Liquid

Hydrogen(lat. Hydrogenium; denoted by the symbol H) - the first element of the periodic table of elements. Widely distributed in nature. The cation (and nucleus) of the most abundant hydrogen isotope, 1 H, is the proton. The properties of the 1 H nucleus make it possible to widely use NMR spectroscopy in the analysis of organic substances.

Three isotopes of hydrogen have their own names: 1 H - protium (H), 2 H - deuterium (D) and 3 H - tritium (radioactive) (T).

Simple substance hydrogen - H 2 - light colorless gas. It is flammable and explosive when mixed with air or oxygen. Non-toxic. Let's dissolve in ethanol and a number of metals: iron, nickel, palladium, platinum.

History

The release of combustible gas during the interaction of acids and metals was observed in the 16th and 17th centuries at the dawn of the formation of chemistry as a science. Mikhail Vasilyevich Lomonosov also directly pointed to its separation, but already definitely realizing that it was not phlogiston. The English physicist and chemist Henry Cavendish investigated this gas in 1766 and called it "combustible air." When burned, the "combustible air" produced water, but Cavendish's adherence to the phlogiston theory prevented him from drawing the correct conclusions. The French chemist Antoine Lavoisier, together with the engineer J. Meunier, using special gas meters, in 1783 synthesized water, and then analyzed it, decomposing water vapor with hot iron. Thus, he established that "combustible air" is part of water and can be obtained from it.

origin of name

Lavoisier gave hydrogen the name hydrogène - "giving birth to water." The Russian name "hydrogen" was proposed by the chemist M.F.

Prevalence

Hydrogen is the most abundant element in the universe. It accounts for about 92% of all atoms (8% are helium atoms, the share of all other elements taken together is less than 0.1%). Thus, hydrogen is the main constituent of stars and interstellar gas. Under conditions of stellar temperatures (for example, the surface temperature of the Sun is ~ 6000 ° C), hydrogen exists in the form of plasma; in interstellar space, this element exists in the form of individual molecules, atoms and ions and can form molecular clouds that differ significantly in size, density and temperature.

Earth's crust and living organisms

The mass fraction of hydrogen in the earth's crust is 1% - this is the tenth most abundant element. However, its role in nature is determined not by mass, but by the number of atoms, the proportion of which among other elements is 17% (second place after oxygen, the proportion of atoms of which is ~ 52%). Therefore, the importance of hydrogen in the chemical processes taking place on Earth is almost as great as oxygen. Unlike oxygen, which exists on Earth in both bound and free states, practically all hydrogen on Earth is in the form of compounds; only a very small amount of hydrogen in the form of a simple substance is contained in the atmosphere (0.00005% by volume).

Hydrogen is a part of almost all organic substances and is present in all living cells. In living cells, hydrogen accounts for almost 50% of the number of atoms.

Receiving

Industrial methods of obtaining simple substances depend on the form in which the corresponding element is found in nature, that is, what can be the raw materials for its production. So, oxygen, which is available in a free state, is obtained by a physical method - by separation from liquid air. Almost all hydrogen is in the form of compounds, therefore, chemical methods are used to obtain it. In particular, decomposition reactions can be used. One of the methods for producing hydrogen is the reaction of water decomposition by electric current.

The main industrial method for producing hydrogen is the reaction of methane with water, which is part of natural gas. It is carried out at a high temperature (it is easy to make sure that no reaction occurs when methane is passed even through boiling water):

CH 4 + 2H 2 O = CO 2 + 4H 2 −165 kJ

In the laboratory, to obtain simple substances, they do not necessarily use natural raw materials, but select those starting materials from which it is easier to isolate the required substance. For example, in a laboratory, oxygen is not obtained from the air. The same applies to the production of hydrogen. One of the laboratory methods for producing hydrogen, which is sometimes used in industry, is the decomposition of water with an electric current.

Usually in the laboratory, hydrogen is produced by the interaction of zinc with hydrochloric acid.

In industry

1.Electrolysis of aqueous solutions of salts:

2NaCl + 2H 2 O → H 2 + 2NaOH + Cl 2

2.Passage of water vapor over red-hot coke at a temperature of about 1000 ° C:

H 2 O + C? H 2 + CO

3.From natural gas.

Steam conversion:

CH 4 + H 2 O? CO + 3H 2 (1000 ° C)

Catalytic oxidation with oxygen:

2CH 4 + O 2? 2CO + 4H 2

4. Cracking and reforming of hydrocarbons in the process of oil refining.

In the laboratory

1.The action of dilute acids on metals. To carry out such a reaction, zinc and dilute hydrochloric acid are most often used:

Zn + 2HCl → ZnCl 2 + H 2

2.Interaction of calcium with water:

Ca + 2H 2 O → Ca (OH) 2 + H 2

3.Hydrolysis of hydrides:

NaH + H 2 O → NaOH + H 2

4.The action of alkalis on zinc or aluminum:

2Al + 2NaOH + 6H 2 O → 2Na + 3H 2

Zn + 2KOH + 2H 2 O → K 2 + H 2

5.By electrolysis. During the electrolysis of aqueous solutions of alkalis or acids, hydrogen is evolved at the cathode, for example:

2H 3 O + + 2e - → H 2 + 2H 2 O

Physical properties

Hydrogen can exist in two forms (modifications) - in the form of ortho- and para-hydrogen. Orthohydrogen molecule o-H 2 (m.p. -259.10 ° C, bp. -252.56 ° C) nuclear spins are directed in the same way (parallel), p-H 2 (m.p. -259.32 ° C, bp. -252.89 ° C) - opposite to each other (antiparallel). Equilibrium mixture o-H 2 and p-H 2 at a given temperature is called equilibrium hydrogen e-H 2.

Hydrogen modifications can be separated by adsorption on active carbon at liquid nitrogen temperature. At very low temperatures, the equilibrium between orthohydrogen and parahydrogen is almost entirely shifted towards the latter. At 80 K, the ratio of forms is approximately 1: 1. Desorbed parahydrogen on heating is converted into orthohydrogen until a mixture equilibrium at room temperature is formed (ortho-pair: 75:25). Without a catalyst, the transformation occurs slowly (under conditions of the interstellar medium - with characteristic times up to cosmological), which makes it possible to study the properties of individual modifications.

Hydrogen is the lightest gas; it is 14.5 times lighter than air. Obviously, the smaller the mass of the molecules, the higher their speed at the same temperature. As the lightest, hydrogen molecules move faster than molecules of any other gas and thus can transfer heat faster from one body to another. It follows that hydrogen has the highest thermal conductivity among gaseous substances. Its thermal conductivity is about seven times higher than the thermal conductivity of air.

The hydrogen molecule is diatomic - Н 2. Under normal conditions, it is a colorless, odorless and tasteless gas. Density 0.08987 g / l (n.u.), boiling point −252.76 ° C, specific heat of combustion 120.9 × 10 6 J / kg, slightly soluble in water - 18.8 ml / l. Hydrogen is readily soluble in many metals (Ni, Pt, Pd, etc.), especially in palladium (850 volumes per 1 volume of Pd). The solubility of hydrogen in metals is associated with its ability to diffuse through them; diffusion through a carbonaceous alloy (eg steel) is sometimes accompanied by the destruction of the alloy due to the interaction of hydrogen with carbon (so-called decarbonization). Practically insoluble in silver.

Liquid hydrogen exists in a very narrow temperature range from -252.76 to -259.2 ° C. It is a colorless liquid, very light (density at -253 ° C 0.0708 g / cm 3) and fluid (viscosity at -253 ° C 13.8 cpoise). The critical parameters of hydrogen are very low: the temperature is −240.2 ° C and the pressure is 12.8 atm. This explains the difficulties in liquefying hydrogen. In the liquid state, equilibrium hydrogen consists of 99.79% para-H 2, 0.21% ortho-H 2.

Solid hydrogen, melting point −259.2 ° C, density 0.0807 g / cm 3 (at −262 ° C) - snow-like mass, crystals of hexagonal system, space group P6 / mmc, cell parameters a=3,75 c= 6.12. At high pressure, hydrogen transforms into a metallic state.

Isotopes

Hydrogen occurs in the form of three isotopes, which have individual names: 1 H - protium (H), 2 H - deuterium (D), 3 H - tritium (radioactive) (T).

Protium and deuterium are stable isotopes with mass numbers 1 and 2. Their content in nature is, respectively, 99.9885 ± 0.0070% and 0.0115 ± 0.0070%. This ratio may vary slightly depending on the source and method of producing hydrogen.

The hydrogen isotope 3 H (tritium) is unstable. Its half-life is 12.32 years. Tritium is found in nature in very small quantities.

The literature also contains data on hydrogen isotopes with mass numbers 4–7 and half-lives of 10–22–10–23 s.

Natural hydrogen consists of H 2 and HD (hydrogen deuteride) molecules in a ratio of 3200: 1. The content of pure deuterium hydrogen D 2 is even less. The ratio of the concentrations of HD and D 2 is approximately 6400: 1.

Of all the isotopes of chemical elements, the physical and chemical properties of hydrogen isotopes differ from each other the most. This is due to the largest relative change in atomic masses.

Deuterium and tritium also have ortho and para modifications: p-D 2, o-D 2, p-T 2, o-T 2. Heteroisotopic hydrogen (HD, HT, DT) have no ortho and para modifications.

Chemical properties

Fraction of dissociated hydrogen molecules

The hydrogen molecules H 2 are quite strong, and a lot of energy must be expended in order for hydrogen to react:

H 2 = 2H - 432 kJ

Therefore, at ordinary temperatures, hydrogen reacts only with very active metals, for example with calcium, forming calcium hydride:

Ca + H 2 = CaH 2

and with the only non-metal - fluorine, forming hydrogen fluoride:

With most metals and non-metals, hydrogen reacts at elevated temperatures or under other influences, for example, under lighting:

О 2 + 2Н 2 = 2Н 2 О

It can "take" oxygen from some oxides, for example:

CuO + H 2 = Cu + H 2 O

The written equation reflects the reducing properties of hydrogen.

N 2 + 3H 2 → 2NH 3

Forms hydrogen halides with halogens:

F 2 + H 2 → 2HF, the reaction proceeds with an explosion in the dark and at any temperature,

Cl 2 + H 2 → 2HCl, the reaction proceeds with an explosion, only in the light.

Reacts with soot under strong heating:

C + 2H 2 → CH 4

Interaction with alkali and alkaline earth metals

When interacting with active metals, hydrogen forms hydrides:

2Na + H 2 → 2NaH

Ca + H 2 → CaH 2

Mg + H 2 → MgH 2

Hydrides- salty, solid substances, easily hydrolyzed:

CaH 2 + 2H 2 O → Ca (OH) 2 + 2H 2

Interaction with metal oxides (usually d-elements)

Oxides are reduced to metals:

CuO + H 2 → Cu + H 2 O

Fe 2 O 3 + 3H 2 → 2Fe + 3H 2 O

WO 3 + 3H 2 → W + 3H 2 O

Hydrogenation of organic compounds

Molecular hydrogen is widely used in organic synthesis for the reduction of organic compounds. These processes are called hydrogenation reactions... These reactions are carried out in the presence of a catalyst at elevated pressure and temperature. The catalyst can be either homogeneous (e.g. Wilkinson's catalyst) or heterogeneous (e.g. Raney nickel, palladium-carbon).

So, in particular, during the catalytic hydrogenation of unsaturated compounds such as alkenes and alkynes, saturated compounds are formed - alkanes.

Hydrogen Geochemistry

Free hydrogen H 2 is relatively rare in terrestrial gases, but in the form of water it plays an extremely important role in geochemical processes.

Hydrogen can be part of minerals in the form of ammonium ion, hydroxyl ion and crystal water.

In the atmosphere, hydrogen is continuously produced by the decomposition of water by solar radiation. Having a small mass, hydrogen molecules have a high speed of diffusion movement (it is close to the second cosmic speed) and, falling into the upper layers of the atmosphere, can fly into space.

Features of treatment

When mixed with air, hydrogen forms an explosive mixture - the so-called explosive gas. This gas is most explosive when the volumetric ratio of hydrogen and oxygen is 2: 1, or hydrogen and air is approximately 2: 5, since the air contains about 21% oxygen. Also hydrogen is fire hazardous. Liquid hydrogen can cause severe frostbite if it comes into contact with the skin.

Explosive concentrations of hydrogen with oxygen arise from 4% to 96% by volume. When mixed with air from 4% to 75 (74)% by volume.

Economy

The cost of hydrogen for large-scale wholesale supplies fluctuates in the range of $ 2-5 per kg.

Application

Atomic hydrogen is used for atomic hydrogen welding.

Chemical industry

• In the production of ammonia, methanol, soap and plastics
• In the production of margarine from liquid vegetable oils
• Registered as a food additive E949(packing gas)

Food industry

Aviation industry

Hydrogen is very light and always rises up in the air. Once airships and balloons were filled with hydrogen. But in the 30s. XX century there were several disasters, during which the airships exploded and burned. Nowadays, airships are filled with helium, despite its significantly higher cost.

Fuel

Hydrogen is used as propellant.

Research is underway on the use of hydrogen as a fuel for cars and trucks. Hydrogen engines do not pollute the environment and only emit water vapor.

Hydrogen-oxygen fuel cells use hydrogen to directly convert energy from a chemical reaction into electrical energy.

"Liquid hydrogen"("LH") is a liquid aggregate state of hydrogen, with a low specific gravity of 0.07 g / cm³ and cryogenic properties with a freezing point of 14.01 K (−259.14 ° C) and a boiling point of 20.28 K (−252.87 ° C). It is a colorless, odorless liquid which, when mixed with air, is classified as explosive with a flammability range of 4-75%. The spin ratio of isomers in liquid hydrogen is: 99.79% - parahydrogen; 0.21% - orthohydrogen. The expansion coefficient of hydrogen when changing the state of aggregation to gaseous is 848: 1 at 20 ° C.

As with any gas, the liquefaction of hydrogen leads to a decrease in its volume. After liquefaction, "LH" is stored in thermally insulated containers under pressure. Liquid hydrogen (rus. Liquid hydrogen, LH2, LH 2) is actively used in industry, as a form of gas storage, and in the space industry, as a rocket fuel.

History

The first documented use of artificial cooling in 1756 was carried out by the English scientist William Cullen, Gaspard Monge was the first to obtain the liquid state of sulfur oxide in 1784, Michael Faraday was the first to obtain liquefied ammonia, the American inventor Oliver Evans was the first to develop a refrigeration compressor in 1805, Jacob Perkins was the first to patent a cooling machine in 1834, and John Gorey was the first to patent an air conditioner in the United States in 1851. Werner Siemens proposed the concept of regenerative cooling in 1857, Karl Linde patented equipment for producing liquid air using the cascade Joule-Thomson expansion effect and regenerative cooling in 1876. In 1885, the Polish physicist and chemist Sigmund Wrobblewski published a critical hydrogen temperature of 33 K, a critical pressure of 13.3 atm. and a boiling point at 23 K. Hydrogen was first liquefied by James Dewar in 1898 using regenerative refrigeration and his invention, the Dewar vessel. The first synthesis of the stable isomer of liquid hydrogen - parahydrogen - was carried out by Paul Hartek and Karl Bonhoeffer in 1929.

Spin isomers of hydrogen

Hydrogen at room temperature consists mainly of the spin isomer, orthohydrogen. After production, liquid hydrogen is in a metastable state and must be converted to a parahydrogenic form in order to avoid the explosive exothermic reaction that occurs when it changes at low temperatures. Conversion to the parahydrogen phase is usually performed using catalysts such as iron oxide, chromium oxide, activated carbon, platinum-coated asbestos, rare earth metals, or by using uranium or nickel additives.

Usage

Liquid hydrogen can be used as a form of fuel storage for internal combustion engines and fuel cells. Various submarines (projects 212A and 214, Germany) and hydrogen transport concepts have been created using this aggregate form of hydrogen (see for example "DeepC" or "BMW H2R"). Due to the proximity of the structures, the creators of the equipment on "ZhV" can use or only modify systems using liquefied natural gas ("LNG"). However, due to the lower bulk energy density, combustion requires a higher volume of hydrogen than natural gas. If liquid hydrogen is used instead of "CNG" in reciprocating engines, a more cumbersome fuel system is usually required. With direct injection, the increased intake losses reduce the cylinder filling.

Liquid hydrogen is also used to cool neutrons in neutron scattering experiments. The masses of the neutron and the hydrogen nucleus are practically equal; therefore, the exchange of energy in an elastic collision is most effective.

Advantages

The advantage of using hydrogen is the "zero emission" of its use. The product of its interaction with air is water.

Obstacles

One liter of "ZhV" weighs only 0.07 kg. That is, its specific gravity is 70.99 g / l at 20 K. Liquid hydrogen requires cryogenic storage technology, such as special thermally insulated containers, and requires special handling, which is typical for all cryogenic materials. It is close in this respect to liquid oxygen, but requires more caution due to the fire hazard. Even with thermally insulated containers, it is difficult to keep it at the low temperature required to keep it liquid (it usually evaporates at a rate of 1% per day). When handling it, you must also follow the usual safety precautions when working with hydrogen - it is cold enough to liquefy air, which is explosive.

Rocket fuel

Liquid hydrogen is a common component of rocket fuels that is used for jet acceleration of launch vehicles and spacecraft. In most hydrogen-fueled liquid propellant rocket engines, it is first used to regeneratively cool the nozzle and other parts of the engine, before it is mixed with an oxidizer and burned to produce thrust. Used modern H 2 / O 2 engines consume a re-enriched fuel mixture, which results in some unburned hydrogen in the exhaust. In addition to increasing the specific impulse of the engine by reducing the molecular weight, it further reduces the erosion of the nozzle and combustion chamber.

Such obstacles to the use of "LH" in other areas, such as cryogenic nature and low density, are also a limiting factor for use in this case. For 2009, there is only one launch vehicle (LV "Delta-4"), which is entirely a hydrogen rocket. Basically, "ZhV" is used either on the upper stages of rockets, or on blocks, which perform a significant part of the work on putting the payload into space in a vacuum. As one of the measures to increase the density of this type of fuel, there are proposals to use slushy hydrogen, that is, the semi-frozen form of "ZhV".

Precision comes first

The relative mass of the light hydrogen isotope was determined with fantastic accuracy: 1.007276470 (if we take the mass of the carbon 12C isotope equal to 12.0000000). If, for example, the length of the equator were measured with such accuracy, the error would not exceed 4 cm!

But why is such precision needed? After all, each new figure requires more and more efforts from experimenters ... The secret is revealed simply: protium nuclei, protons, take part in many nuclear reactions. And if the masses of the reacting nuclei and the masses of the reaction products are known, then using the formula, you can calculate its energy effect. And since the energy effects of even nuclear reactions are accompanied by only a slight change in mass, then these masses have to be measured as accurately as possible.

Isotope effects

For many years, deuterium and more recently tritium have been widely used as tagged atoms differences in mass or radioactivity allow them to be detected and separated, and chemically they are similar to ordinary hydrogen atoms. For most elements, a change in the mass of the nucleus by one or several units leads to a very small percentage change in the atomic weight, which determines only an indirect effect of the mass of the nucleus on the chemical behavior of matter; in general, a chemical difference between isotopes cannot be detected. However, for the lightest elements B, C, N and especially H, reactions with substances containing different isotopes of the same element proceed at low but measurably different rates. This often gives good results when studying the reaction mechanisms in detail. The replacement of deuterium with hydrogen in biological systems can significantly change the delicate equilibrium processes. In the case of deuterium, this difference is not large enough to diminish its value as a tagged atom, although care must be taken in interpreting the data. Tritium, however, is so much heavier than hydrogen that any given tritium compound cannot be expected to react in the same way as its hydrogen counterpart. However, it can still be assumed that even for hydrogen, the chemistry of all isotopes is the same.

The lightest isotope of hydrogen

Protium is the name of the lightest isotope of hydrogen, denoted by the symbol. The protium nucleus consists of one proton, hence the name of the isotope.

Table 5.1.

Protium makes up 99.9885 ± 0.0070% of the total number of hydrogen atoms in the Universe and is the most abundant nuclide in nature among the isotopes of all chemical elements. It is not a metal. Under normal conditions, it always arrives in a gaseous state, without color, taste or odor, but if desired, hydrogen can be brought to a liquefied or solidified state, but this requires an incredibly low temperature and very high pressure.

Hydrogen has three isotopes with mass numbers 1, 2, and 3.

The most common isotope of hydrogen is the usual, familiar to us hydrogen " 1 H"With a nucleus consisting of one single proton. There are no neutrons in this nucleus at all. By default, when we say "hydrogen", they mean just such an isotope, but when we talk about different isotopes of hydrogen, then the term "hydrogen" will be incomprehensible - whether we mean this isotope without neutrons, or any isotope of hydrogen ... Therefore, such an isotope has its own name: “ protium».

Another isotope that occurs in nature is “ deuterium» - « 2 H". The deuterium nucleus consists of one proton and one neutron. The deuterium content in nature is very small - about 0.01% of all hydrogen atoms. Deuterium is also denoted for brevity by the letter " D»

The third isotope - "tritium" - " 3 H". For brevity, it is also referred to as “ T»

Hydrogen occurs naturally in the form of molecules H 2 and HD in a ratio of 3200: 1.

If we take different chemical elements and see how strongly the physical properties of their isotopes differ, we will see that hydrogen isotopes differ from each other the most. This can be easily explained, because there is only one proton in the hydrogen nucleus, and the addition of a neutron to one proton increases the mass of the nucleus by as much as 100%! That is, the mass of the nucleus changes very strongly, respectively, and the physical properties also change greatly.

Hydrogen is a chemical element with the symbol H and atomic number 1. With a standard atomic weight of about 1.008, hydrogen is the lightest element on the periodic table. Its monatomic form (H) is the most abundant chemical in the universe, accounting for approximately 75% of the baryon's total mass. Stars are mostly made of hydrogen in a plasma state. The most common isotope of hydrogen, called protium (this name is rarely used, the symbol 1H), has one proton and no neutrons. The ubiquitous appearance of atomic hydrogen first occurred in the era of recombination. At standard temperatures and pressures, hydrogen is a colorless, odorless, tasteless, non-toxic, non-metallic, flammable diatomic gas with the molecular formula H2. Because hydrogen readily forms covalent bonds with most non-metallic elements, most of the hydrogen on Earth exists in molecular forms such as water or organic compounds. Hydrogen plays a particularly important role in acid-base reactions because most acid-based reactions involve the exchange of protons between soluble molecules. In ionic compounds, hydrogen can take the form of a negative charge (i.e., anion), in which it is known as a hydride, or as a positively charged (i.e., cation) species denoted by the symbol H +. The hydrogen cation is described as consisting of a simple proton, but in reality, the hydrogen cations in ionic compounds are always more complex. Being the only neutral atom for which the Schrödinger equation can be solved analytically, hydrogen (namely, the study of energy and the bonding of its atom) played a key role in the development of quantum mechanics. Hydrogen gas was first produced artificially in the early 16th century by the reaction of acids with metals. In 1766-81. Henry Cavendish was the first to recognize that hydrogen gas is a discrete substance and that it produces water when it is burned, which is why it was named so: in Greek, hydrogen means "water producer." Industrial hydrogen production is mainly associated with the steam conversion of natural gas and, less commonly, more energy intensive methods such as water electrolysis. Most hydrogen is used close to where it is produced, with the two most common uses being fossil fuel processing (eg hydrocracking) and ammonia production, mainly for the fertilizer market. Hydrogen is a concern in metallurgy because it can brittle many metals, making it difficult to design pipelines and storage tanks.

Properties

Combustion

Hydrogen gas (dihydrogen or molecular hydrogen) is a flammable gas that will burn in air over a very wide concentration range from 4% to 75% by volume. The enthalpy of combustion is 286 kJ / mol:

2 H2 (g) + O2 (g) → 2 H2O (l) + 572 kJ (286 kJ / mol)

Hydrogen gas forms explosive mixtures with air in concentrations from 4-74% and with chlorine in concentrations up to 5.95%. Explosive reactions can be caused by sparks, heat or sunlight. The autoignition temperature of hydrogen, the spontaneous ignition temperature in air, is 500 ° C (932 ° F). Pure hydrogen-oxygen flames emit ultraviolet radiation and with a high oxygen mixture are almost invisible to the naked eye, as evidenced by the faint plume of the space shuttle's main engine compared to the highly visible plume of the space shuttle solid rocket amplifier that uses an ammonium perchlorate composite. A flame detector may be required to detect burning hydrogen leaks; such leaks can be very dangerous. The hydrogen flame is blue under other conditions, and resembles the blue flame of natural gas. The sinking of the Hindenburg airship is a notorious example of the burning of hydrogen, and the case is still under debate. The visible orange flame in this incident was caused by exposure to a mixture of hydrogen and oxygen combined with carbon compounds from the airship's skin. H2 reacts with every oxidizing element. Hydrogen can react spontaneously at room temperature with chlorine and fluorine to form the corresponding hydrogen halides, hydrogen chloride and hydrogen fluoride, which are also potentially hazardous acids.

Electron energy levels

The energy level of the ground state of an electron in a hydrogen atom is -13.6 eV, which is equivalent to an ultraviolet photon with a wavelength of about 91 nm. The energy levels of hydrogen can be calculated fairly accurately using Bohr's model of the atom, which conceptualizes the electron as an "orbiting" proton, similar to the Earth's orbit of the Sun. However, an atomic electron and a proton are held together by electromagnetic force, while planets and celestial objects are held together by gravity. Due to the discretization of angular momentum postulated in early quantum mechanics by Bohr, an electron in Bohr's model can only occupy certain allowable distances from the proton and therefore only certain allowable energies. A more accurate description of the hydrogen atom comes from purely quantum mechanical processing that uses the Schrödinger equation, Dirac's equation, or even the Feynman integrated circuit to calculate the probability density of an electron around a proton. The most sophisticated processing methods allow obtaining small effects of the special theory of relativity and vacuum polarization. In quantum machining, an electron in a ground state hydrogen atom has no torque at all, which illustrates how a "planetary orbit" differs from the motion of an electron.

Elementary molecular forms

There are two different spin isomers of diatomic hydrogen molecules, which differ in the relative spin of their nuclei. In orthohydrogen form, the spins of the two protons are parallel and form a triplet state with a molecular spin quantum number of 1 (1/2 + 1/2); in the form of parahydrogen, the spins are antiparallel and form a singlet with the molecular spin quantum number 0 (1/2 1/2). At standard temperature and pressure, hydrogen gas contains about 25% para-form and 75% ortho-form, also known as "normal form". The equilibrium ratio of orthohydrogen to parahydrogen depends on temperature, but since the ortho-form is an excited state and has a higher energy than the para-form, it is unstable and cannot be purified. At very low temperatures, the state of equilibrium consists almost exclusively of the para-form. The thermal properties of the liquid and gas phases of pure parahydrogen differ significantly from the properties of the normal form due to differences in the rotational heat capacities, which is discussed in more detail in the spin isomers of hydrogen. The ortho / pair difference also occurs in other hydrogen-containing molecules or functional groups such as water and methylene, but this is of little consequence to their thermal properties. The uncatalyzed interconversion between vapor and ortho H2 increases with increasing temperature; thus, the rapidly condensed H2 contains large amounts of the high energy orthogonal form, which is very slowly converted to the para form. The ortho / vapor ratio in condensed H2 is an important factor in the preparation and storage of liquid hydrogen: the conversion from ortho to vapor is exothermic and provides enough heat to vaporize some of the hydrogen liquid, resulting in the loss of liquefied material. Ortho-para conversion catalysts such as iron oxide, activated carbon, platinized asbestos, rare earth metals, uranium compounds, chromium oxide or some nickel compounds are used for cooling with hydrogen.

Phases

Hydrogen gas

Liquid hydrogen

Slime hydrogen

Solid hydrogen

Metallic hydrogen

Connections

Covalent and organic compounds

While H2 is not very reactive under standard conditions, it forms compounds with most elements. Hydrogen can form compounds with elements that are more electronegative, such as halogens (eg F, Cl, Br, I) or oxygen; in these compounds, hydrogen takes on a partial positive charge. When bonded with fluorine, oxygen, or nitrogen, hydrogen can participate in the form of a medium-strength non-covalent bond with other similar molecules, a phenomenon called hydrogen bonding that is critical to the stability of many biological molecules. Hydrogen also forms compounds with less electronegative elements such as metals and metalloids, where it takes on a partial negative charge. These compounds are often known as hydrides. Hydrogen forms a vast array of compounds with carbon, called hydrocarbons, and an even greater variety of compounds with heteroatoms, which, because of their common bond with living things, are called organic compounds. Their properties are studied in organic chemistry, and their study in the context of living organisms is known as biochemistry. According to some definitions, "organic" compounds must contain only carbon. However, most of them also contain hydrogen, and because it is the carbon-hydrogen bond that gives this class of compounds most of their specific chemical characteristics, carbon-hydrogen bonds are required in some definitions of the word "organic" in chemistry. Millions of hydrocarbons are known and are usually formed by complex synthetic pathways that rarely involve elemental hydrogen.

Hydrides

Hydrogen compounds are often referred to as hydrides. The term "hydride" implies that the H atom has acquired a negative or anionic character, designated H-, and is used when hydrogen forms a compound with a more electropositive element. The existence of the hydride anion, proposed by Gilbert N. Lewis in 1916 for salt-containing hydrides of groups 1 and 2, was demonstrated by Moers in 1920 by electrolysis of molten lithium hydride (LiH), producing a stoichiometric amount of hydrogen per anode. For hydrides other than Group 1 and 2 metals, this term is misleading given the low electronegativity of hydrogen. An exception in Group 2 hydrides is BeH2, which is polymeric. In lithium aluminum hydride, the AlH-4 anion carries hydride centers firmly attached to Al (III). Although hydrides can be formed in almost all elements of the basic group, the number and combination of possible compounds vary greatly; for example, more than 100 binary borane hydrides and only one binary aluminum hydride are known. Binary indium hydride has not yet been identified, although large complexes exist. In inorganic chemistry, hydrides can also serve as bridging ligands that bind two metal centers in a coordination complex. This function is especially characteristic for elements of group 13, especially in boranes (boron hydrides) and aluminum complexes, as well as in clustered carboranes.

Protons and acids

Oxidation of hydrogen removes its electron and gives H +, which does not contain electrons and a nucleus, which usually consists of one proton. This is why H + is often referred to as a proton. This view is central to the discussion of acids. According to the Bronsted-Lowry theory, acids are proton donors, and bases are proton acceptors. A naked proton, H +, cannot exist in solution or in ionic crystals due to its irresistible attraction to other atoms or molecules with electrons. Except for the high temperatures associated with plasma, such protons cannot be removed from the electron clouds of atoms and molecules and will remain attached to them. However, the term "proton" is sometimes used metaphorically to refer to positively charged or cationic hydrogen attached to other species in this way, and as such, is referred to as “H +” without any meaning that any individual protons exist freely as a species. To avoid the appearance of a naked "solvated proton" in solution, acidic aqueous solutions are sometimes thought to contain a less unlikely fictitious species called "hydronium ion" (H 3 O +). However, even in this case, such solvated hydrogen cations are more realistically perceived as organized clusters that form species close to H 9O + 4. Other oxonium ions are found when water is in acidic solution with other solvents. Although exotic on Earth, one of the most abundant ions in the Universe is H + 3, known as protonated molecular hydrogen or trihydrogen cation.

Isotopes

Hydrogen has three naturally occurring isotopes, designated 1H, 2H, and 3H. Other highly unstable nuclei (from 4H to 7H) have been synthesized in the laboratory, but have not been observed in nature. 1H is the most abundant hydrogen isotope with a prevalence of over 99.98%. Since the nucleus of this isotope consists of only one proton, it is given a descriptive but rarely used formal name protium. 2H, another stable isotope of hydrogen, is known as deuterium and contains one proton and one neutron in its nucleus. It is believed that all the deuterium in the universe was produced during the Big Bang and has existed since that time. Deuterium is not a radioactive element and does not pose a significant toxicity hazard. Water enriched with molecules that include deuterium instead of normal hydrogen is called heavy water. Deuterium and its compounds are used as a non-radioactive label in chemical experiments and in solvents for 1H-NMR spectroscopy. Heavy water is used as a neutron moderator and as a coolant for nuclear reactors. Deuterium is also a potential fuel for commercial nuclear fusion. 3H is known as tritium and contains one proton and two neutrons in its nucleus. It is radioactive, decays into helium-3 through beta decay with a half-life of 12.32 years. It is so radioactive that it can be used in luminous paint, which makes it useful in making watches with a luminous dial, for example. The glass prevents a small amount of radiation from escaping. A small amount of tritium is formed naturally by the interaction of cosmic rays with atmospheric gases; tritium was also released during nuclear weapons tests. It is used in nuclear fusion reactions as an indicator of isotope geochemistry and in specialized self-powered lighting devices. Tritium has also been used in chemical and biological labeling experiments as a radioactive label. Hydrogen is the only element that has different names for its isotopes, which are widely used today. During the early study of radioactivity, various heavy radioactive isotopes were given their own names, but such names are no longer used, with the exception of deuterium and tritium. The symbols D and T (instead of 2H and 3H) are sometimes used for deuterium and tritium, but the corresponding symbol for protium P is already used for phosphorus and is therefore not available for protium. In its nomenclature guidelines, the International Union of Pure and Applied Chemistry allows the use of any characters from D, T, 2H and 3H, although 2H and 3H are preferred. The exotic muonium atom (symbol Mu), made up of an anti-muon and an electron, is also sometimes regarded as a light radioisotope of hydrogen due to the mass difference between the anti-muon and an electron, which was discovered in 1960. During the lifetime of a muon, 2.2 μs, muonium can enter into compounds such as muonium chloride (MuCl) or sodium muonide (NaMu), similarly to hydrogen chloride and sodium hydride, respectively.

History

Discovery and use

In 1671, Robert Boyle discovered and described the reaction between iron filings and dilute acids, which leads to the production of hydrogen gas. In 1766, Henry Cavendish was the first to recognize hydrogen gas as a discrete substance, calling this gas "flammable air" due to its metal-acid reaction. He suggested that "flammable air" was virtually identical to a hypothetical substance called "phlogiston" and again discovered in 1781 that gas produces water when it is burned. It is believed that it was he who discovered hydrogen as an element. In 1783, Antoine Lavoisier named this element hydrogen (from the Greek ὑδρο-hydro meaning water and -γενής genes, meaning creator), when he and Laplace reproduced Cavendish's data that burning hydrogen produces water. Lavoisier produced hydrogen for his mass conservation experiments by reacting a stream of steam with metallic iron through an incandescent lamp heated in a fire. Anaerobic oxidation of iron by protons of water at high temperatures can be schematically represented by a set of the following reactions:

Fe + H2O → FeO + H2

2 Fe + 3 H2O → Fe2O3 + 3 H2

3 Fe + 4 H2O → Fe3O4 + 4 H2

Many metals, such as zirconium, undergo a similar reaction with water to produce hydrogen. The hydrogen was first liquefied by James Dewar in 1898 using regenerative refrigeration and his invention, the vacuum flask. The following year, he produced solid hydrogen. Deuterium was discovered in December 1931 by Harold Urey, and tritium was prepared in 1934 by Ernest Rutherford, Mark Oliphant, and Paul Harteck. Heavy water, which consists of deuterium instead of ordinary hydrogen, was discovered by Yurey's group in 1932. François Isaac de Rivaz built the first Rivaz engine, an internal combustion engine powered by hydrogen and oxygen, in 1806. Edward Daniel Clarke invented the hydrogen gas tube in 1819. The Doebereiner Flame (the first full-fledged lighter) was invented in 1823. The first hydrogen cylinder was invented by Jacques Charles in 1783. Hydrogen provided the rise of the first reliable form of air traffic after the invention of Henri Giffard's first hydrogen-powered airship in 1852. The German Count Ferdinand von Zeppelin promoted the idea of ​​rigid airships lifted into the air by hydrogen, which were later called the Zeppelin; the first of these took off for the first time in 1900. Regularly scheduled flights began in 1910 and by the outbreak of World War I in August 1914, they had carried 35,000 passengers without major incident. During the war, hydrogen airships were used as observation platforms and bombers. The first non-stop transatlantic flight was made by the British R34 airship in 1919. Regular passenger service resumed in the 1920s and the discovery of stocks of helium in the United States was supposed to improve flight safety, but the US government refused to sell gas for this purpose, so H2 was used in the Hindenburg airship, which was destroyed in a fire in Milan in New Jersey May 6, 1937 The incident was broadcast live on radio and filmed. It has been widely speculated that the ignition was due to a hydrogen leak, but subsequent research indicates that the aluminized fabric covering could ignite with static electricity. But by this time, hydrogen's reputation as a lift gas had already been damaged. In the same year, the first hydrogen-cooled turbine generator with hydrogen gas as a refrigerant in the rotor and stator entered service in 1937 in Dayton, Ohio, by Dayton Power & Light Co; Due to the thermal conductivity of hydrogen gas, it is the most common gas for use in this field today. The nickel-hydrogen battery was first used in 1977 aboard the US Navigation Technology Satellite 2 (NTS-2). The ISS, Mars Odyssey and Mars Global Surveyor are all powered by nickel-hydrogen batteries. In the dark part of its orbit, the Hubble Space Telescope is also powered by nickel-hydrogen batteries that were finally replaced in May 2009, more than 19 years after launch and 13 years after their design.

Role in quantum theory

Because of its simple atomic structure, consisting only of a proton and an electron, the hydrogen atom, together with the spectrum of light created from or absorbed by it, has been central to the development of the theory of atomic structure. In addition, the study of the corresponding simplicity of the hydrogen molecule and the corresponding H + 2 cation led to an understanding of the nature of the chemical bond, which soon followed the physical treatment of the hydrogen atom in quantum mechanics in mid-2020.One of the first quantum effects that were clearly observed (but were not understood at the time), there was Maxwell's observation of hydrogen half a century before the full quantum mechanical theory emerged. Maxwell noted that the specific heat of H2 irreversibly departs from a diatomic gas below room temperature and begins to increasingly resemble the specific heat of a monatomic gas at cryogenic temperatures. According to quantum theory, this behavior arises from the distance of the (quantized) levels of rotational energy, which are especially widely spaced in H2 due to its low mass. These widely spaced levels prevent an equal division of thermal energy into rotational motion in hydrogen at low temperatures. Diatom gases, which are made up of heavier atoms, do not have such widely spaced levels and do not exhibit the same effect. Antihydrogen is an antimaterial analogue of hydrogen. It consists of an antiproton with a positron. Antihydrogen is the only type of antimatter atom that has been produced as of 2015.

Being in nature

Hydrogen is the most abundant chemical element in the Universe, accounting for 75% of normal matter by mass and over 90% by the number of atoms. (Most of the mass in the universe, however, is not in the form of this chemical element, but is believed to have as yet undiscovered forms of mass, such as dark matter and dark energy.) This element is found in great abundance in stars and gas giants. Molecular H2 clouds are associated with star formation. Hydrogen plays a vital role in turning stars on through the proton-proton reaction and nuclear fusion of the CNO cycle. All over the world, hydrogen is found mainly in atomic and plasma states with properties quite different from those of molecular hydrogen. As a plasma, the electron and proton of hydrogen are not bonded to each other, resulting in very high electrical conductivity and high emissivity (generating light from the sun and other stars). Charged particles are strongly influenced by magnetic and electric fields. For example, in the solar wind, they interact with the Earth's magnetosphere, creating Birkeland currents and auroras. Hydrogen is in a neutral atomic state in the interstellar medium. It is believed that large amounts of neutral hydrogen found in damped Lyman-alpha systems dominate the cosmological baryon density of the Universe up to redshift z = 4. Under normal conditions on Earth, elemental hydrogen exists as a diatomic gas, H2. However, hydrogen gas is very rare in the earth's atmosphere (1 ppm by volume) due to its light weight, which makes it easier to overcome Earth's gravity than heavier gases. However, hydrogen is the third most abundant element on the Earth's surface, existing primarily in the form of chemical compounds such as hydrocarbons and water. Hydrogen gas is produced by some bacteria and algae and is a natural component of flute, just like methane, which is an increasingly important source of hydrogen. A molecular form called protonated molecular hydrogen (H + 3) is found in the interstellar medium, where it is generated by ionizing molecular hydrogen from cosmic rays. This charged ion has also been observed in the upper atmosphere of the planet Jupiter. Ion is relatively stable in the environment due to its low temperature and density. H + 3 is one of the most abundant ions in the Universe and plays a prominent role in the chemistry of the interstellar medium. Neutral triatomic hydrogen H3 can exist only in an excited form and is unstable. In contrast, the positive molecular ion of hydrogen (H + 2) is a rare molecule in the universe.

Hydrogen production

H2 is produced in chemical and biological laboratories, often as a by-product of other reactions; in industry for the hydrogenation of unsaturated substrates; and in nature as a means of displacing reducing equivalents in biochemical reactions.

Steam reforming

Hydrogen can be produced in several ways, but the economically most important processes involve the removal of hydrogen from hydrocarbons, since about 95% of hydrogen production in 2000 came from steam reforming. Commercially, large volumes of hydrogen are typically produced by steam reforming of natural gas. At high temperatures (1000-1400 K, 700-1100 ° C, or 1300-2000 ° F), steam (water vapor) reacts with methane to produce carbon monoxide and H2.

CH4 + H2O → CO + 3 H2

This reaction works best at low pressures, but nevertheless, it can be carried out at high pressures (2.0 MPa, 20 atm or 600 inches of mercury). This is because high pressure H2 is the most popular product and pressure superheat cleaning systems perform better at higher pressures. The product mixture is known as "syngas" because it is often used directly to produce methanol and related compounds. Hydrocarbons other than methane can be used to produce synthesis gas with different product ratios. One of the many complications of this highly optimized technology is the formation of coke or carbon:

CH4 → C + 2 H2

Consequently, steam reforming typically uses excess H2O. Additional hydrogen can be recovered from the steam using carbon monoxide through a water gas displacement reaction, especially using an iron oxide catalyst. This reaction is also a common industrial source of carbon dioxide:

CO + H2O → CO2 + H2

Other important methods for H2 include partial oxidation of hydrocarbons:

2 CH4 + O2 → 2 CO + 4 H2

And the reaction of coal, which may serve as a prelude to the shear reaction described above:

C + H2O → CO + H2

Sometimes hydrogen is produced and consumed in the same industrial process, without separation. In the Haber process for the production of ammonia, hydrogen is generated from natural gas. Brine electrolysis to produce chlorine also produces hydrogen as a by-product.

Metallic acid

In the laboratory, H2 is usually produced by reacting dilute non-oxidizing acids with some reactive metals such as zinc with a Kipp apparatus.

Zn + 2 H + → Zn2 + + H2

Aluminum can also produce H2 when treated with bases:

2 Al + 6 H2O + 2 OH- → 2 Al (OH) -4 + 3 H2

Water electrolysis is an easy way to produce hydrogen. A low voltage current flows through the water and oxygen gas is generated at the anode, while hydrogen gas is generated at the cathode. Typically, the cathode is made from platinum or another inert metal in the production of hydrogen for storage. If, however, the gas is to be burnt in situ, the presence of oxygen is desirable to promote combustion, and therefore both electrodes will be made of inert metals. (For example, iron is oxidized and therefore reduces the amount of oxygen released.) The theoretical maximum efficiency (electricity used in relation to the energy value of hydrogen produced) is in the range of 80-94%.

2 H2O (L) → 2 H2 (g) + O2 (g)

An alloy of aluminum and gallium in the form of granules added to water can be used to produce hydrogen. This process also produces aluminum oxide, but the expensive gallium, which prevents the formation of oxide skin on the granules, can be reused. This has important potential implications for the hydrogen economy, as hydrogen can be produced locally and does not need to be transported.

Thermochemical properties

There are over 200 thermochemical cycles that can be used to separate water, about a dozen of these cycles, such as the iron oxide cycle, cerium (IV) oxide cycle, cerium (III) oxide, zinc oxide zinc, sulfur iodine cycle, copper cycle, etc. chlorine and the hybrid sulfur cycle are in the research and testing stages to produce hydrogen and oxygen from water and heat without the use of electricity. A number of laboratories (including those in France, Germany, Greece, Japan and the USA) are developing thermochemical methods for producing hydrogen from solar energy and water.

Anaerobic corrosion

Under anaerobic conditions, iron and steel alloys are slowly oxidized by the protons of water, while being reduced in molecular hydrogen (H2). Anaerobic corrosion of iron leads first to the formation of iron hydroxide (green rust) and can be described by the following reaction: Fe + 2 H2O → Fe (OH) 2 + H2. In turn, under anaerobic conditions, iron hydroxide (Fe (OH) 2) can be oxidized by water protons to form magnetite and molecular hydrogen. This process is described by the Shikorr reaction: 3 Fe (OH) 2 → Fe3O4 + 2 H2O + H2 iron hydroxide → magnesium + water + hydrogen. Well-crystallized magnetite (Fe3O4) is thermodynamically more stable than iron hydroxide (Fe (OH) 2). This process takes place during the anaerobic corrosion of iron and steel in anoxic groundwater and during the restoration of soils below the water table.

Geological origin: serpentinization reaction

In the absence of oxygen (O2) in deep geological conditions prevailing far from the Earth's atmosphere, hydrogen (H2) is formed in the process of serpentinization by anaerobic oxidation of iron silicate (Fe2 +) by protons of water (H +) present in the crystal lattice of fayalite (Fe2SiO4, minal olivine -gland). The corresponding reaction leading to the formation of magnetite (Fe3O4), quartz (SiO2) and hydrogen (H2): 3Fe2SiO4 + 2 H2O → 2 Fe3O4 + 3 SiO2 + 3 H2 fayalite + water → magnetite + quartz + hydrogen. This reaction is very similar to the Schikorr reaction observed during the anaerobic oxidation of iron hydroxide in contact with water.

Formation in transformers

Of all the hazardous gases produced in power transformers, hydrogen is the most abundant and generated in most fault conditions; thus, the formation of hydrogen is an early sign of serious problems in the life cycle of a transformer.

Applications

Consumption in various processes

Large amounts of H2 are required in the petroleum and chemical industries. Mostly, H2 is used for the processing ("modernization") of fossil fuels and for the production of ammonia. In petrochemical plants, H2 is used in hydrodealkylation, hydrodesulfurization, and hydrocracking. H2 has several other important uses. H2 is used as a hydrogenating agent, in particular to increase the saturation level of unsaturated fats and oils (found in items such as margarine) and in the production of methanol. It is also a source of hydrogen in the production of hydrochloric acid. H2 is also used as a reducing agent for metal ores. Hydrogen is highly soluble in many rare earth and transition metals and is soluble in both nanocrystalline and amorphous metals. The solubility of hydrogen in metals depends on local distortions or impurities in the crystal lattice. This can be useful when hydrogen is purified by passing through hot palladium discs, but the high solubility of the gas is a metallurgical problem, contributing to the embrittlement of many metals, complicating the design of pipelines and storage tanks. In addition to being used as a reagent, H2 has a wide range of applications in physics and technology. It is used as a shielding gas in welding methods such as hydrogen atomic welding. H2 is used as a rotor coolant in electric generators in power plants because it has the highest thermal conductivity of all gases. Liquid H2 is used in cryogenic research, including superconductivity research. Since H2 is lighter than air, at just over 1/14 of the density of air, it was once widely used as a lift gas in balloons and airships. In newer applications, hydrogen is used neat or mixed with nitrogen (sometimes called forming gas) as an indicator gas for instantaneous leak detection. Hydrogen is used in the automotive, chemical, energy, aerospace and telecommunications industries. Hydrogen is an approved food additive (E 949) that allows leak testing of foodstuffs, in addition to other antioxidant properties. Rare isotopes of hydrogen also have specific uses. Deuterium (hydrogen-2) is used in nuclear fission applications as a slow neutron moderator and in nuclear fusion reactions. Deuterium compounds are used in the field of chemistry and biology to study the isotope effects of a reaction. Tritium (hydrogen-3), produced in nuclear reactors, is used in the manufacture of hydrogen bombs, as an isotope marker in the biological sciences, and as a radiation source in glowing paints. The triple point of equilibrium hydrogen is the defining fixed point in the ITS-90 temperature scale at 13.8033 Kelvin.

Cooling medium

Hydrogen is commonly used in power plants as a refrigerant in generators due to a number of beneficial properties that are a direct result of its light diatomic molecules. These include low density, low viscosity and the highest specific heat and thermal conductivity of all gases.

Energy carrier

Hydrogen is not an energy resource, except in the hypothetical context of commercial fusion power plants using deuterium or tritium, and this technology is currently far from being developed. The energy of the Sun comes from the nuclear fusion of hydrogen, but this process is difficult to achieve on Earth. Elemental hydrogen from solar, biological, or electrical sources requires more energy to produce it than is consumed when burning it, so in these cases hydrogen functions as an energy carrier, by analogy with a battery. Hydrogen can be obtained from fossil sources (such as methane), but these sources are depleted. The energy density per unit volume of both liquid hydrogen and compressed hydrogen gas at any practically achievable pressure is significantly less than that of traditional energy sources, although the energy density per unit mass of fuel is higher. However, elemental hydrogen has been widely discussed in the energy context as a possible future energy carrier for the entire economy. For example, CO2 sequestration followed by carbon capture and storage can be carried out at the point of production of H2 from fossil fuels. The hydrogen used in transportation will burn relatively cleanly, with some NOx emissions, but no carbon emissions. However, the infrastructure cost associated with a full conversion to a hydrogen economy will be substantial. Fuel cells can convert hydrogen and oxygen directly into electricity more efficiently than internal combustion engines.

Semiconductor industry

Hydrogen is used to saturate the dangling bonds of amorphous silicon and amorphous carbon, which helps stabilize material properties. It is also a potential electron donor in various oxide materials including ZnO, SnO2, CdO, MgO, ZrO2, HfO2, La2O3, Y2O3, TiO2, SrTiO3, LaAlO3, SiO2, Al2O3, ZrSiO4, HfSiO4, and SrZrO3.

Biological reactions

H2 is a product of several types of anaerobic metabolism and is produced by several microorganisms, usually through reactions catalyzed by iron or nickel-containing enzymes called hydrogenases. These enzymes catalyze a reversible redox reaction between H2 and its components, two protons and two electrons. The creation of hydrogen gas occurs by transferring the reducing equivalents formed during the fermentation of pyruvate into water. The natural cycle of production and consumption of hydrogen by organisms is called the hydrogen cycle. Water splitting, the process by which water breaks down into its constituent protons, electrons and oxygen, occurs in light reactions in all photosynthetic organisms. Several such organisms, including the algae Chlamydomonas Reinhardtii and cyanobacteria, have developed a second stage in dark reactions in which protons and electrons are reduced to form H2 gas by specialized hydrogenases in the chloroplast. Attempts have been made to genetically modify cyanobacterial hydrases to efficiently synthesize H2 gas even in the presence of oxygen. Efforts have also been made using genetically modified algae in a bioreactor.