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ExposeExciteIgniteMay2012

3 Hot Hydrogen

3 Hot Hydrogen Applications Hydrogen is an important gas as it is the most abundant in the universe (75%), has the highest energy content per unit weight of all fuels and produces only water and energy when reacting with oxygen. It is also easy and cheap to prepare (see previous chapter) and can produce a number of safe Whizz-Bang demonstrations with no nasty residues or flying pieces that may compromise students’ safety. All in all hydrogen is a very useful chemical for the resourceful teacher. So what can we do with a number of hydrogen filled balloons? “Let’s celebrate the joys of chemistry in a big way!” We will focus in this chapter on two of hydrogen’s most useful properties and find some fun classroom applications: ! Hydrogen has a low density and a lifting power 7% greater than Helium. It’s density is only 0.090 g/L (Air is 1.205g /L at 20!). ! Hydrogen produces a highly exothermic reaction when it combusts with oxygen to form water and lots of energy. 33

A. Floating Hydrogen on Air Balloons and airships are lighter-than-air (LTA), and fly because they are buoyant: the total weight of the craft is less than the weight of the air it displaces. The Greek philosopher Archimedes (287 BC – 212 B.C.) was the first to establish the basic principle of buoyancy. Buoyancy is the upwards thrust acting on a body immersed in a fluid (air or liquid). It is equal to the weight of the fluid displaced. Buoyancy = Upthrust = (mg) fluid = (!Vg )fluid Two equal sized balloons (of 14 litres) floating in air filled with hydrogen and helium, will experience the same upthrust (they displace the same weight of air): WeightAIR = mg = !Vg = (1.205 g/L)(14L)(9.8 m/s 2 ) = 165.3 N But as the two gases have different densities, their weights will be different: WeightH2 = mg = !Vg = (0.090 g/L)(14L)(9.8 m/s 2 ) = 12.3 N WeightHe = mg = !Vg = (0.179 g/L)(14L)(9.8 m/s 2 ) = 24.6 N (We assume that the rubber balloons are of the same weight and thus are not included in the calculation. Densities: H2 0.090 g/L and He 0.179 g/L) So what difference does this make in lifting power (resultant force)? Lifting power H2: 12.3 - 165.3 = -153.0 N He: 24.6 - 165.3 = -140.7 N (the negative signs indicate that it is an upwards force) 153.0 - 140.7 / 165.3 x 100 = 7.44% This gives hydrogen a 7.4% lifting power advantage over helium. Safety ! No flames or heat should be allowed close to the balloons ! Never inhale hydrogen gas to demonstrate the “Donald Duck” voice effect. Use only helium for this demonstration ! Check with the local aviation authority if permission is required to launch balloons 34