EXPERIMENT 16
[oxygen.lab]
OBJECTIVES:
1. To be able to prepare oxygen gas in the laboratory.
2. To experience some of the common reactions of O2.
3. To recognize the dangers of O2 reactions and to take proper precautions.
4. To be able to identify O2 if given as an unknown gas.
DISCUSSION:
Oxygen (O2) is a colorless and odorless gas that makes up about 20% of the air you breathe. Nitrogen makes up most of the rest of our atmosphere. Oxygen is the most common element on earth, comprising about 50% of the earth's crust. Oxygen never exists in nature for long as single atoms. It is always in molecular form as O2 or combined with other elements. When combined, it usually has an oxidation number of -2. The most common compounds are probably SiO2 and H2O.
You will prepare this gas by decomposition of potassium chlorate (KClO3) and then study the properties of oxygen and reactions of oxygen with several substances. Because oxygen is not very soluble in water, it is very easy to collect oxygen in bottles by water displacement. This ensures that the O2 will not be mixed with other gases.
It requires a very high temperature to decompose pure KClO3. Adding MnO2 lowers the decomposition temperature of the KClO3. The MnO2 does NOT change or increase the yield of O2. When a substance behaves this way, it is called a catalyst.
| NOTE: Goggles MUST be worn throughout the whole lab period !! |
EXPERIMENTAL PROCEDURE:
A. PREPARING AND COLLECTING O2
1. Work in pairs. Set up the apparatus as shown by the Instructor.
2. Fill a wide-mouth bottle with water, cover with a glass square and invert it in a water trough 2without losing any water. Do this for four bottles. Be sure that water overflow goes into a large beaker of another trough.
3. Add about 5 centimeters of potassium chlorate (KClO3) to a large pyrex (or other hard glass) test tube. Then add a small amount of manganese dioxide (MnO2) and gently mix together in the test tube.
4. Connect the test tube to the apparatus you set up. Set one of the bottles over the delivery tube. Get your instructor's approval for your set-up.
Instructor's Approval ______
5. Heat the test tube CAREFULLY and GENTLY. When a bottle is almost filled with a gas, cover the bottle's mouth with a glass square and remove the bottle from the trough. Set the bottle upright, leaving the glass square on the top. The purpose of the glass square is to hold the oxygen gas inside the bottle.
6. Quickly place another water-filled bottle over the delivery tube and repeat Step 5 until all four bottles are filled with oxygen. (If the O2 gas should stop coming before the 4 bottles are filled, disconnect the delivery tube, let the test tube cool. Then add a little more KClO3 and reheat.)
7. After the fourth bottle is filled, FIRST disconnect the delivery tube from the test tube and THEN stop heating the KClO3/MnO2 mixture.
B. PROPERTIES OF O2
1. Hold a wooden splint in a Bunsen burner flame until it catches fire. Remove the splint and let it burn for about 5 seconds. Blow the flame out and place the glowing splint in one of the bottles. Describe your observation in the Lab Report sheet (Question 1).
2. Using tongs, hold a small piece of charcoal (C) in Bunsen burner flame until it starts glowing. Put it in a deflagrating spoon and lower the spoon into the second bottle.
3. Hold a small strip of magnesium (Mg) with crucible tongs and ignite it with a Bunsen burner and quickly thrust it into the third bottle. DO NOT LOOK directly at the bottle!!!
4. Hold a small ball of steel wool (Fe) in a Bunsen burner flame. When it glows red, immediately thrust it into the fourth bottle.
5. Clean up your work area. Do not dump materials into the sinks. Use the waste containers provided. Answer the questions on your Lab Report.
Experiment 16: LAB REPORT - OXYGEN Name ______________ Sec____
Partner ___________________
A. PREPARATION
1. Write the balanced equation for the reaction used to prepare oxygen.
2. What is the purpose of adding the MnO2 ?
3. Does the manganese dioxide undergo a chemical change in the above reaction?
4. If more MnO2 is added, will we get more O2 ?
5. Why not just deliver the O2 into an upright glass bottle instead of collecting the oxygen "over water" ?
B. PROPERTIES
1. What happens to the glowing splint when you put it in the first bottle of oxygen?
2. In the second, third and fourth steps, what happens when you put a glowing or burning object into a bottle of oxygen?
3. Write the formulas of the oxides on the right side and then balance the equation.
a. O2 + C ==>
b. O2 + Mg ==>
c. O2 + Fe ==>
4. If someone gave you a bottle of an unknown gas, how would you test it to see if it is oxygen or not?
5. In the l970's Apollo spacecrafts were filled with pure oxygen so that our astronauts could breathe easily while in the space. In 1976 before the lift-off, a small electrical spark occurred and everything inside the spacecraft suddenly caught on fire. The three astronauts quickly died. Explain what happened.