Hydrogen : Hydrogen From Water

The name hydrogen was given to the element by Lavoisier, from the Greek hudor, water. Water may be decomposed with the liberation of hydrogen in several ways.

By electrolysis both hydrogen and oxygen are produced, theoretically in the volume ratio of 2 to 1. In practice rather less oxygen is evolved, partly on account of the greater solubility of oxygen in water compared with hydrogen (1.8 to 1), and partly owing to oxidation of the sulphuric acid, added to the water "to make it conduct the current," to persulphuric acid, H₂S₂O₄, at the anode. Some hydrogen peroxide, H₂O₂, is formed at the cathode. The presence of these oxidising agents "in the liquid may be shown by adding absolution of potassium iodide and starch, when a blue colour, due to liberation of iodine, appears. The oxygen evolved may also contain a little ozone, O₃. If the liquid is electrolysed hot or phosphoric used instead of sulphuric acid, no ozone is formed and the volumes are nearly in the ratio 2:1.

Hydrogen is set free by the action of certain metals on water, Sodium and potassium react violently with cold water: 2Na + 2H₂O = 2NaOH + H₂

Expt. 1. - A small piece of potassium thrown on water floats, and the hydrogen takes fire and burns with a lilac flame, due to potassium vapour. A small fused globule of caustic potash (KOH) is left in the spheroidal condition; this is often projected from the water on cooling. The water contains caustic potash and turns red litmus blue. Sodium reacts in a similar way, but the hydrogen does not take fire unless the metal is kept in one place by throwing it on starch-jelly: the hydrogen then burns with a yellow flame, owing to the presence of sodium vapour.

The action of sodium amalgam on water is much less vigorous than that of sodium itself. The amalgam, if it contains more than i per cent, of sodium, is solid at the ordinary temperature.

Expt. 2. - Small pieces of clean sodium are pressed one by one under the surface of dry mercury in an iron mortar. Each piece dissolves with a flash of light, poisonous fumes of mercury vapour being evolved. The amalgam is placed in a porcelain crucible in a basin of water, under an inverted jar of water. Gradual evolution of hydrogen occurs, metallic mercury being left in the crucible.

H. B. Baker and L. H. Parker (1913) found that if the amalgam and water are very pure, the action is slow, bubbles of gas appearing only at isolated points on the surface of the amalgam. If ordinary distilled water is added, the evolution of gas is accelerated, apparently owing to the presence of hydrogen peroxide in the water.

Calcium decomposes cold water slowly:

Ca + 2H₂O = Ca(OH)₂ + H₂.

Boiling water is decomposed readily by ordinary magnesium powder, by aluminium powder, and by copper-zinc couple, prepared by the action of copper sulphate solution on zinc:

Zn + 2H₂O = Zn(OH)₂ + H₂.

Expt. 3. - Pour a solution of copper sulphate over about 25 gm. of zinc dust in a 250 c.c. flask. When a deposit of copper forms, pour off the solution and fill up the flask with previously boiled water. Fit a cork and delivery tube and heat. Hydrogen is evolved.

Steam is decomposed by sodium, and by heated magnesium, zinc, iron, cobalt, lead, tin, and nickel, but not by silver or copper.

Expt. 4. - Insert a piece of burning magnesium ribbon into a large flask in which water is boiling vigorously. The metal burns brightly in the steam, and the escaping hydrogen may be burnt at the mouth of the flask: Mg + H₂O = MgO + H₂.

In the decomposition of steam by red-hot iron, black oxide of iron, or ferroso-ferric oxide, Fe₃O₄, is formed, but the decomposition of the steam is incomplete. A state of chemical equilibrium is set up:

3Fe+4H₂O <=> Fe₃O₄ + 4H₂

The reaction is reversible, and if hydrogen is passed over heated oxide of iron, metallic iron and steam are formed. The same mixture of hydrogen and steam results at a given temperature whether steam is passed over heated iron, or hydrogen over heated oxide of iron, the proportion of hydrogen decreasing with rise of temperature:

Ratio H₂/H₂O by volume	20.9	5.6	2.78	2.00
Temperature	200°	444°	860°	918°

This process is used in the technical preparation of hydrogen. In the Lane process reduced iron, from spathic iron ore, is heated at 600°-850° in vertical iron retorts, and steam blown through. The iron oxide formed is reduced again with water-gas, a mixture of hydrogen and carbon monoxide, formed by passing steam over red-hot coke: C + H₂O = CO + H₂. The steaming and reduction processes alternate. The gas contains 98 per cent, of hydrogen and a little carbon monoxide. In the Bosch process water gas mixed with steam is passed over a heated catalyst consisting of ferric oxide with "promoters," such as chromium oxide (Cr₂O₃), which increase the activity of the catalyst. The carbon monoxide is decomposed by steam: CO + H₂O <=> CO₂ + H₂. The carbon dioxide is removed by washing with water at 25 atm. pressure, and the remaining trace of carbon monoxide by washing the gas, under 200 atm. pressure, with ammoniacal cuprous formate solution and with 25 per cent. caustic soda solution at 260°, in steel towers: CO + NaOH = HCOONa (sodium formate).

Special processes for the production of smaller quantities of hydrogen include the action of water on hydrolith, or calcium hydride:

CaH₂ + 2H₂O = Ca(OH)₂ + 2H₂.

Hydro genite is a mixture of 25 parts of silicon, 60 parts of caustic soda, and 20 parts of slaked lime: when ignited it burns, evolving 270-370 litres of hydrogen per kgm., and leaving sodium and calcium silicates.

In the silicol process, powdered silicon, or an alloy of silicon with iron, is treated with a strong solution of caustic soda:

Si + 2NaOH + H₂O = Na₂SiO₃ (sodium silicate) + 2H₂. Hydrogen is also produced in the electrolysis of caustic soda solution with iron or nickel electrodes, and as a by-product in alkali manufacture.

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