Dioxygen

Introduction

Chemical elements can be recognized by a knowledge of their physical and chemical properties. In this experiment you will prepare the element oxygen in order to observe some of its chemical properties.

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Procedure
  1. Pick up a Beral pipet, a 250-mL beaker, and three small test tubes with rubber stoppers.
  2. Fill the beaker about three-fourths full with water and fill the three small test tubes completely full and set them in your test tube rack.
  3. Squeeze your pipet to push out the air and draw up a small amount of MnO2 (manganese dioxide) into the pipet from the stock provided by your teacher.
  4. Half fill a test tube with 3% H2O2 (hydrogen peroxide).
  5. Close one of the small, water-filled test tubes with your thumb and invert it into the beaker of water. Remove your thumb, making sure that there are no air bubbles in the test tube. Rest the inverted tube on the bottom of the beaker.
  6. Carefully squeeze your pipet to push out the air, being careful not to do it so rapidly that the manganese dioxide is blown out in the process. Place the tip of the pipet into the H2O2 and draw up enough hydrogen peroxide so that the pipet is about half filled. (Oxygen will begin to form as soon as the hydrogen peroxide touches the manganese dioxide. The manganese dioxide acts as a catalyst in this reaction. What is the purpose of a catalyst?)
  7. Place a 60° glass bend of 8 mm tubing on the edge of the beaker. Insert the tip of the Beral pipet through the glass bend into the water in the beaker, directing the stream of oxygen bubbles into your test tube holding the test tube so that the mouth is slightly below the water level. (Oxygen is commonly collected by water displacement as you are doing in this experiment. What does that tell you about the solubility of oxygen in water?)
  8. When your test tube is filled with oxygen, stopper it and collect a second and third in the same way. If your oxygen generator slows down too much, you may need to add a little more hydrogen peroxide.
  9. Observe the physical properties of the oxygen you have produced. What is its state, color, odor (waft a small amount of the gas from the tip of the generator towards your nose - does it have a distinct odor?)
  10. You will use the three test tubes of oxygen to investigate the most important property of this element. Light a small wooden splint with a match or Bunsen burner and allow it to burn briefly so that the tip is glowing red. Blow out the flame and insert the glowing tip into one of the test tubes of oxygen.
  11. Repeat this with the second test tube but this time restopper the test tube after the wood splint has burned. Place 1 mL of saturated aqueous Ca(OH)2 (calcium hydroxide) into the test tube and shake it with the gas formed by the combustion.
  12. Roll a few strands of steel wool into a small cylinder. Grasp the cylinder with a metal tweezers and heat for 12 seconds in a match flame. Hold the hot steel wool in the third test tube of oxygen and observe.

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Data Sheet
  1. You have prepared oxygen by the decomposition of hydrogen peroxide. Write the complete balanced equation for this decomposition.
  2. The manganese dioxide acts as a catalyst. What is the purpose of a catalyst?
  3. What are the physical properties of oxygen which you observed?
  4. You have investigated the most important chemical reaction of oxygen, what is it?
  5. One of the products of the combustion of wood in oxygen is carbon dioxide. A common test for the presence of carbon dioxide is the formation of CaCO3(calcium carbonate), a white insoluble substance. Did you see evidence that carbon dioxide was produced in your reaction? Write the balanced equation for the reaction between calcium hydroxide and carbon dioxide to form calcium carbonate.
  6. Write the complete balanced equation for the combustion of iron in oxygen. In an abundance of oxygen, such as in this reaction, you may assume that the product is iron (III) oxide.
  7. When you travel on an airplane there is always a warning to put out all cigarettes if you should have to use the oxygen masks. Using the evidence gathered in your experiment, explain this caution.

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Safety

Wear safety glasses at all times. Wear an apron. Escaping dioxygen can cause mixtures of manganese dioxide and hydrogen peroxide solution to spray around the work area.

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TG Answers
  1. See reactions for the balanced equation.
  2. The purpose of a catalyst is to change the rate of the reaction. In this case, the decomposition of hydrogen peroxide is sped up by the addition of the manganese dioxide as a catalyst. Students should be reminded that no oxygen is lost by the manganese dioxide.
  3. Physical properties of oxygen: colorless, odorless, a gas at room temperature.
  4. The ability to support combustion is the most important chemical property of oxygen.
  5. The formation of white, insoluble CaCO3 when carbon dioxide is bubbled through Ca(OH)2 (commonly known as limewater) is evidence that carbon dioxide was formed when the wood burned in oxygen. See above for the balanced reaction.
  6. See reactions for the balanced equation.
  7. If you wish to have to students ignite a cigarette in pure oxygen, direct them to hold it in the edge of a Bunsen burner flame for several seconds until it is smoldering. When they insert it into a test tube of oxygen it will then burst into flame.

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TG Background

MnO2

2 H2O2 --> 2 H2O + O2

CO2 + Ca(OH)2 --> CaCO3 + H2O

4 Fe + 3 O2 --> 2 Fe2O3

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TG Lab Hints

Hydrogen peroxide concentrations should be adjusted on the day of the experiment so that oxygen is produced at a suitable rate for the student. If the concentration is too high, the gas is produced too fast. Too low a concentration leads to a long and boring procedure. Hydrogen peroxide has poor storage qualities. Bottles of 3% H2O2 obtained from a pharmacy usually have concentrations lower than 3%. Using a diluted solution of 6% peroxide (beauty supply stores) can work around this difficulty.

While the problem is lessened when 30% H2O2 is diluted ten-fold, the handling and storage of that chemical presents so many problems that we recommend against its use.

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TG Materials

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TG Reference

The Woodrow Wilson microscale version of this lesson was developed by:

Dianne N. Epp
Lincoln East High School
1000 S. 70th St.
Lincoln, NE 68510

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