Hydrogen

V

Preparation and Uses

Pure hydrogen gas is rare, so chemists produce it in the laboratory and in chemical factories. They can produce it in a variety of ways. Producing extremely pure hydrogen gas involves a process called hydrolysis. In this process, a chemist passes an electrical current through water to break the water molecules up into hydrogen gas and oxygen gas:

2 H₂O + electrical energy → 2H₂ + O₂

This chemical equation shows that two water molecules (H₂O), with electricity, form two molecules of hydrogen gas (H₂) and one molecule of oxygen gas (O₂). Early chemists made hydrogen gas by reacting a metal with an acid. One example of such a reaction occurs between zinc (Zn) and hydrochloric acid (HCl). The chemical equation for this reaction is the following:

Zn + 2HCl → ZnCl₂ + H₂

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In the chemical industry, hydrogen forms in other reactions, such as in the production of chlorine (Cl₂) and sodium hydroxide (NaOH) from sodium chloride dissolved in water (NaCl in H₂O). In petroleum refineries, hydrogen forms as a by-product from hydrocarbon processing.

The chemical industry uses hydrogen gas in many industrial chemical processes. The most important of these processes uses hydrogen to make ammonia (NH₃); it is called the Haber process after German chemist Fritz Haber, who developed it in 1908. The industry can then use ammonia to make other important products, such as explosives and fertilizers. Industrial chemists also use hydrogen in large amounts to make compounds such as the fuel methane (CH₄) and the alcohol methanol (CH₃OH), which is used as antifreeze and to make other chemicals. The food industry hydrogenates (adds hydrogen to) liquid oils (see Hydrogenation). When hydrogen atoms are added to the molecules of liquid oils, the oils become solid fats, such as margarine or vegetable shortening (for example, Crisco). Metallurgists use hydrogen to separate pure metals from their oxides. For example, hydrogen bonds with and removes oxygen from copper oxide, leaving pure copper.

Physicists use liquid hydrogen, which is extremely cold, to study elementary particles and low-temperature effects. Elementary particles, the smallest building blocks of matter, form in nuclear reactions, but they are too small and move too quickly to be visible to scientists. Scientists can view them indirectly by looking at the evidence the particles leave behind. In a device called a bubble chamber, this evidence is a little ripple pattern, or a track, in liquid hydrogen. Laboratory scientists also use liquid hydrogen to cool objects to extremely low temperatures to study effects such as superconductivity, which is the ability of a material to conduct electricity with no resistance (no loss of energy). Substances usually only become superconducting at very low temperatures.

Hydrogen gas, because it is lighter than air, floats upward in the atmosphere. People once used it to lift zeppelins and other airships into the sky, allowing trans-Atlantic voyages by air. However, because the gas is so flammable, it contributed to many explosive accidents, including the Hindenberg explosion in 1937. Airships now use helium gas because it is nonflammable and therefore a safer lifting gas.

Industries can use hydrogen’s reaction with oxygen, the reverse reaction to hydrolysis, to create energy:

2H₂ + O₂ → 2H₂O + energy

People may someday use hydrogen as fuel for automobiles, refrigerators, and airplanes, if it becomes easier to distribute, store, and use. Automobile manufacturers are developing vehicles that are powered by hydrogen fuel cells, devices that use hydrogen to produce electricity. The aerospace industry, the industry that designs and builds airplanes and spacecraft, already uses liquid hydrogen as a fuel for rockets. Aerospace engineers are interested in using hydrogen fuel for airplanes because of its low density. Conventional hydrocarbon fuels add much weight to an aircraft. Low-weight, high-energy hydrogen would decrease the amount of fuel needed to lift the airplane at takeoff and increase the distance the airplane could fly without stopping. Hydrogen fuel could also cut pollution, since it mostly produces water when it burns. Spacecraft use hydrogen as a primary rocket fuel that reacts with fluorine or oxygen to produce energy.

Nuclear engineers and scientists use the hydrogen isotope deuterium and deuterium oxide (D₂O), which is also called heavy water, to help control nuclear power plants and to perform experiments. Deuterium is twice as heavy as the more common protium isotope of hydrogen, so its water compound is also heavier. Nuclear power plants based on natural uranium reactors use D₂O to slow the particles (neutrons) involved in the nuclear reaction, thus slowing the reaction itself (see Nuclear Energy: Light and Heavy Water Reactors). The more common protium oxide or H₂O (water) molecules absorb too many neutrons and allow the reaction to go too fast.

Processors obtain deuterium oxide by making use of the fact that deuterium oxide boils at a slightly higher temperature and is harder to separate by electrolysis than protium water. Scientists can boil off or use electrolysis to drive off the protium water in a sample of regular water. In either method, the liquid left behind gets heavier and heavier as the concentration of deuterium oxide rises.

Research scientists use deuterium to follow the movement of materials in chemical and biochemical research (see Isotopic Tracer). Chemical reactions that use deuterium are often much slower than reactions of protium are, so chemists can study these reactions in more detail. Deuterium and tritium are also used in nuclear weapons, because they combine into helium and release energy more readily than protium does.

VI

Discovery of Hydrogen

Early chemists confused hydrogen with other gases until British physicist and chemist Henry Cavendish described the properties of hydrogen gas in the mid-1700s. Many scientists before Cavendish had made the flammable gas by mixing metals with acids. Cavendish called the gas flammable air and studied it. He demonstrated in 1766 that sulfuric acid reacted with metals to produce flammable air. Later, Cavendish burned his flammable air in regular air to produce water, and only water. Many historians consider Cavendish to be the principle discoverer of hydrogen gas, although Scottish engineer James Watt reported that he had produced water at about the same time as Cavendish.

The isotopes deuterium and tritium were discovered in the 20th century. Shortly after World War I (1914-1918), British physicist Francis W. Aston invented a mass spectrograph (see Mass Spectrometer), a device that separates atoms by their masses. He found atoms with masses that were unusual, namely the isotopes. This provided the first clue to the existence of deuterium. In 1932 American chemist Harold C. Urey and his associates isolated and discovered deuterium. Urey predicted that water made with deuterium would evaporate more slowly than would water made with protium and was, in this way, able to separate and isolate the deuterium. Scientists first produced tritium in 1935 by bombarding deuterium with deuterium nuclei (one proton and one neutron). Scientists have since found tritium in very small amounts in ordinary water. Tritium forms naturally in some atmospheric reactions.

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