Introduction
The purpose of the balloon investigation was for students to use the accumulation of their chemistry knowledge to construct apparatus that would fill a balloon with enough hydrogen to lift a 50g weight. Throughout the previous terms students were taught the theories and formulae that pertain to the task. Students performed experiments to find out the lifting ability of several gases and were asked to choose the most suitable one.
Lighter-than-air ballooning was first invented in 1783 by Professor Jacques Charles. He used a gas filled balloon to rise to an altitude of 3000m. Manned gas balloons currently can reach altitudes of tens of kilometres while unmanned balloons can roughly double the distance; the record heights are 34.7km and 53km respectively. Their most common use today is lifting scientific instruments into particular sections of the atmosphere to record data or perform experiments. Conversely, hot air ballooning uses the heating of air to rise. Hot air balloons are slower and more precise. As a result, it is more common to use hot air balloons for recreational purposes and gas balloons for scientific purposes.
The main principle that this experiment relied on was buoyancy. Buoyancy is the upward force (upthrust) exerted on an object by a surrounding fluid. The upward force is due to the pressure difference between the bottom and top of the object. Essentially, a less dense object placed in a more dense fluid will have an upward thrust equal to the magnitude of the weight of the displaced fluid. In the case of air and hydrogen, the density of dry air at sea level is approximately 1.292g/L (Wikipedia: Density of air, 2007). The density of hydrogen is considerably less then that; H2 has a density of only 0.08988g/L (Wikipedia: Hydrogen, 2007). Containing the hydrogen in a bag allows the gas to become a single object and displace air. The upthrust then pushes the balloon skywards.
Hydrogen is the first element on the periodic table. It is the most abundant chemical element. In the atmosphere, it is a colourless and odourless gas. The reason hydrogen was chosen for this task is because it is has a lower molecular weight than the common constituents of air and also is reasonably easy to synthesise. The reaction chosen to generate the hydrogen was to combine aluminium and sodium hydroxide to produce sodium aluminate, hydrogen and water. The equation for the reaction is
2Al(s) + 6NaOH(aq) → 3H2(g) + 2Na3AlO3(aq)
The only problem with this reaction is that it is quite fast and has a high heat of reaction (ΔH). As a result, when it is scaled up to generate large amounts of hydrogen, the reaction becomes quite hot and unmanageable. If the reaction is to be feasible, the heat must be countered.
One way of countering the heat is to use more solution then needed by the reaction. That way, the heat can spread out over a larger volume and thus have less effect on any one point. The added bonus of this method is that a higher volume has a higher surface area and thus the heat can be conducted by another material faster than if there was just enough solution for the reaction.