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ExposeExciteIgniteMay2012

2 Hydrogen from Kitchen

2 Hydrogen from Kitchen Stuff Two ingredients from the kitchen will prove that kitchen science can be a hot topic! Many years ago my life as a chemist started out with this amazing ‘kitchen science’ chemistry. I spent hours preparing hydrogen filled balloons and polluting the air with colourful home-made weather balloons. I still recall the backyard ‘lab’ with endless rolls of pocket money aluminium foil. We did many things with our balloons . . . . some acted as moving targets for our air gun and some took scientific and secret messages over long distances. I even had a map in my room indicating the areas from which phone calls were received as people phoned in after collecting the tags attached to the balloons. On windless days we launched gliders from the rising balloons using the ice switch method. All in all – we had lots of fun and even though Science Fairs didn’t exist at the time, we performed some “research” and even had controls built into our systems. Let’s call this “informal science”. The Background Hydrogen is the most abundant and lightest element in the universe, it is colourless, odourless and a non-toxic gas consisting of H2 molecules. The density of hydrogen (0.0899 g/l) is less than that of air (which is a mixture of different gasses) and therefore a hydrogen balloon rises in air. Discovered by Henry Cavendish in 1766, hydrogen was originally named “inflammable air” as it combusted when mixed with air. Something we will pursue in this chapter. There are many ways of preparing hydrogen gas in the laboratory. The best known procedure being the reaction of an acid (usually hydrochloric 23

acid) with a metal (usually zinc metal). The chemical reaction being: 2HCl (aq) + Zn (s) ! H2 (g) + ZnCl2 (aq) Another method is that of electrolysis – the breaking up of water into hydrogen and oxygen when an electrical current is passed through it. In this chapter we will describe a less known method which has certain advantages: The reacting reagents are less hazardous to handle than acid and readily available from the supermarket. Furthermore, it does not require batteries and special collection techniques as with the electrolysis. Aluminium foil (Al) is a well-known product in the kitchen and is one of the rare ‘pure’ chemical elements we encounter in our daily lives. Because of its reactivity, pure aluminium instantly gets covered in a thin outer transparent oxide layer known as corundum (aluminium oxide). Caustic soda, known in the lab as sodium hydroxide (NaOH) or traditionally as lye, is found in the laundry section of the supermarket as it is used to make soap or used as a drain cleaner. It is one of only a few chemicals that will penetrate the strong protective aluminium oxide layer to get to the reactive aluminium. The chemical reaction for the preparation is: 2Al (s) + 2NaOH (aq) + 2H2O (l) ! 2NaAlO2 (aq) + 3H2 (g) + energy Sodium aluminate (black compound), hydrogen gas and heat are the reaction products. Hydrogen and Oxygen Hydrogen gas combines explosively with air oxygen in concentrations ranging from 4 to 75 % hydrogen by volume. It requires a form of activation energy to get going. The reaction produces water in an exothermic reaction: 2H2 (g) + O2 (g) ! 2H2O (g) + 572 kJ For every milliliter of water produced, 15,900 J of energy is released, in the form of sound, heat, light and mechanical energy. The rapid release of a considerable amount of energy causes the pocket of water vapour formed to expand rapidly, resulting in a loud “Bang”. It is also known as the oxidation of hydrogen to hydrogen oxide (water!). When igniting a hydrogen filled balloon with a candle flame, the flame first pops the rubber and then provides the activation energy needed to 24