Materials Included In Kit

Additional Materials Required

Safety Precautions

Gases in the syringe may be under pressure and could spray liquid chemicals. Follow the instructions and only use the quantities suggested. Hydrogen gas is a flammable gas and forms explosive mixtures with air or oxygen. Magnesium powder is a serious fire hazard. Keep magnesium away from all open flames. Hydrochloric acid solution is toxic by inhalation, ingestion and is corrosive to all body tissues. Use care when using matches. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Please consult current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Excess H₂ can be released into the air. Excess reagents can be rinsed down the drain with plenty of water according to Flinn Suggested Disposal Method #26b.

Teacher Tips

• Enough materials are provided for a class of 30 students working in pairs, or 15 groups of students. All parts of the lab can be completed in a standard 50-minute lab period if post-lab questions are completed either outside of class or on day 2.
• An excess of magnesium and 1 M hydrochloric acid solution are provided in case students need to make extra hydrogen to repeat some of the tests.
• The hydrogen gas that is generated can be stored in the sealed syringe for extended periods of time.
• Demonstrate the procedure for washing hydrogen. Washing the gas removes HCl from the syringe, which could affect any of the experiments involving hydrogen.
• Supply a waste beaker at each lab table for the wastewater from the washings.
• Remind students that hydrogen gas is lighter than air. Therefore, when the syringe cap is released, the tip should be facing downward so the hydrogen will not be lost.
• For best results on Part 4, have students perform the experiment in a darkened room. Hydrogen’s flame is nearly impossible to see in a well-lit room.

Sample Data

Answers to Questions

Part 1. Preparation of Hydrogen Gas

1. Write the balanced chemical equation for the reaction occurring in the syringe.
   Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g) 
2. Determine the number of moles of magnesium used to prepare the hydrogen in this experiment. Show your work.
   0.070 g Mg x 1 mol/24.3 g = 0.0029 moles of Mg 
3. Use the molar concentration and volume of HCl used to determine the number of moles of HCl you used to make hydrogen gas.
   Using 5.0 mL of 1.0 M HCl gives 0.0050 moles of HCl
4. Which is the limiting reactant, Mg or HCl?
   Using the balanced equation and the answers from questions #2 and 3, it is seen that HCl is the limiting reactant.
5. Use the balanced equation to determine the number of moles of hydrogen gas expected.
   Since HCl is the limiting reactant, and the ratio of HCl to H₂ is 2:1, then 0.0025 moles of H₂ gas are expected.
6. What volume in mL of hydrogen is expected from the 0.0050 moles of HCl? (Hint: use the Ideal Gas Law and assume P = 1.00 atm, T = 298 K, and R = 0.0821 L•atm/mol•K).
   V = nRT/P = (0.0025 mol)(0.0821 L atm/mol K)(298 K) / 1.00 atm 
   V = 0.061 L or 61 mL of H₂ gas were expected.

Part 2. Classic Test for Hydrogen

7. Write the balanced chemical equation for the combustion of hydrogen. What is the familiar product that was formed when H₂ gas was ignited?
   2H₂(g) + ₂(g) →  H₂O(l)
   Water is the familiar product, which could be seen by observing the inside of the test tube after ignition of the gas.

8. Which gas has the lower density, hydrogen or air? (Note: Air has an average molar mass of 29 g/mol.)
   The molar mass of hydrogen gas is 2.0 g/mol. The element hydrogen, H₂(g), has the lowest density of all the elements (0.090 g/L). If we have the same amount of any gas, then each gas will have the same volume. Since density is mass/volume, then using 1 mole of any gas means that H₂(g) will have the lowest mass (2.0 g) and therefore the lowest density. For ideal gas behavior, the density of a gas is directly proportional to the molar mass of a gas (from the ideal gas law).

9. Why were the test tubes containing hydrogen gas stored upside down in the water?
   Hydrogen is less dense than air. If the tubes were held upright, the H₂ gas would quickly escape.

Part 3. Hydrogen Bubbles

10. What are the two major gases found in air? Which one is reacting with the hydrogen?
    Air is about 79% nitrogen and 20% oxygen. Oxygen is reacting with the hydrogen. Nitrogen is an “inert” gas.
11. Write the balanced chemical equation for the reaction occurring in this experiment.
    2H₂(g) + O₂(g) → 2H₂O(l)
12. What differences did you notice between pure H₂ bubbles and H₂/air bubbles? Explain.
    H₂/air bubbles gave a louder explosion than pure H₂ bubbles. With H₂/air bubbles, oxygen was present to react with the hydrogen in the combustion reaction above.
13. Propose a set of experiments to determine how to produce the loudest bang from a constant amount of H₂ gas added to soap bubbles.
    Answers will vary, although a possible proposal might be to repeat the experiment with the volume of hydrogen held constant. Vary the amounts of air, remembering that air is only 20% oxygen. The loudest explosion would be observed when the mole (and therefore volume) ratio of H₂ to O₂ is exactly 2:1, according to the balanced equation in question #11.

Part 4. Candle Ka-Pow

14. Often there is an initial pop when the candle is raised into the syringe. Why?
    The oxygen in the air around the base of the syringe reacted with some of the escaping hydrogen to form water.
15. After the initial “ka-pow,” what happened when the lighted candle was moved farther up the hydrogen-filled syringe? What happened when you lowered the syringe? Explain.
    The farther the lighted candle is moved up inside the syringe barrel, the smaller the flame becomes. It may even be extinguished. The closer the flame is to the mouth of the syringe, the larger it is because of the oxygen mixing with the hydrogen.
16. How does the flame re-ignite?
    There are several possible explanations. Most likely, the candle’s wick is glowing hot enough to re-ignite the hydrogen once enough oxygen is available again. In pure hydrogen, further inside the syringe barrel, the candle does not burn. Hydrogen needs oxygen to burn.
17. Does pure hydrogen burn in the absence of air? What is required in order to make hydrogen burn?
    Hydrogen by itself is not flammable or combustible. Combustion or burning is a reaction of a substance with oxygen.

References

Special thanks to Bruce Mattson, Creighton University, Omaha, Nebraska for the microscale gas generation and testing procedures used in this kit. For more experiments on microscale gas generation and testing, please purchase Chemistry of Gases: A Microscale Approach, AP4849, from Flinn Scientific, Inc.

Mattson, Bruce; Anderson, Michael; Schwennsen, Cece Chemistry of Gases: A Microscale Approach, Flinn Scientific: Batavia, IL; Chapter 4.

Introduction

Explore the explosive properties of hydrogen gas and observe the effect of mixing hydrogen gas with air.

Concepts

• Gases
• Preparation of hydrogen gas
• Properties of hydrogen gas

Background

Hydrogen is one of the most unique elements of the periodic table. As a molecule, hydrogen has the formula of H₂. It is a colorless, odorless, and tasteless gas that is insoluble in water. It has a freezing point of –252.7 °C (20.3 K). It is one of the most important of the chemical elements. Most people live their entire lives without ever encountering H₂, yet we are constantly in contact with and dependent on compounds containing hydrogen. The most familiar compound of hydrogen is water, H₂O, which is 2⁄18 or 11% hydrogen by mass but ⅔ hydrogen by atom count. Almost all organic compounds contain hydrogen. Compounds of hydrogen are well represented on the list of major chemicals sold in the United States.

It is estimated that hydrogen makes up over 92% of all the atoms or 73.9% of the mass of the Universe. Our sun, like many other stars, consists mostly of hydrogen. All of the other elements were (and still are) formed from hydrogen and helium fusion reactions occurring in stars. As the star ages, the percentage of hydrogen drops. The current hydrogen content of our sun is 30% by mass.

Naturally occurring elemental hydrogen, H₂, is relatively rare on Earth. In the early stages of the Earth’s history, considerable amounts of hydrogen and carbon dioxide made up the atmosphere. Hydrogen’s mass is so light that hydrogen escapes the gravitational attraction of the Earth. Elemental hydrogen is still found near volcanic areas. Compounds of hydrogen, especially water, are very common in the Earth’s crust. Others include compounds with sulfur (H₂S), nitrogen (ammonia and amines) and carbon (organic compounds).

Small samples of hydrogen are conveniently prepared in the laboratory by reacting dilute HCl with metals such as zinc, magnesium, or aluminum. Metals that react with HCl (and other acids) generally produce hydrogen and the metal salt. For these experiments, hydrogen will be produced by reacting powdered magnesium with HCl 

Materials

Safety Precautions

Gases in the syringe may be under pressure and could spray liquid chemicals. Follow the instructions and only use the quantities suggested. Hydrogen gas is a flammable gas and forms explosive mixtures with air or oxygen. Magnesium powder is a serious fire hazard. Keep magnesium away from all open flames. Hydrochloric acid solution is toxic by inhalation, ingestion and is corrosive to all body tissues. Use care when using matches. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory.

Procedure

Note: Remember that hydrogen gas is lighter than air. Therefore, any time the syringe cap is released, the tip should be facing downward so the hydrogen will not be lost.

Part 1. Preparation of Hydrogen Gas

1. Inspect the syringe making certain that the plunger moves freely in the syringe, and that both the plunger seal and syringe are free from cracks. If the plunger moves with difficulty, it may be necessary to lubricate the rubber seal with a thin film of silicone oil. (Lubricate only the edge that makes contact with the inner barrel wall.)
2. Measure out 0.07 g of magnesium powder and place it in a plastic vial cap. Avoid getting any chemical on the sides of the vial cap.
3. Remove the plunger from the syringe barrel. Hold your finger over the tip of the syringe. Fill syringe completely with tap water. The water should be even with the top of the syringe.
4. Carefully place the vial cap containing the solid reagent on the surface of the water face up so that it floats.
5. Remove your finger from the syringe opening and allow the water to flow out of the syringe into a flask, a beaker, or into the sink. As the water level decreases, the vial cap will be lowered to the bottom of the syringe. When successfully completed, the cap should rest upright on the bottom of the syringe with all of the reagent still in the cap. If the vial cap tips over and the solid spills out, clean out the syringe and start over.
   {11920_Procedure_Table_2}
6. Carefully replace the plunger while maintaining the syringe in a vertical position. Gently push the plunger in as far as it will go. It will anchor the vial cap into the depression at the base of the syringe. (Note: There is a point just after the plunger’s rubber diaphragm enters the barrel where there will be resistance. Gently, but firmly, push the plunger past this point.)
7. Pour about 10 mL of 1 M HCl into a small beaker.
8. Draw about 5 mL of 1 M HCl from the beaker into the syringe. Be careful that the vial cap does not tip over since it will cause the reaction to begin prematurely.
9. Secure the latex syringe cap on the tip of the syringe by setting the syringe cap on the counter and quickly pushing the syringe into the cap. The latex syringe cap will push on.
   {11920_Procedure_Table_3}
10. Read step 11 now to understand how to stop the reaction. Do this before going on. Perform the reaction by shaking the syringe vigorously. The reagents will mix causing the reaction to proceed. The plunger will move outward as it is displaced by the gas. Do not leave syringe unattended. This reaction is quite rapid.
11. The next three steps (11, 12 and 13) should be performed quickly to minimize any loss of gas. When the reaction is completed or the volume of gas is about 50–60 mL, tip the syringe up to stop the reaction and remove the syringe cap. If the reaction is occurring too rapidly, or generating more than 50–60 mL of gas, stop gas collection by using the tilt, twist and release procedure. Tilt the syringe so the tip is pointing upward but away from anyone. Twist off the syringe cap with a slight twist, and release the pressure.
12. Hold the syringe with the tip pointing downward. Discharge the liquid reagents into the sink or a beaker. Use caution during this step so that none of the gas is discharged.
13. Secure the latex syringe cap back on the tip of the syringe.

1. Remove the syringe cap with the tip of the syringe pointing up. (A)
2. Draw a few mL of water into the syringe. (B)
3. Recap the syringe. (C)
4. Shake the syringe to wash the inside surfaces. (D)
5. Remove the syringe cap again. (A)
6. Discharge the water only into the sink or a beaker. (Note: Do not depress the plunger fully or the gas will be lost.) (E)
7. Recap the syringe. (C)

1. Use about 60 mL of H₂ gas from Part 1 for this part of the experiment.
2. Stand up a small candle in a one-hole rubber stopper. Light the candle.
3. Fill a small test tube with tap water and place it upside down in a 250-mL beaker half-filled with tap water.
4. Remove the syringe cap and quickly attach a piece of latex tubing to the syringe. Keep the syringe tip pointing down to prevent loss of H₂ gas.
   {11920_Procedure_Table_6}
5. Displace the water in the test tube with the H₂. A small bubble of air in the test tube is not a problem.
6. Keeping the open end down, remove the tube containing H₂ gas from the water. Hold the inverted tube near the lit candle until the hydrogen combusts. It may be necessary to slightly tilt the test tube so that the open end is directed slightly upward. (Note: Be sure to observe the inside of the test tube after igniting the gas. What do you observe?)
7. Repeat steps 3–6 for a second test tube.
8. Record your observations on the data sheet.

1. Prepare another syringeful of H₂ gas by repeating the procedure from Part 1. About 60 mL of H₂ gas is needed for this part of the experiment.
2. Pour about 10 mL of bubble solution into a small weighing boat.
3. Remove the syringe cap and quickly attach a piece of latex tubing to the syringe. Keep the syringe tip pointing down to prevent loss of H₂ gas.
4. Place the free end of the tubing into the bubble solution and slowly depress plunger discharging about 20 mL of the H₂ gas. Once a rounded mound of bubbles has been produced, remove the tubing from the solution, replace the latex syringe cap, and set the syringe aside. Save the rest of the hydrogen for use in steps 7–10.
5. Light a wood splint with a match and ignite the bubbles with the burning wood splint.
6. Record your observations on the data sheet.
   {11920_Procedure_Table_8}
7. Remove the syringe cap from the syringe containing the remaining 30–40 mL of H₂ gas. Draw about 20 mL of air into syringe and recap the syringe.
8. Place the free end of the tubing into the bubble solution and slowly depress plunger discharging about 20 mL of the H₂/air gas. (Caution: Do not add more than 20 mL of gas.) Once a rounded mound of bubbles has been produced, remove the tubing from the solution, replace the latex syringe cap, and set the syringe aside. (Note: The syringe cap may fit over the end of the latex tubing.)
9. Light a wood splint with a match. (Caution: WARN OTHERS BEFORE IGNITING!) Ignite the bubbles with the burning wood splint. This may be considerably louder than in step 5.
10. Record your observations on the data sheet.
11. Repeat steps 8–10, if desired, or release the remaining H₂/air mixture into the atmosphere.

1. Prepare a third syringeful of H₂ gas by repeating the procedure from Part 1. This experiment requires 60 mL of H₂ gas.
2. Stand up a small candle in a one-hole stopper. Light the candle.
3. With the latex cap directed upward, clamp the syringe into position using a ring stand setup. Remove the plunger from the syringe barrel. (Note: The plastic vial cap will also be removed with the plunger.)
4. Immediately and quickly insert the burning candle into the barrel of the syringe until the flame is completely in the mouth of the barrel. Observe what happens to the flame. As soon as the flame goes out, lower the candle below the barrel. Observe what happens. Repeat this raising and lowering of the candle a few more times.
5. Record your observations on the data sheet.
6. Excess H₂ can be released into the air. Consult your instructor for appropriate disposal procedures for all other materials.

Student Worksheet PDF

11920_Student1.pdf