Platinum Catalyzed Reaction of Hydrogen and Oxygen

Equipment

Disposable glass pipet, rubber hose 3-6 feet long to fit pipet and regulator, ringstand, iron ring, wire mesh.

Reagents

Tank of hydrogen gas with regulator and 0.5% platinized 1/8 inch alumina pellets

Presentation

1. Pound a small indentation into the center of the wire mesh large enough to contain 2-3 of the alumina pettets. The rounded end of a ball peen hammer works well for this. Without the indentation, the hydrogen stream will blow the pellets off of the screen.
2. Attach the iron ring to the ringstand. The height is not critical. Place the wire mesh on the ring and place 2-3 pellets in the indentation.
3. Attach the rubber hose to the pipet and the regulator outlet.
4. Open the hydrogen cylinder and adjust the outlet pressure to approximately 10 psi.
5. Dim the lights.
6. Hold the pipet approximately 6-10 cm from the pellets and allow the hydrogem stream to flow over the pellets. Note: Hold the pipet too close and all of the oxygen will be displaced from around the pellets, hold the pipet too far away and the hydrogen concentration will be too dilute around the pellets, either way there will be no reaction.
7. The pellets will begin to glow red hot if the exothermic reaction between hydrogen and oxygen begins to occur. You may use the glow to judge how to adjust the hydrogen stream. Adjust your position for maximum glow. When this is achieved, the hydrogem stream will ignite shortly thereafter.
8. Display the flame to the audience. Closing the outlet valve on the regulator will cause the flame to go out. Repeat if desired.
9. Close the cylinder valve.

Hazards

Hydrogen is an explosive gas, use in a well ventilated room and do not allow quantities of unreacted hydrogen to build up. The flame produced in this demonstration is very hot and can be up to 25 cm long. Do not allow the flame to come near anyone or any combustibles.

Discussion

The combination of hydrogen and oxygen to form water is an exothermic process. Despite this fact, hydrogen and oxygen will not react automatically when mixed together. The reason for this is the rather large activation energy needed to begin the reaction. The mechanism is very complex (2-4). However we do know that it is a free radical mechanism and that one of the initiation steps is:

H₂ (g)® 2H• (g)

Breaking the bond between the two hydrogen atoms requires 432 kJ/mole. This energy is typically initially provided by a spark or a flame. After the reaction begins, the energy produced from it will provide the necessary energy to continue breaking apart the hydrogen molecules. A catalyst provides an alternative mechanism that has a lower activation energy, this allows the reaction to proceed without the requirement of the initial addition of energy via a flame or spark. Hydrogen molecules will adsorb to the platinum surface. The energy of the interaction between the hydrogen atoms and the platinum surface contributes to the breaking of the bond between the hydrogen atoms. The separate hydrogen atoms are then free to react at the surface or leave the surface and participate in the water forming steps.


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References

1. Developed in house.
2. Eggers, D.F.,Jr.; Gregory, N.W.; Halsey, G.D., Jr.; Rabinovitch, B.S. Physical Chemistry, Wilely: New York, 1954, p. 475.
3. Nicholas, John. Chemical Kinetics, Halsted: 1976, p. 143
4. Pannetier, G.; Souchay, P. Chemical Kinetics, Elsevier: New York, 1967, p. 211.