Demonstration 15
Water Synthesis
The mention of condensation on the inside of the 20 litre flask in experiment 14 (MH was a little embarrassed to find that he had said water vapour!) brings us to a series of experiments dealing with the chemical combination of dihydrogen with dioxygen to make the compound dihydrogen monoxide (water).
15a The Marvel Tin
‘Marvel’ was the name of a brand of dehydrated powdered milk, supplied in tins with push-on lids, that MH purchased in large numbers during his bachelor days. That was long ago, but he still has a number of them because they once formed part of a carefully catalogued system of odds and ends (or rubbish as most other people called it). This particular tin, No.18, once contained ‘Grommets -- Various’. Little did he realise that it would, one day, play such a major role in chemistry demonstrations. Sadly, it is now rusty inside due to the water synthesised there, and battered after having its lid blown off nearly 100 times! MH says that he will soon have to use tin No.19, ‘Odds & Sods -- Various’!
The tin has a hole in its lid and another in the base. It is filled with dihydrogen from the hole in the top by downward displacement of the air inside. There are two easy ways of determining when it is full of hydrogen. If the room is reasonably quiet, you can hear the sound of the gas entering the can and when it’s almost full the pitch rises, or if you place the back of your hand close to the outlet hole in the bottom, the hydrogen feels significantly colder than the air.
When the vessel is full, turn off the hydrogen, remove the tube and place your finger over the top hole to prevent hydrogen escaping. With a taper, set light to the hydrogen at the top hole (having first removed your finger). The hydrogen will burn with an almost invisible diffusion flame. As the hydrogen is lost from the top it is replaced by air entering through the bottom hole, which by diffusion mixes with the hydrogen thus increasing the density of the mixture inside the tin (air is approximately seven times heavier than dihydrogen). Two things happen: as the density increases the flame gets smaller, and it becomes increasingly more premixed. Mixtures of hydrogen and air are explosive between 4.1 to 71.5% hydrogen and the ignition temperature is between 580 and 590C. Close to the upper explosive limit the flame ignites the mixture inside the vessel and a violent explosion ensues, largely because the mixture is confined. c.f. experiments 4 & 5.
15b Oxygen Filled Bubbles
Demonstration. Hydrogen and oxygen lines are fed from compressed gas cylinders separately though a system of ‘bubble’ jars to a ‘blow torch’. The bubble jars serve to indicate the flow rates of the gasses, to the audience. The blow torch burner is used normally during the Limelight experiment (Demonstrationn19) or, with the jet nozzle removed, tubing attached as in the previous experiment or with a bubble funnel attachment as in this experiment and 15b and 15c.
Dioxygen is very slightly heavier (ca. 1.05 times) than the air of which it is a component gas.
MH regrets that he omitted to show that oxygen does not, itself, burn. This is an important point to make, and he usually does so by involving his audience in a deductive process as in the next experiment. The two propositions being that when a flame is put to a bubble of oxygen it will:-
1. Burn violently or explode. or
2. Do nothing.
15c Hydrogen Filled Bubbles
Dihydrogen is ca. 0.066 times as heavy as air i.e. over 15 times lighter, and so the weight of the detergent film is not significant here. It works against the proposition that hydrogen is lighter. In other words the hydrogen filled bubbles rise to the ceiling despite the weight of the detergent film.
Although there was sufficient evidence from demonstrations 4 and 5 and 15a to enable the children to know that ‘hypothesis 1’ was correct, it was unlikely that they would do so. Their guess was more probably based upon a belief that I would go for something dramatic, like an explosion. However, the procedure of deduction (that is often ascribed to the Frenchman Rene Descartes) was clearly demonstrated.
The teacher might wish to mention the alternative rational of induction (a method favoured by the Englishman Francis Bacon), where the researcher collects, and compares, many observations of the effect before seeking to generalise and form some theory.
It is important to point out that advances in scientific knowledge and thought have not only resulted from deductive and inductive reasoning but also from sheer inspiration!
15d Hydrogen + Oxygen Filled Bubbles
Although the combustibility of hydrogen is one of its most characteristic properties, perfectly dry hydrogen ignites with difficulty, if at all, when mixed with perfectly dry oxygen. According to H.B. Baker (1902) [5] the dried mixture may be heated to the melting point of silver (960.5 o) without appreciable combination. Note, however, that moisture is a product of the reaction. The moisture is here said to act as a catalytic agent.
H. Davy [1] and T. von Grotthus found that a mixture of hydrogen gas and air heated to a temperature below visible redness, rapidly unites to form water without the evolution of light or heat. The temperature at which detonating gas (a mixture of two volumes of hydrogen and one volume of oxygen) inflames has been measured by E. Mallard and H. le Chetelier (1880) [2] and many others. The numbers are widely divergent. By plunging a bulb containing mixed gases in a bath at a constant temperature, numbers ranging from 518 o to 650 o have been obtained, and higher results are obtained if an excess of either gas is present. By measuring the adiabatic compression required to just ignite the gas, and calculating the corresponding temperature, K.G. Falk [3] obtained 540 o for the mixture 2H2+O2; 514 o, for H2+O2; and 530 o, for H2+2O2. According to H.B. Dixon (1910) [4], the ignition temperatures of mixtures of 100 vols. of hydrogen with n vols. of oxygen, by adiabatic compression, are:
|
Vols. of oxygen |
33.33 |
40 |
50 |
100 |
200 |
300 |
400 |
|
Ignition point |
557 o |
542 o |
536 o |
530 o |
520 o |
512 o |
507 o |
Hence, the most easily ignited mixture is not one in which the proportion of hydrogen to oxygen is as 2 : 1, but when the ratio is 1 : 4. The ignition temperatures obtained with the gases in sealed bulbs are rather lower than the ignition points of flowing gases.
Demonstration. Increasing stepwise, oxygen was added to the hydrogen bubbles and, at each stage, some of them were ignited. The ideal or stoichiometric mixture was obtained when the bubble jars indicated two bubbles of hydrogen to one of oxygen.
15e Hydrogen + Air Rockets
This experiment demonstrates the effect of diluting the reactants. In this gas phase reaction the dioxygen and dihydrogen molecules are separated by the dinitrogen molecules.