Preparation & Examination of OXYGEN
BACKGROUND INFORMATION
Oxygen constitutes over 50% of the mass of the earth's crust and
some 23% of the mass of the atmosphere. This abundant element
was not isolated and characterized until about 1773. Joseph Priestly
is generally credited with the discovery of oxygen, but C. W. Scheele
conducted and published similar findings at about the same time as
Priestly. This "dephlogisticated air", as oxygen was referred to,
became the key in gaining an understanding of combustion and led
to the final overthrow of the phlogiston theory.
Besides its obvious importance in life processes, oxygen has
become an important industrial material following only sulfuric acid
and lime in volume used in the United States. Its primary use has
been to achieve higher temperatures for the processing and
production of iron and steel. Among its other varied uses are sewage
treatment and treatment of low-grade infections, where it is toxic to
anaerobic bacteria.
8
15.9994
-2
O
1s22s22p4
Oxygen
Atmospheric oxygen exists as a diatomic molecule. This means that two oxygen atoms join together
to make one molecule of oxygen gas (O2). An allotrope of oxygen called ozone, occurs as a triatomic
molecule (O3). This pungent smelling gas is produced by electrical discharges in the atmosphere,
primarily lightning, and by exposure of ordinary oxygen to ultraviolet light (U.V.). An important
property of ozone is its high absorption in the U.V. region of the spectrum. This protects the surface of
the earth from intense solar radiation that contains large amounts of U.V. light. The depletion of
ozone in the atmosphere has caused great concern in recent decades. Fluorinated hydrocarbons or
CFCs have been identified as a causal factor in this ozone depletion, and their use has become more
limited due to them being banned in many countries including the U.S.A. CFCs are used as
refrigerant gases and liquids in air conditioners, refrigerators and freezers, and were formerly used as
spray can propellants.
Your textbook contains additional information about oxygen. Look in the index for the pages where
“oxygen” and “ozone” are covered. Mr. Scott’s website has some special resources about oxygen
and of course the Internet makes available a vast assortment of information on every topic.
Purpose
Prepare oxygen gas and investigate some of its properties.
Equipment
Goggles
Support rod
Clamp
Gas generator setup
Gas-collecting bottles
Pneumatic trough
Glass squares or watch glasses
Crucible tongs
Bunsen burner
Materials
hydrogen peroxide, 3%, H2O2
manganese dioxide, MnO2
water
candle & wire
steel wool
wood splint
Hazards
Combustion in pure oxygen proceeds rapidly in a spectacular fashion. H2O2 can bleach
and burn skin and clothing. GOGGLES MUST BE WORN AT ALL TIMES and other standard
precautions relating to fire and corrosive substances must be followed.
Procedure
1. Remove the stopper from the gas generator and place about 3-5 g of MnO2 into the bottom
of the gas generator bottle.
2. Position the tubing as shown in the
diagram at the right. The gas
collecting bottles should all be filled
with water and inverted in the
trough in order to reduce the time
involved with switching from one to
another.
3. Obtain 80 – 100 mL of 3% H2O2
When the setup is completed as
shown at the right, pour all of the
H2O2 into the funnel at the top of
the thistle tube.
4. In order to be sure the system is purged of air, the first bottle of collected gas should be
discarded, the bottle refilled with water, and used again to collect another volume of gas.
5. Three full bottles of gas should be collected. Cover the bottles with a glass plate or watch
glass as they are removed from the trough and set them aside one by one being sure to
keep them covered. If the gas production stops before three bottles of gas are collected,
obtain another 80-100 mL of 3% H2O2 and pour it into the gas generator through the thistle
tube as before.
6. After all three bottles are filled with oxygen gas, its examination can begin.
a) Produce a glowing splint by first igniting one end of the wood splint and allowing it to
burn lengthwise 1-2 cm. Carefully tap out the flame so that glowing embers remain
attached to the splint (this is called a glowing splint). Remove the glass plate from
one of the bottles and quickly thrust the glowing splint into the oxygen gas inside.
DON’T DROP the splint down into the bottle since there
will likely be some water remaining in the bottom that will
extinguish the embers. Make and record your
observations.
b) Obtain a wire and a candle as shown at the right. Light the
candle, remove the glass plate from the second bottle, and
with out dropping it, lower the candle into the oxygen gas
in the bottle. Make and record your observations.
c) Obtain some steel wool.
Roll the steel wool
between your hands until
it is shaped like a hot
dog. Hold one end of
the steel wool with
crucible tongs (as shown above) and heat it over a Bunsen burner. When the
interior of the steel wool begins to glow, QUICKLY insert the glowing steel wool into
the oxygen gas in the third bottle. Make and record your observations.
7.
Clean up.
• Empty the water from the trough into the sink.
• Rinse the gas collecting bottles with water and place upside down on paper
towel.
• Rinse the glass squares or watch glasses, dry them and them return to your
drawer.
• Steel wool waste goes into the trash after being doused with water.
• Used wood splints go into the trash after being doused with water.
• Return the candle and wire to its proper location.
• Rinse the gas generator with water and flush all of its contents down the drain.
• Reassemble the gas generator and leave it at your lab area.
• Return all other equipment to its proper storage location.
• Wipe up any spills or splashes from your lab bench