chlorine with steel wool, assuming the steel wool is pure iron.

7. Would you expect fluorine to react similarly to chlorine in the above tests?  Why?

8. Would you expect bromine to react similarly to chlorine in the above tests?  Why?

Chlorine Gas Preparation

The excess chlorine gas is absorbed into NaOH solution rather than into air. [Franz and Malm. (1968, 1969). Fundamental experiments for college chemistry (2nd Ed.). San Francisco, CA: Freeman.]

Major Chemical Concept

This is an introductory laboratory activity, designed to be descriptive in nature, involving the reactivity of chlorine gas.

Level

Activity appropriate for basic, general, and honors students.

Expected Student Background

The major prerequisite for this laboratory activity is the ability to follow written instructions, make observations, and handle hazardous materials safely. You may choose to include this activity early in the year, since very little chemical knowledge is required.

Related concepts somewhat depend upon when this activity is used and what principles are being developed. Related concepts needed for understanding this activity include atomic structure, ions, formula writing, and equations. The activity is useful in developing an understanding of oxidation-reduction and periodicity.

Time

While the time to complete the activity depends on the availability of hood space and the number of students trying to use each hood, work can be completed in one 45-min period. Teacher preparation time will be about one hour, provided the glass tubing is already prepared. If not, then about a half-hour extra is required.

Safety

1. If insufficient hood space is available, consider conducting this activity as a demonstration. The chlorine is generated and stored in the hood; you then carry individual gas bottles (covered) back to the demonstration bench as needed. The bottles are only uncovered long enough to insert materials and then immediately recovered. Items to be investigated can be taped to other cover glasses, which then replace the original cover glasses.

2. Be certain the laboratory has adequate ventilation and is vented to outside of the school before you do this activity. The NaOH traps are very effective in preventing chlorine from entering the atmosphere.

3. Allow each student adequate room in the fume hood. Crowding can lead to accidents. The need for a burner in Step 6b means that only two students should work in a normal 4-foot hood.

4. An acid pump is recommended for dispensing the 12 M HCl to avoid the hazard of spills. Alternatively, a buret makes a very safe dispenser for small amounts of acid if you lack an acid pump. You must cover, but not seal, the top of the buret; students should still dispense the acid into a graduated cylinder. The reason for this is that it will be quicker and safer than trying to use buret markings to determine the volume and, more importantly, the acid must be added to the bleach quickly since the reaction is so rapid. Recognize, however, that the buret will need to be refilled often if you are supplying a large class.

5. CAUTION: Chlorine bleaches and ammonia-containing cleansers should never be mixed, since they react to form very toxic chloramine (NH 2 Cl). No bottles of aqueous ammonia should be present on the benches during this activity. An inadvertent error could cause a serious problem.

Materials (For 24 students working in pairs)

• 48 Wide-mouth bottles, 250-mL capacity
• 12 Rubber stoppers fitted with glass tubes
• Chlorine bleach, such as Clorox^TM
• 2 M Sodium hydroxide, NaOH, 1 L (80 g diluted to 1 L)
• 12 M Hydrochloric acid, HCl, 250 mL
• 12 Graduated cylinders, 10- and 100-mL sizes
• 36 Pyrex watch glasses or flat glass plates to cover the bottles
• 12 Strips of denim and/or other dyed cloth (used or prewashed so it wets easily)
• 12 Strips of newsprint
• 12 Flower petals
• Other items to be bleached—add your own items to the list, but test them first!
• A small amount of fine steel wool. (NOTE: Inexpensive commercial steel wool is often oil coated and gives poor results.)
• 12 Test-tubes, 15- x 150-mm
• 12 Test-tube holders
• 12 Burners and lighters
• 12 Pairs forceps

Advance Preparation

Complete the activity yourself with the bleach you intend to use. It would be wise to use a fresh bottle of bleach. If the bleach is old you may not get much reaction, and thus not much chlorine. Should you find yourself without any bleach, consult the school custodian.

Pre-Laboratory Discussion

Insure that students understand the safety instructions. Demonstrate proper placement of the stopper in the chlorine generator and the glass tube in the NaOH solution. Also focus on the difference between chlorine and chloride.

Teacher-Student Interaction

Monitor the generation of gas in the hood. This way you can prevent the release of unwanted gas into the atmosphere and ensure that things are working properly.

Expect students to want to test the reactivity of chlorine with such unorthodox items such as hair, gum, or anything else that they can find. This must be prevented!

Anticipated Student Results
(Data Analysis)

(These answers correspond to Data Analysis Questions 1, 2, and 3.)

1. Step 4: Wet strips of cloth will discolor more quickly and completely than the dry ones. From this observation it is concluded that the wet strips reacted more with the chlorine.

2. Step 5: The blue inks will turn brown. Felt-tipped marker behavior must be determined by trial. Do this yourself. The pencil mark will not discolor. These observations help determine which inks reacted more with the chlorine.

3. Step 6: Cool steel wool will not react; hot steel wool will react vigorously with evolution of much brown smoke. From these observations it can be concluded that the hot steel wool reacted more vigorously with chlorine.

Answers to Implications and Applications

1.     2HCl(aq) + NaClO(aq) ---> Cl₂(g) + NaCl(aq) + H₂O(l)

2. The sides must be dry so the strips of tested material are not able to react with the bleach or acid—the observed results are thus due only to reactionwith chlorine gas.

3. The wet cloth reacts faster and more completely than does the dry cloth because the gas can dissolve in the water and be brought into chemical contact with the dye.

4. It is expected that some inks will be more reactive than others. Therefore, you will need to determine the answer to this question by testing the actual inks used. (We anticipate that the wet paper, and “wetter” inks should bemore reactive than the dry ones.)

The pencil “lead” is actually graphite (carbon), and carbon is not reactive with chlorine at room temperature. (It is only reactive with chlorine at very high temperatures.)

5. a-b. The cool steel wool (iron) does not react rapidly due to the activation energy required. Hot steel wool possesses the needed activation energy. The steel wool was not heated directly in the flame because it would be oxidized in the flame—there could be some confusion regarding what caused the reaction.

6. 3Cl₂ (g) + 2Fe(s) ---> 2FeCl₃ (s). Iron in the III oxidation state is expected because chlorine is an oxidizing agent, although above 500 °C FeCl₃ begins to spontaneously reduce to FeCl₂ :

2FeCl₃ (s) ---> 2FeCl₂ (s) + Cl₂ (g)

7. Yes. It would be expected, based upon element positions on the Periodic Table, that fluorine would be even more reactive than chlorine toward the test materials. Note that the following side reaction also occurs with fluorine:

2F₂ (g) + 2H₂ O(l) ---> O₂ (g) + 4HF(g)

8. It would be expected, based upon positions in the Periodic Table, that bromine would be less reactive than chlorine toward all test materials.

Post-Laboratory Activities

Allow students ample time to answer the questions before discussing the laboratory activity.

Possible Extensions

1. You may repeat Steps 4 through 6 with chlorine bleach and compare the results with those involving chlorine gas. Note that the oxidizing agent in bleach is the hypochlorite (ClO^– ) ion.

2. You may also repeat Steps 4 through 6 with nonchlorine bleach and compare results with those of the chlorine gas and chlorine bleach.

3. a). If this activity comes after a study of kinetics, students can understand that the vigor of an oxidation-reduction reaction can depend on the concentrations of reactants. For example, to illustrate redox and the concept of relative oxidation strengths, heat the steel wool directly in the flame and compare the reaction to steel wool in chlorine. This is evidence that chlorine is more reactive than the oxygen in the air. (Be sure to compare the reactivities with the same grade (fineness) and compactness of “wad.”)
    b.) To further reinforce this concept, point out that oxygen in air does not oxidize the dye in their clothing or any other materials they tested.
    c.) Recognizing that the Cl₂ concentration in the bottle is higher than that of O₂ in air, you could drift into a discussion that includes more than you had probably intended. The whole concept of concentration effects and their importance in reaction kinetics is probably beyond most students.  You may recall, however (even though your students will not) that the concentrated oxygen environment of the first Apollo spacecraft was blamed for the fire that killed three men on the launch pad and resulted in the destruction of much of the space capsule contents.

Assessing Laboratory Learning (Written Examination)

1. Write equations to represent the reactions of chlorine gas with the materials tested. What is the main feature of all of the reactions?

2. Comment on the reactivity of chlorine gas and predict how fluorine gas, bromine vapor, or iodine vapor would react with the same materials.