The Discovery and Properties of Hydrogen

Hydrogen is the lightest element of the Periodic Table. Hydrogen was first formed as the universe began to cool down after the Big Bang, and still represents 90% of the atoms present in the cosmos, the rest of it being mostly helium. By fusion reactions in the stars, hydrogen subsequently formed the heavier elements, and can thus be considered as their common ancestor. Hydrogen is the fuel of the stars. Every second, 600 million tonnes of hydrogen are converted into helium in our Sun alone by nuclear fusion, releasing enormous amounts of energy, providing also the light and heat which makes life on our Earth possible.

Hydrogen is also one of the most widespread and plentiful elements on Earth. Due to its high reactivity however, hydrogen combines with other elements. As our atmosphere contains 20% oxygen, molecular hydrogen (H2) is not present in it except in small amounts in the upper atmosphere. In nature, hydrogen is nearly always found combined with other elements. In every water molecule (H2O), covering 70% of the Earth's surface, two hydrogen atoms are attached to an oxygen atom. Hydrogen can also be found in hydrocarbons as well as in every living organism, plants or vegetation.

Figure 9.1 The Sun converts 600 million tonnes of hydrogen to helium every second (photo source: NOAA).

Table 9.1 Properties of hydrogen.

Figure 9.1 The Sun converts 600 million tonnes of hydrogen to helium every second (photo source: NOAA).

Table 9.1 Properties of hydrogen.

Chemical formula Molecular weight Appearance Melting point Boiling point Density at 0 °C

Density as a liquid at -253 °C Energy content

Octane number Autoignition temperature Flammability limits in air Explosive limits in air Ignition energy

2.0159

colorless and odorless gas

28 670 kcal kg-1

57.7 kcal mol-1

130+

0.005 milli calorie

Hydrogen as a distinct element was first identified, and some of its properties described, in 1766 by the English scientist Henry Cavendish who called it "inflammable air" (see Table 9.1). By applying a spark to hydrogen, water was produced. This later led the French chemist Antoine Lavoisier to name the gas hydrogen from the Greek "hydro" and "genes" meaning "water" and "born of". Shortly after the French revolution (during which Lavoisier literally lost his head on the guillotine), the first practical use for hydrogen was found in the military in the form of reconnaissance balloons filled with hydrogen gas able to fly high above enemy lines. The large quantities of hydrogen gas needed were initially produced by passing steam at high temperature over iron filings. The possibility to generate hydrogen and oxygen gases by water electrolysis was discovered in the early 1800s by two Englishmen, William Nicholson and Anthony Carlisle. William Grove, in 1839, found how to reverse the electrolysis process and generate electricity by combining hydrogen and oxygen in what would be later called a fuel cell.

As mentioned, unlike wood, coal, oil or natural gas, hydrogen is not found in its free form on Earth and thus cannot be collected for combustive energy production. A significant amount of energy must first be expended to produce hydrogen, which is bound to other elements such as in water or hydrocarbons, in order to be able to use it as a fuel. Hydrogen is thus not a primary energy source but only an energy carrier. Some of its physical characteristics, however, are not well-suited for this purpose, especially as a transportation fuel. Paradoxically there is presently great interest for it for such use. The lightness of hydrogen (indeed, it is the lightest of all elements) represents a handicap for its storage, transmission, and use in its gaseous form. The small H2 molecule can also diffuse through most materials and make for example steel brittle, especially at high pressure or/and temperature. Being a volatile gas, it can only be condensed to a liquid at a very low temperature of -253 °C, only 20 °C over absolute zero. Hydrogen can also ignite or explode in contact with air and should thus be handled with substantial care.

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