Explosive Reaction of Hydrogen and Oxygen Using Balloons
Six balloons are filled with different gases and ignited. Different intensities of sound are emitted, with the loudest sound occurring when a hydrogen/oxygen mixture is ignited.
Illustrates the properties of lighter than air gases and mixtures, and provides an opportunity to show students various aspects of the scientific method.
When ignited, hydrogen gas combines with oxygen gas explosively in proportions ranging from 4.1 - 71.5% hydrogen - ignition temperature is ca. 580 °C. The gaseous reaction is
2 H2(g) + O2(g) → 2 H2O(g)
This exothermic reaction yields 232 kJ/mol of water formed. The rapid release of a considerable amount of energy causes the surrounding air to expand suddenly, resulting in a sharp expolsion. When pure hydrogen is ignited, the reaction with the surrounding air is less rapid, the sound is less loud, and a significantly larger flame is produced. The comparatively mild sounds emitted when the helium balloons are ignited are caused by the rupturing of the balloons and the escape of the gas. The explosions are caused by a sudden pressure effect through the action of heat on produced or adjacent gases.
Six balloons (two red, two yellow, and two blue - colors can vary) are prepared along with ca. 1.5 - 2.0 m lengths of string attached to weights. Sources of helium, hydrogen and oxygen should be available. The blue balloons are filled with helium, tied off and attached to the strings attached to weights - plastic balloon closures can be used to attach the balloons to the strings. The yellow balloons are filled with hydrogen, and the red balloons are filled with a 2:1 mixture of hydrogen and oxygen - each balloon being secured in the same manner as the helium balloons. Each of the floating balloons is placed on a lab bench ca. 2.0 m apart. A birthday candle is attached to a meter stick or metal rod. A source of flame is prepared - a stationary candle in a glass holder can be lit with a barbecue lighter which can be used to light the birthday candle on the meter stick, or the barbecue lighter can be used exclusively - experience has shown the stationary candle preparation provides a smoother demonstration flow. Since the demonstrator is the closest to the balloons and loud sounds are generated, ear protection is strongly recommended.
Ask the audience to make observations and to make comments about the six balloons. When asked about the colors of the balloons, audience members usually are not specific about what they see - reinforce the fact that there are a specific number of red, yellow and blue balloons - not all balloons are all colors - this is important to emphasize so that know the importance of good observations in experiments. Ask them to comment about their buoyancy - they may volunteer that the balloons are filled with helium, based on previous experience. Ask them if this is the only possible gas the balloons can contain - more informative people may mention hydrogen. Mention these are the first two elements in the periodic table, and show this if a table is available. In order to test the hypothesis that the balloons contain helium, inform them of the fact that hydrogen is a flammable gas while helium is not. Using the lit candle on the meter stick, carefully touch the blue balloon filled with helium with the flame. Ask the audience about what they saw. They will say the balloon popped, and that it made a sound, but ask them if there is anything else they should comment on. Invariably, most people neglect the fact that the candle flame has been extinguished as well - inform them that this observation lends support to the fact that helium was contained in the balloon, is not combustible and that the escaping gas extinguishes the flame - one of the three things needed to support combustion has been eliminated. Repitition of the experiment is also part of the scientific method - proceed to repeat the experiment, and reinforce the fact that their observational powers have improved, since they now know what to look for. Proceed to the yellow balloons, inform audience members that they may want to protect their ears by placing fingers in them, put on ear protection and ignite the first one - many audience members will be startled when they now experience seeing a flame and hearing a louder sound. Ask the audience what they saw that time - engaging the audience enhances the demonstration experience. Inform them that the yellow balloon contained hydrogen. The hydrogen mixes with oxygen in the air forming an explosive mixture. Because the hydrogen and oxygen must mix before an explosion can occur, the explosion is relatively slow and diffuse. Point out that the remote candle is still lit. Since this experiment resulted in light energy being emitted as well as sound energy, repeat the experiment with the second yellow balloon with the lights turned down - after the experiment, ask if the experiment is performed better in the light or in the dark. Proceed to the first red balloon, and ignite it, always reminding the audience to protect their ears. The result is a louder sound accompanied by a brief flash of bright light, which audience members may not expect but nevertheless enjoy. Again engage the audience in a discussion of their observations. Inform them the last balloon had a mixture of hydrogen and oxygen - enough hydrogen was present to keep the balloon afloat - the explosion occurs rapidly in a compact area producing a louder, shorter bang. Inform them what they have seen in the yellow and red balloons are examples of an uncontrolled combustion - ask them to provide another word for this, to which they should reply, "an explosion." Inform the audience that the experiment is to be repeated with the lights turned down - proceed to do so - the audience is usually pleased with the grand finale.
Hydrogen gas is very flammable and yields explosive mixtures with air and oxygen. The explosion of the mixture of hydrogen and oxygen is quite loud. Tests should be made prior to the demonstration to adjust the total volume in the balloons so that the sound of the explosion is tolerable in the room.
Balloon remnants may be disposed of in the trash. Left over string and weights may be used again.