PREPARATION AND PROPERTIES OF OXYGEN

PURPOSE

DESCRIPTION

TIME REQUIRED

MATERIALS

Chemicals:

cobalt(II) chloride

magnesium ribbon

charcoal

sulfur

distilled or deionized water

chlorine bleach

steel wool

Equipment:

candle

one hole stopper

250-mL Erlenmeyer flask

Bunsen burner

ring stand

utility clamp

pneumatic trough

eye dropper

rubber tubing

litmus paper

forceps

small gas collecting bottles

glass plates

deflagrating spoon

HAZARDS

MODIFICATIONS/SUBSTITUTIONS

1. Magnesium sparklers such as those used on the Fourth of July may be used in place of magnesium ribbon.
2. Charcoal is available in a variety of forms. Old fashioned stick charcoal is preferred.
3. Sulfur is available from garden supply stores in products such as Ortho's "Flotex".
4. A 12-oz soda bottle may be substituted for the Erlenmeyer flask.
5. Any type of fairly deep tray or pan can be used in place of a pneumatic trough.
6. Baby food jars and their lids may be substituted for the gas collecting bottles and the glass plates.
7. An old bent spoon or piece of bent copper foil can be used in place of the deflagration spoon.
8. If cobalt(II) chloride is not available or too expensive, an alternate reaction can be carried out in which 3 or 6% hydrogen peroxide, H₂0₂, is used in place of bleach and allowed to react with controlled amounts of the dark royal blue copper amine complex, Cu(NH₃)₆²⁺. The 3% hydrogen peroxide is available from grocery or drug stores, while 6% hydrogen peroxide can be obtained from drug or beauty supply stores under the brand name "Clairoxide." The copper complex can be prepared by mixing an ammonia solution (household ammonia works fine) with 25 mL of 0.1 M copper(II) sulfate (available under the name "Rooteater" from a garden supply store) until the copper hydroxide precipitate dissolves, leaving the transparent, dark blue amine complex ion. If this alternate preparation is used, a two-hole stopper should be substituted for the one-hole stopper in the flask. The copper amine complex is added to the hydrogen peroxide from an eye dropper inserted in the second hole in the stopper. A trial run should be done by the teacher to determine the amounts of copper amine complex and hydrogen peroxide necessary to fill the five jars with oxygen.

PROCEDURE

1. Clamp the neck of a 250-mL Erlenmeyer flask to a ringstand. This will prevent it from tipping over.
2. Insert the glass tube from an eye dropper in a one-hole stopper that fits the flask and attach a 0.5-m piece of rubber tubing.
3. Fill five gas collecting bottles to the very top with water; cover with glass plates. Fill the trough 1/4 full of water, and invert the bottles of water into the water. Carefully remove the glass plates, keeping the mouth of the bottle below the water level in the tray.
4. Put 150 mL of bleach into the Erlenmeyer flask.
5. Measure out approximately 4 grams of cobalt(II) chloride. Crush any chunks.
6. When all is ready, add the cobalt(II) chloride to the flask and quickly insert the stopper/hose assembly into the bottle's mouth. Lead the rubber hose from the generator into the tray of water and allow bubbles to escape for a short time to purge the system of air.
7. Insert the end of the rubber hose under the mouth of one of the bottles of water. As the oxygen bubbles from the hose, it will displace the water from the bottle. When one bottle is full of oxygen, move the hose to one of the remaining bottles.
8. Lift each bottle of oxygen just enough to slip a glass plate over the mouth. Do not lift the bottle completely out of the water while this is being done. Once the glass plate is over the mouth of the bottle, the bottle can be removed from the trough.
9. If the reaction stops or slows, swirl the contents of the flask to mix the reactants. When this no longer works, measure another 50 mL of bleach, quickly remove the stopper and add it to the generator. For steps 10-15 an observation table should be prepared.
10. Light a candle. Remove the plate from one inverted bottle of oxygen just enough to let out any water. Then hold the inverted bottle over a burning candle and remove the glass plate. Observe the flame of the candle.
11. With forceps hold a small tuft of steel wool in the flame of a Bunsen burner for a few seconds. As soon as it begins to glow red, quickly remove the cover from a second upright bottle of oxygen and hold the hot steel wool in the oxygen gas. While holding the steel wool, use the glass plate to close off as much of the mouth of the bottle as possible. You want to collect any products from the reaction between the oxygen and steel wool. Observe the steel wool. When the reaction stops add 10 mL of distilled water to the bottle, replace the glass plate and shake up the water and collected gas.
12. In a similar fashion, heat the magnesium in the flame. When the magnesium begins to burn, test it in a similar fashion as that used with the steel wool. Don't forget to add the distilled water before shaking.
13. Repeat step 11 with a small piece of charcoal.
14. Use a deflagration spoon to test the reaction of burning sulfur in oxygen. Put a small piece of sulfur about the size of a pea on the spoon. Hold the Bunsen burner soon as the sulfur begins burning, hold the spoon in a bottle of oxygen, and cover the mouth of the bottle as much as possible with a glass plate. When the reaction is over, add the distilled water, replace the plate and shake.
15. Test the water in each of the bottles with litmus paper, and record your observations.

SUGGESTED QUESTIONS

1. What physical properties must oxygen gas possess to allow it to be collected by water displacement instead of air displacement? (Remember ALL gases are less dense than water.) Would this method of gas collection be good for collecting ammonia gas too?
2. What physical and chemical properties of oxygen were illustrated in step 10 when the oxygen-was poured over the candle?
3. Write a balanced equation for each burning reaction which occurred in steps 11-14. Write a second balanced equation for those reactions with water which produced a color change in litmus paper.
4. Give a reason for the difference between how things burned in the air compared to how they burned in oxygen.
5. Compare the litmus paper results in steps 11 through 14. Suggest a reason why the results may have been the same or different.
6. Suggest how you can "predict" the litmus paper results based upon the substance you heat in the Bunsen burner.

DISPOSAL

DISCUSSION

2 NaOCl _(aq) ----> 2 NaCl _(aq) + O_{2 (g)}

Other reactions for this experiment are listed in order below.

4 Fe _(s) + 3 O_{2 (g)} ----> 2 Fe₂O_{3 (s)}

2 Mg _(s) + O_{2 (g)} ----> 2 MgO _(s)

C _(s) + O_{2 (g)} ----> CO_{2 (g)}

S _(s) + O_{2 (g)} ----> SO_{2 (g)}

TIPS

1. If the magnesium oxide produced by the burning of magnesium ribbon proves to be insoluble in water, suggest that students transfer the mixture to a test tube and warm gently before testing with litmus paper.
2. Do not use activated charcoal. After heating, it produces very little change when held in the oxygen gas.
3. The z-shaped spoons used to burn sulfur should be made up ahead of time and tested to be certain they'll fit easily into the gas collection jars. If old spoons are not available, a piece of copper foil or light gauge copper metal can be bent into a spoon shape and used.
4. The experiment should not be done with more than the 150 mL of bleach called for or the reaction may force black cobalt oxide into the delivery tube and into the collection bottles.

REFERENCES

Summerlin, L.R., Chemistry of Common Substances, Silver Burdett Co., New York, 1979, p. 25. -- Describes experiments with oxygen.

Metcalfe, H.C., Williams, J.E., and Castka, J.F., Modern Chemistry, Holt, Rinehart, and Winston Publishers, New York, 1982, p. 185. -- This work describes the chemistry of oxygen.

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