Hydrogen on top

• Hydrogen is by far the commonest element, making up 98% of the universe. 70% of the earth’s surface consists of hydrogen.
• Hydrogen is the simplest of all elements; it is colourless, odourless and tasteless and 14 times lighter than air. Hydrogen becomes liquid at –252.9° C. When burnt, hydrogen releases energy and reacts with oxygen in the air to form nothing but water.
• Hydrogen never occurs naturally in isolation; it is always combined with other elements. The commonest compounds are those with carbon, in the form of natural gas, and with oxygen in the form of water. To be able to use hydrogen, we have to isolate it.
• Most of the hydrogen used nowadays (48%) is produced by reforming natural gas. Only 4% of hydrogen is currently obtained by pure electrolysis – a method whereby water is broken down into its constituent elements of hydrogen and oxygen through the addition of electrical energy.
• Approximately 95% of the hydrogen currently being produced is used immediately by the consuming industry. Worldwide hydrogen production currently stands at 45 million tons per year.

Physical properties

• Being the lightest gas, hydrogen diffuses easily through metals such as iron, platinum or palladium. It escapes easily through porous partition walls. So although it is very light, hydrogen is not the ideal gas for filling balloons, for example.
• Hydrogen has significantly higher thermal conductivity than air. Light hydrogen molecules move much faster than heavy nitrogen and oxygen molecules.
• Hydrogen has low solubility in water and somewhat higher solubility in alcohol. Many metals, on the other hand, have a remarkably high solvent power in relation to hydrogen. Spongy palladium, for example, can absorb 850 times its own volume of hydrogen.

Chemical reactions

• The chief characteristic of hydrogen is its combustibility. It burns in air with a pale, hot flame to produce water. At room temperature, hydrogen and oxygen combine to form water so slowly that the reaction cannot be measured. A measurable reaction can only be obtained at high temperatures.
• Hydrogen reacts not only with free oxygen. It can also extract oxygen from oxygen compounds to form water.
• Hydrogen also combines with many other elements besides oxygen. With the non-metallic elements chlorine, sulphur, nitrogen and carbon, the volatile compounds HCl, H₂S, NH₃ and CH₄ are formed. The metals sodium, calcium, lanthanum and thorium form the salts (hydrides) NaH, CaH₂ , LaH₃ and ThH₄. Particularly worth mentioning is the synthesis of ammonia from hydrogen and nitrogen, on which the large-scale anorganic industry is based.
• Another important process is the addition of hydrogen to “unsaturated” carbon compounds, known as hydrogenation. The most important activities in this area are the hydrogenation of coal, mineral oil and tar to make petrol, the hydrogenation of carbon monoxide to make alcohols or hydrocarbons and the hydrogenation of oily greases to make solid greases.

Hydrogen generation

• During electrolysis, water is broken down into its constituent elements of hydrogen and oxygen through the addition of electrical energy.
• In chlorine-alkaline electrolysis, large quantities of hydrogen are produced through the generation of caustic soda or chlorine
• In steam reforming, hydrogen is produced through the partial oxidation of light hydrocarbons such as methane, propane or butane to form a mixture of H₂ and CO which is separated with the aid of a catalyst.
• In ammonia splitting, hydrogen is obtained through the thermal separation of ammonia into N₂ and H₂

Characteristics