Hydrogen Chemical Properties

Although a very reactive substance under certain conditions, hydrogen is characterized by its inertness at ordinary temperatures. An exception to this rule was observed by Moissan and Dewar, who found that even at - 210° C. it ignites spontaneously when brought into contact with solid or liquid fluorine, forming hydrogen fluoride. With chlorine it combines slowly at the ordinary temperature, with production of hydrogen chloride: direct sunlight so accelerates the reaction as to cause an explosion. This photochemical reaction has been investigated by Baly and Barker. By employing activating light of varying intensity, they found that with a given light-intensity the proportion of hydrogen chloride formed in unit time is small at first, and then rapidly increases up to a constant maximum. Reference is also made to an unpublished observation by Campbell that the constant maximum rate of formation of hydrogen chloride is not proportional to the intensity of the light.

Combination with the other halogens and with the elements of the oxygen group has a measurable velocity only at elevated temperature. Dixon and Coward have determined the ignition-temperature of hydrogen in oxygen and air, the mean result for both gases being 585° C. They define the ignition-temperature as the temperature to which the gases must be heated separately so as to cause them to inflame immediately when brought into contact." Within 10° C. the ignition-temperature determined by Dixon's apparatus for hydrogen in oxygen or air is independent of the velocity of the gases within wide limits. It is unaffected by the material and surface-character of the apparatus, and also by the velocity of heating. The results of later work on electrolytic gas gave the ignition-temperature 526° C. Fiesel found the minimum ignition-temperature for the mixture 3H₂+2O₂ to be 397.5° C., that for equal volumes of the dry gases to be 407° C., and that for the mixture 4H₂+O₂ to be 433° C. An interesting account of the subject was given by Dixon in his presidential address to the Chemical Society of London.

For the lower dilution-limit of inflammability of hydrogen in air, Somermeier gives 5.9 per cent., and Coward and Brinsley give 4.1 per cent, of hydrogen. Terres gives the limits of inflammability in air as between 9.5 and 65.2 per cent., and in oxygen as between 9.2 and 91.6 per cent.

One of the lessons taught by the Great European War was the extreme vulnerability of airships and balloons filled with hydrogen, due to the ready inflammability of mixtures of the gas and air. During the war it was found possible to develop the production of helium within the British Empire on a commercial basis, and to employ this gas as a safe substitute for hydrogen in connexion with warfare in the air. Within certain limits it has been found practicable to utilize non- inflammable mixtures of helium and hydrogen for this purpose. Such mixtures have the advantage of possessing a greater lifting power than pure helium. Ledig found that a jet of helium with more than 14 per cent, of hydrogen can be ignited in air, but that in balloon practice from 18 to 20 per cent, of hydrogen can be employed, as the mixture does not burn with a persistent flame. With a proportion of hydrogen exceeding 20 per cent., the mixture is unsafe for use in war aeronautics.

von Wartenberg and Sieg found that the union of hydrogen and oxygen between 600° and 1000° C. is attended by the formation of a considerable proportion of hydrogen peroxide, this product rapidly decomposing into water and oxygen. Ozone is formed by condensation of a part of the oxygen. The velocity of decomposition of the ozone being less than that of the hydrogen peroxide, a greater proportion of ozone is found in the mixture. Fiesel states that with moist hydrogen and oxygen the reaction is bimolecular, and that hydrogen peroxide may be an intermediate product; with the absolutely dry gases the reaction is termolecular.

The combination of hydrogen and nitrogen under pressure was effected by Le Chatelier in 1901, but owing to an explosion the method was not worked commercially. In 1905 the subject was further studied by Haber and van Oordt, who found that at red heat the velocity of combination is too slow to admit of measurement, and that at higher temperatures the amount of ammonia formed is small, either on account of rapid dissociation or because the reaction-affinity is small.

In 1910 Haber found that in presence of metallic osmium 1 volume of nitrogen unites with 3 volumes of hydrogen at 550° C. and 200 atmospheres, the yield being 8 per cent, of the mixed gases. On the manufacturing scale, osmium can be replaced by the less costly uranium, and a continuous circulation method is employed, with a temperature about 500° C. Any carbon monoxide present in the hydrogen should be removed.

Methods for producing ammonia from its elements under the influence of the silent discharge and of the electric spark have not proved commercially successful.

The other members of the nitrogen group do not form hydrides by direct heating in hydrogen, the cause of this phenomenon with phosphorus being the rapid increase in the dissociation of the hydride with rise of temperature.

The nature of the products obtained by the interaction of hydrogen and carbon depends on the experimental conditions. Bone and Coward found that hydrogen unites with very pure carbon at 1100° to 1200° C., producing 75 per cent, of the theoretical yield of methane. By means of an arc-discharge between carbon poles in an atmosphere of hydrogen at a temperature of about 4000° C., Berthelot synthesized acetylene, an endothermic substance.

Gautier has studied the interaction of carbon monoxide and hydrogen at 300° to 1250° C. A mixture of 3 volumes of hydrogen and 1 volume of the monoxide passed through a porcelain tube begins to react at 400° C., water, carbon dioxide, and a small proportion of methane being formed. The action attains its maximum at 1000° C.

Under the influence of ultraviolet rays, carbon dioxide and nascent hydrogen yield formaldehyde, a reaction with an important bearing on the formation of carbohydrates in the plant.

Hydrogen Reduction

According to theory, hydrogen should precipitate from their salts all metals below it in the potential series. When it is passed into a solution of silver nitrate, silver is slowly deposited: with palladous chloride there is a rapid, quantitative precipitation of the metal. Although at ordinary temperatures the rate of deposition of the metal from solutions of cupric sulphate, platinum chloride, and auric chloride is too slow to be measured, the reduction of these and other salts is greatly accelerated by introducing the hydrogen occluded in palladium. In some instances reduction by hydrogen is facilitated by increase of pressure, examples being afforded by solutions of mercuric chloride and mercurous nitrate, which are reduced at 100 atmospheres, but not at the ordinary pressure.

Similar conditions govern the interaction of hydrogen and oxidizers. Although the number of observed instances of direct reduction of oxidizing-agents by gaseous hydrogen is small in comparison with those theoretically possible, such reactions are often induced by the presence of finely divided platinum or palladium, and especially by palladium-occluded hydrogen. Gladstone and Tribe cite examples such as the reduction of chlorates to chlorides, nitrates to ammonia, sulphurous acid to hydrogen sulphide, ferricyanides to ferrocyanides, arsenic acid to arsenious acid, nitrobenzene to aniline, and indigo to indigo-white. Further instances of reduction by hydrogen and colloidal palladium are given by other investigators.

Gaseous substances also react with hydrogen occluded by metals, the increased activity being probably due to the existence of the hydrogen partly in the atomic state and partly as hydride. The combination of hydrogen with the halogens and oxygen is promoted by the catalytic action of platinum and palladium, the same effect being noted by Kuhlmann for the interaction of hydrogen and nitric oxide to form ammonia. The direct combination of nitrogen and hydrogen is not induced by these catalysts. It is noteworthy that hydrogenation cannot be induced by either the spongy or colloidal form of platinum or palladium completely freed from oxygen, and that hydrogen desorbed from these metals retains activity for some time.

A most important method for the application of hydrogen as a reducing agent has been discovered and elucidated by the researches of Sabatier and Senderens. A very succinct summary of their work and that of other investigators has been given by Sabatier. The method is simple, and consists in passing a mixture of the gaseous substance and hydrogen through a tube containing the finely divided metallic catalyst, obtained by previous reduction of the oxide in the same tube. For each reaction there is a suitable temperature, sometimes that of the atmosphere, but more usually 150° to 200° C. The neighbourhood of 180° C. has been found well adapted for many reactions. The metals employed have been platinum-black, nickel, cobalt, iron, and copper. Of these catalysts nickel is the most active, and shares with cobalt the power of inducing reactions not promoted by the other metals. Copper is the least useful of the five, platinum and iron occupying an intermediate position.

The preparation of the catalyst can be exemplified by a description of the operations involved in the case of nickel. Unglazed biscuit-ware, broken to the size of peas, and freed from iron by boiling for several days with dilute hydrochloric acid, is rendered more porous by heating to redness for half an hour. After immersion in a concentrated solution of nickel nitrate, and evaporation of the liquid, the material is dried at 100° C., and subsequently heated until evolution of oxides of nitrogen ceases -

2Ni(NO₃)₂=2NiO+4NO₂+O₂.

The reduction to metal is effected in the hydrogenation apparatus, the oxide being placed in a hard-glass tube 1 metre long, and with a bore of 2 cm., supported in a slanting position, and surrounded by an iron tube having holes drilled in it for the insertion of thermometers. The hydrogen employed is washed successively with an acid solution of potassium permanganate, concentrated sodium hydroxide, and concentrated sulphuric acid, and finally passed over heated, palladized asbestos. During the operation the temperature of the reduction tube is maintained at 300° C., the end of the reaction being indicated by cessation in the production of water.

The reduction processes effected by this method can be classed in four groups -

1. Simple reduction, without addition of hydrogen, exemplified by the conversion of nitrous oxide into water and nitrogen.
2. Reductions accompanied by addition of hydrogen, such as the formation of methane from the oxides of carbon, effected by nickel or cobalt only, or of benzene from halogen-benzenes. In these reactions the hydrogen replaces oxygen or halogen. The reduction of nitric oxide and nitrogen peroxide to ammonia, and of nitrobenzene to aniline, belong to the same group.
3. Addition of hydrogen to compounds with multiple bonds, such as the formation of ethane from ethylene or acetylene. Rideal found 137° C. to be the optimum temperature for the hydrogenation of ethylene, and considers the mechanism of the process to be explicable on the hypothesis of Langmuir that the reaction proceeds in a unimolecular film on the surface of the nickel catalyst.
4. Sometimes the metallic catalyst exerts a more or less powerful tendency to break down the molecule, hydrogen being added not only to the initial substance, but also to its fission-products. Examples are the conversion of benzene at 300° C. into methane; of acetylene at 200° C. into a liquid resembling American petroleum, above red heat into one like Caucasian petroleum, and at red heat into a product similar to a mixture of the two varieties.

The two factors essential to success in the application of the method are purity of material and a suitable temperature.

Orloff found that in presence of coke coated with nickel or palladium hydrogen reacts at 95° to 100° C. with carbon monoxide, yielding ethylene and water. The reaction can be assumed to take place in two stages:

CO+2H₂=CH₂+H₂O; 2CH₂=C₂H₄.

Gauthier prepares the finely divided nickel by depositing nickel oxide on glass beads of 2 mm. diameter, and reducing at 330° C.

Nascent Hydrogen

At the moment of liberation from its compounds, hydrogen displays much greater chemical activity than it does in its ordinary condition, and is able to effect reductions beyond the power of gaseous hydrogen at atmospheric temperature, the method being specially suited to the reduction of organic compounds. When in this state, hydrogen is termed nascent. The method employed for preparing nascent hydrogen depends in each instance on the nature of the reduction contemplated. The action of an amalgam of sodium, magnesium, or aluminium on water serves for the reduction of aldehydes and ketones to alcohols; that of metals such as zinc, tin, and iron on acids is employed to convert nitro-compounds into amines; the decomposition of a concentrated solution of hydrogen iodide in a sealed tube at 150° to 275° C. is applied to the hydrogenation of unsaturated hydrocarbons. Ostwald attributes the reducing power of nascent hydrogen to an increase in free energy, just as in electrolytic reductions the chemical activity of the evolved hydrogen depends on the pressure at which it is discharged at the cathode.

Zenghelis proved the chemical activity of hydrogen to be much increased by bringing the gas in very minute bubbles into contact with solutions. His process consisted in forcing the gas into paper cartridges under such conditions as to inhibit bubbling through the paper, but so as to facilitate reaction with the dissolved substance in the pores of the cartridge. At 90° C. an appreciable reduction of mercuric chloride to mercurous chloride was observed, and between 80° and 85° C. a similar reduction of potassium chlorate to potassium chloride. Contact for three days at the ordinary temperature, and more rapidly at 65° C., yielded evidence of the conversion of carbon dioxide into formaldehyde and substances with characteristic sugar properties. At ordinary temperature, potassium nitrate was reduced to potassium nitrite; and under similar conditions of temperature an experiment lasting half an hour transformed sufficient nitrogen into ammonia to give the Nessler test.

The energy characteristic of the nascent state is attributed by Zenghelis to the very fine state of division of the reacting gas.

Reduction of Metallic Oxides

Hydrogen can displace many metals from their oxides, the reduction taking place at the ordinary temperature, as with silver and palladium oxides, or on heating, as with the oxides of copper, cadmium, lead, antimony, nickel, cobalt, and iron. Sometimes these reductions are incomplete, an equilibrium being attained. Such equilibria depend on the experimental conditions, an example being the action of steam on heated iron.

Raschig observed that a mixture of hydrogen and nitrogen peroxide passed through a heated tube reacts with such violence as to cause explosion.

The influence of the silent electric discharge on mixtures of hydrogen and other gases has been studied by Losanitch. Sulphur dioxide is rapidly reduced, with liberation of sulphur -

SO₂+2H₂=2H₂O+S.

Nitric oxide reacts fairly rapidly, forming ammonium nitrite. Two stages may be assumed:

2NO+2H₂=2H₂O+N₂; N₂+2H₂O=NH₄NO₂.

At ordinary temperature, pure hydrogen slowly reduces concentrated sulphuric acid to sulphur dioxide and water. Carbon disulphide forms a brown, insoluble solid of the formula 3CS₂,H₂. Acetylene produces a light yellow mass containing two substances: one is a thick liquid with the formula (C₂H₂,2C₂H₄)₂, is soluble in ether, and has a pleasant odour; the other is an insoluble solid, (2C₂H₂,C₂H₄)_x, of pungent odour and high molecular weight.