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PREPARATION AND PROPERTIES OF OXYGEN
 PURPOSE
The objectives of this experiment are to prepare, collect and
investigate the properties of oxygen gas.
DESCRIPTION
This experiment is appropriate for a general or first-year
college-prep course. Many common household bleaches contain oxygen
in the form of sodium hypochlorite, NaClO. If the hypochlorite ion,
ClO-, is decomposed, oxygen gas can be produced. In this
experiment, cobalt(II) chloride, a catalyst, will be used to
increase the rate of production of oxygen. The properties of oxygen
will then be investigated.
TIME
REQUIRED
Two lab periods.
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
*See Modifications / Substitutions.
HAZARDS
Chlorine bleach has a strong odor and should be handled carefully.
Never mix ammonia with chlorine bleach; chlorine, which causes
severe lung damage is produced. Avoid breathing the sulfur dioxide
produced when sulfur burns; it is poisonous. Do not look at burning
magnesium; retinal damage may result. Goggles must be worn
throughout the experiment.
MODIFICATIONS/SUBSTITUTIONS
- Magnesium sparklers
such as those used on the Fourth of July may be used in place of
magnesium ribbon.
- Charcoal is available
in a variety of forms. Old fashioned stick charcoal is
preferred.
- Sulfur is available
from garden supply stores in products such as Ortho's "Flotex".
- A 12-oz soda bottle
may be substituted for the Erlenmeyer flask.
- Any type of fairly
deep tray or pan can be used in place of a pneumatic trough.
- Baby food jars and
their lids may be substituted for the gas collecting bottles and
the glass plates.
- An old bent spoon or
piece of bent copper foil can be used in place of the
deflagration spoon.
- 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
- Clamp the neck of a
250-mL Erlenmeyer flask to a ringstand. This will prevent it
from tipping over.
- 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.
- 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.
- Put 150 mL of bleach
into the Erlenmeyer flask.
- Measure out
approximately 4 grams of cobalt(II) chloride. Crush any chunks.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- Repeat step 11 with a
small piece of charcoal.
- 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.
- Test the water in
each of the bottles with litmus paper, and record your
observations.
SUGGESTED
QUESTIONS
- 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?
- What physical and
chemical properties of oxygen were illustrated in step 10 when
the oxygen-was poured over the candle?
- 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.
- Give a reason for the
difference between how things burned in the air compared to how
they burned in oxygen.
- Compare the litmus
paper results in steps 11 through 14. Suggest a reason why the
results may have been the same or different.
- Suggest how you can
"predict" the litmus paper results based upon the substance you
heat in the Bunsen burner.
DISPOSAL
Decant all solids and place in chemical waste receptacle. The
remaining liquid waste may be flushed down the drain with copious
quantities of water.
DISCUSSION
This experiment is a variation of the preparation of oxygen used in
kinetic studies. The reaction for the preparation is:
2 NaOCl (aq) ----> 2 NaCl (aq) + O2 (g)
Other reactions for this experiment are listed in order below.
4 Fe (s) + 3 O2 (g) ----> 2 Fe2O3
(s)
2 Mg (s) + O2 (g) ----> 2 MgO (s)
C (s) + O2 (g) ----> CO2 (g)
S (s) + O2 (g) ----> SO2 (g)
TIPS
- 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.
- Do not use
activated charcoal. After heating, it produces very little
change when held in the oxygen gas.
- 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.
- 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
Alyea, H.N. and Dutton, F.B., Tested Demonstrations in Chemistry,
American Chemical Society, Washington, 1960 pp. 7, 87, 91. --
Experiments using oxygen are described.
Joseph, A., Bandwein, P.F., Marhott, E., Pollack, J.F., Castka, J.F.,
A Sourcebook for the Physical Sciences, Harcourt, Brace and
World, Inc., New York, 1961, pp. 32, 136, 150. -- The preparation of
oxygen from hypochlorite and hydrogen peroxide are described.
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. |
