Hydrogen


Contents

  1. Introduction
  2. The Hydrogen Atom
  3. The 21-cm Line
  4. Ortho- and Parahydrogen
  5. Water and Hydrogen Ions
  6. Preparing Hydrogen
  7. Using Hydrogen
  8. Lighter-than-Air Craft
  9. References

Introduction

Hydrogen has been receiving interest as a "source of energy" recently. Besides, it is interesting in itself, so its nature and properties will be discussed here. Hydrogen is such a simple system that it has been important in the development of physics, a test bed for quantum mechanics, which it has amply confirmed. When we talk about hydrogen, we must distinguish between the hydrogen atom, hydrogen gas (the diatomic molecule), the proton (its nucleus), and the hydrogen that is part of many chemical compounds.

Hydrogen is apparently the most common substance in the universe, because it has no competitors for the title, except perhaps for helium. It exists mainly in the interstellar medium, with an average density of 10 atoms or less per cubic centimetre, and is by no means uniformly distributed. It has no effect on visible light, and was only discovered when ultraviolet observations could be made outside the earth's atmosphere, since it absorbs the far-ultraviolet Lyman alpha line at 12.15 nm. It also emits radiation of wavelength 21 cm, from a transition between hyperfine levels of its ground state, that has also been observed. Hydrogen molecules do not seem to have been observed in the interstellar medium, but hydrogen atoms are generally taken as indicating the existence of a wide variety of other molecules.

Hydrogen exists on the Sun, because its absorption lines are observed in the solar spectrum. Although there are hydrogen atoms in the solar atmosphere, hydrogen itself does not exist in the Sun, but only a plasma of protons and a few other nuclei, rendered neutral by a sea of electrons. If these protons are called hydrogen, then hydrogen is by far the most abundant element in the Sun, with helium second and everything else much rarer. The Sun's energy is produced by fusion of protons to form helium (alpha particles), in the proton-proton chain. Two protons react to form deuteron, consisting of a proton and a neutron, after emitting a neutrino and a positron. A proton reacts with a deuteron to form helium-3, which has two protons and one neutron, emitting a characteristic gamma-ray. Two helium-3 nuclei then react to form an alpha particle, or helium-4 nucleus, and two protons. A temperature of 107K is necessary to achieve the kinetic energies to bring the positively-charged particles close enough together to react. 90% of the Sun's energy comes from the proton-proton chain, and 10% from other fusion processes. The neutrinos released should be observable on earth, but at first seemed to be absent. Recently, it was discovered that they changed from electron neutrinos to muon neutrinos on the way to the earth, and then would not be detected by counters of electron neutrinos. The absence of the characteristic gamma rays in the cold-fusion claims some years ago was a sure sign that no fusion was going on. These gammas are absorbed deep in the Sun, of course, fortunately for us.

The Sun emits a plasma consisting mainly of protons and electrons at high speed in all directions, which is called the solar wind and can be considered as the outer part of the Sun's corona, rushing outward at 400 km/s or so from areas of weak magnetic fields, called corona holes. This plasma interacts in complicated ways with the magnetic field of the earth to give the aurora borealis, magnetic storms, and other effects. Comets are surrounded by a cloud of hydrogen that can be as large as the Sun. This cloud results from the decomposition of water by ultraviolet radiation.

On the Earth, hydrogen is not very common, existing only in the thin sheet of water covering 70% of the Earth, as only 0.127% by weight of the lithosphere, and in the small amount of hydrocarbons and organic matter. It is present only to about 5 x 10-5% by volume of air (about the same as nitrous oxide, and less than methane or carbon monoxide). It tends to become more important at very high altitudes, rising to about 1% of a very, very rare atmosphere. The hydrogen atoms at high altitudes may be the result of the solar wind, as well as the hydrogen that has diffused from below, and all this hydrogen is gradually lost, since the Earth's gravity is not sufficient to retain it. Hydrogen gas is present in small amounts in volcanic gases where it is produced by chemical reactions, but more of it appears combined as hydrogen sulphide and nascent water. There are no practical sources of hydrogen gas. All that we use must be manufactured with the expense of energy from hydrogen compounds.

The hydrogen atom is composed of a very light, negatively charged electron, and a heavy, positively charged proton. The weight of the proton is 1.00758 atomic mass units (about 6.02 x 1023 protons weigh 1.00758 grams). The electron weighs about 1/1837 of the proton's weight. The diameter of a proton has a fairly definite meaning, and is 1.6 x 10-13 cm. The diameter of a hydrogen atom in its ground state is around 1 x 10-8 cm, so the ratio of the diameters is about 1.6 x 10-5, which makes the ratio of the volumes 4.1 x 10-15. The volume occupied by the electron is all but a tiny fraction of the volume of the atom, while 99.95% of the atom's mass is in the proton.

The question of the "discovery of hydrogen" only has meaning after the recognition of the significance of chemical elements in the 18th century. As soon as strong mineral acids were discovered, dissolving metals in them often produced bubbles of hydrogen. Paracelsus could very well have made some hydrogen, and even made it explode, but there was no recognition of the significance of these observations, or of the real nature of gases. Henry Cavendish first studied the gas in 1760 or 1766, and showed that it burned to water with Priestley's oxygen. Lavoisier named it hydrogen, "water-former," in 1783, the first year that it was used in balloon ascents.

The Hydrogen Atom

The hydrogen atom may be considered as a mobile package of an electron and a proton, available for any adventures it may encounter. Two atoms can easily come together, since they do not repel one another, and when they do, they find that it is much more comfortable for the two protons to move close together (to 0.7416 Å) and arrange the electrons around themselves to achieve the minimum energy. This forms a hydrogen molecule, H2, whose dissociation energy is 4.476 eV, the energy of a 277 nm far-ultraviolet photon. Since there is radiation of this and smaller wavelengths in space, and collisions are rare, we can expect all the hydrogen there to be atomic. On the Earth's surface, however, protected from ultraviolet and at higher pressures, the diatomic molecule is stable.

There is a paradox here. Although atomic hydrogen is very active, diatomic hydrogen is not; it is quite inert until the molecule is disrupted. There is so little attraction between molecules that hydrogen melts at -259.20°C (14K) and boils at -252.77°C (20.4K), lower temperatures than for any other substance except helium, which we may expect to be inert. The critical temperature is -240°C (33K), the critical pressure 12.8 atm, and the critical density 0.0308 g/cc. Hydrogen is very difficult to liquefy, but Dewar managed it in 1898. In spite of this, hydrogen can explode when mixed with oxygen, the very opposite of inertness. The secret, of course, is reaction mechanism. If just one hydrogen atom H* (an excited state is represented by the *) should be present, then the reactions H* + O2 → HO* + O*; O* + H2 → HO* + H*; HO* + H2 → H2O + H* produce additional H* atoms, which then go on to react further. This is a chain reaction that proceeds more and more rapidly, gobbling up the H2 and O2, both of which are otherwise inert. Hydrogen is inflammable over the very wide range of concentration from 6.2% to 71.4% in air. The ignition temperature is also low, only 580°C to 590°C, 70°C lower than for methane. If a little air leaks into a volume of methane (natural gas) there is little hazard, since mixtures over 13.3% are not inflammable. The same is true of gasoline. If a little air leaks into a volume of hydrogen, the hazard is great, for then the gas will explode, not just burn. Hydrogen is not very soluble in water; at 0°C, 100 ml of water dissolves 1.93 ml of hydrogen.

In energy, according to E = mc2, 1 amu is 931.16 MeV, so the energy equivalent of the proton's mass is 938 MeV. The energy equivalent of the electron's mass is 1/1837 of this, or 0.511 MeV. The ionization energy of a hydrogen atom, the energy required to free the electron from its ground state, is 13.5 eV. The electromagnetic energies released or absorbed as the electron moves between its stationary states in the field of the proton comprise its spectrum. The hydrogen spectrum is by far one of the simplest, but its explanation defied classical physics. In the visible and ultraviolet, only a series of lines, called the Balmer series, were observed. The longest wavelength is 656.3 nm, called Hα, and then 486.1, 434.0, 410.2, 397.0, 388.9, and so on, approaching a limit at 364.6 nm. These could not be explained as harmonics of any fundamental frequency, nor were harmonics of these frequencies observed, as would be the case in any mechanical system. The series is named after Balmer because he found a simple empirical formula, λ = Cn2/(n2 - 4) that gave the wavelengths of every member of the series.

In 1911, however, Bohr created a simple mechanical model of the hydrogen atom, but with the restriction that angular momenta were quantized in units of h/2π = h', where h was Planck's constant from the theory of radiation. This gave a series of stationary states, and it was postulated that the frequencies of the spectral lines were proportional to the differences in the energies of these stationary states, where ΔE = hf. Since the energies of the stationary states were given by E(n) = -R/n2, where n = 1,2,3,..., f = R(1/n2 - 1/n'2). For n < n', there is emission of radiation, and for n > n', absorption. The Balmer series in emission is given by n = 2, n' = 3, 4, 5, ... , or by a "jump" from higher states to the state n = 2. This formula can be modifed to allow for a nucleus of any charge or mass.

The ground state is n = 1. The far ultraviolet series corresponding to n' = 2, 3, ... is the Lyman series, of which the longest wavelength is 121.5 nm. The least excitation of a hydrogen atom in the ground state is from n = 1 to n = 2, which corresponds to this line. Under normal conditions, hydrogen is in its ground state, and so does not absorb radiation of longer wavelengths. Also, far ultraviolet is strongly absorbed by the atmosphere.

The lines of the Balmer series are seen in absorption in the solar spectrum. Hα is the C line, Hβ the F line, and Hγ the G line. The photosphere emits a continuous spectrum that is modified by the relatively cooler atomic hydrogen gas in the chromosphere. The appearance of the Balmer series means that many atoms are in the first excited state, n = 2, in the chromosphere. The Balmer series is strongest in stars of spectral class A, weakening in hotter or cooler stars. The Sun is class G, where lines of ionized calcium are prominent, but the Balmer series is still seen.

Bohr's theory has no firm foundation; it is merely a remarkable coincidence. Quantum mechanics put it on a firm foundation after 1925, however, and verified all its predictions. One fault of Bohr's theory was that it could not be extended to the case of more than one electron, or to molecules. Quantum mechanics gave excellent, though approximate, results for helium, with two electrons around a nucleus of charge Z = 2, and for molecular hydrogen, H2.

A proton and a neutron combine to form the stable deuteron. Adding an electron makes heavy hydrogen, or deuterium D, with an atomic weight of 2.014. D2 has mp -254.60°C, bp -249.7, a little higher than H2. Deuterium was discovered by H. C. Urey in 1931 or 1932. It makes up about 1 part in 5000 of natural hydrogen, and its chemical behavior is identical. Two neutrons can also combine with a proton to form a triton. The triton is not stable, but emits an electron of 18 keV peak energy with a half-life of 12.26 years to become He3. The corresponding atom is called tritium. Tritium is produced only by nuclear reactions, including those of cosmic rays, and its abundance is no greater than 1 ppm.

The free neutron, incidentally, is also radioactive with a half-life of 13 minutes, emitting an electron of peak energy 0.78 MeV to become a proton. That is, it decays into hydrogen. Neutrons in a nucleus do not do this, since they do not get bored there.

The 21-cm Line

It was mentioned above that electromagnetic radiation of about 21 cm wavelength is received from interstellar atomic hydrogen, and gives us information about the interstellar medium. The explanation of this radiation takes us into several interesting topics, and so will be presented here.

The electron and the proton have angular momentum, called "spin," that is a consequence of relativity. The magnitude of the angular momentum is √[s(s+1)](h/2π), where s = 1/2 for both electron and proton (and neutron as well). Relative to any reference direction in space, the spin can take only two orientations, parallel and antiparallel, and the observable momenta are s(h/2π), where s = ±(1/2). Two angular momenta s and s' add to a resultant S = s + s' down to |s - s'|. For example, the angular momentum of the deuteron could be 1 or 0; it is actually 1 in the nuclear ground state. The total angular momentum of a nucleus is denoted by I. Hydrogen has I = 1/2, deuterium I = 1. Nuclei with an odd number of nucleons have half-integral I, while those with an even number have integral I.

Associated with the spin is a magnetic moment. Classically, a particle of mass m and charge q with angular momentum L has a magnetic moment μ = (q/2mc)L. If q = e and L = h/2π, then μ = eh/4πmc. If m is the mass of the electron, then this is the Bohr magneton, and happens to be the magnetic moment of the electron, although the spin angular momentum is only h/4π. Its value is 0.918 x 10-20 erg/gauss. Relativity makes the magnetic moment of spin twice as large as expected. We can write μ = g(q/2mc)L, and say that the g-factor g for the electron spin is 2. For orbital motion, it has the classical value 1.

If m is the proton mass, then μ is the nuclear magneton, 1836 times smaller than the Bohr magneton. Its value is 5.04929 x 10-24 erg/gauss. The proton magnetic moment is 2.79267 nuclear magnetons, the neutron magnetic moment is -1.91315 nuclear magnetons. Note that the neutron has a magnetic moment even though its total charge is zero. Nucleons have a complex internal structure as a system of three quarks. Since the proton and the neutron in a deuteron have their spins aligned, the magnetic moment of the deuteron should be the sum of the individual moments, and indeed it is, very closely, at 0.8574 nuclear magnetons.

Because of the spin and orbital motion of the electron, there is a magnetic field at the nucleus. In the ground state of the hydrogen atom, there is no orbital angular momentum, l = 0, so its spectroscopic designation is 2S1/2, where the total angular momentum J = 1/2 is due to the electron spin alone. The magnetic field H at the nucleus is aligned with the direction of the electron magnetic moment, which is opposite to the direction of the spin. (Classically, it would seem to be in the opposite direction, but a careful quantum-mechanical investigation shows differently). The nuclear magnetic moment can assume two orientations with respect to this field, with interaction energies ±μH. The total difference in energy between these two states, of parallel and antiparallel spins of electron and nucleus, will be 2μH. These energy differences due to the interaction of the nuclear and electronic spins are called hyperfine structure.

In the chill of interstellar space, a hydrogen atom that happens to be in the upper of these two states will eventually jump to the lower state, emitting a photon of frequency f, where hf = 2μH. This is so improbable that it is never observed on earth. Transitions between these two states in the laboratory can, however, be stimulated by radiofrequency radiation, and have been easily observed since the development of atomic beam techniques. The transition frequency is 1.4204057 GHz for hydrogen, 0.3273843 GHz for deuterium (because of the smaller nuclear moment). This gives an energy difference for hydrogen of 9.41 x 10-18 erg. From this, we find that the magnetic field at the proton is 33.4T, a surprisingly large value, much larger than macroscopic fields. The 21-cm radiation is caused by a flip of the electron and proton spins from antiparallel to parallel, or from total angular momentum F from 0 to 1.

This discussion of the 21-cm line is closely related to the general phenomenon of magnetic resonance, which is of more general importance, as in modern medical imaging. Let's consider a classical system with angular momentum J and magnetic moment M. These vectors will be parallel, and related by M = γJ, where γ is the gyromagnetic ratio. In the absence of an external magnetic field H, the moments will be directed randomly. An applied magnetic field exerts a torque MxH on the angular momentum that is always perpendicular to it. Since dJ/dt = MxH, the angular momentum, and with it the magnetic moment, rotates about the direction of the magnetic field. Using the proportionality of magnetic moment to angular momentum, we find the equation of motion for the moment to be dM/dt = γMxH. In an interval of time dt, the change in the vector moment is dM = γH M sin θ at right angles to M. If M is rotating with angular velocity ω, dM = ω M sin θ as well. Equating these expressions, we hav ω = γH. Since H is a constant vector, we find the remarkable result that all the moments rotate at the same angular velocity, whatever angle they make with the magnetic field. This movement is called precession. In an actual system, various mechanisms will exchange energy so that in thermal equilibrium the magnetic moment will eventually point in the direction of H, the state of minimum energy. This is consistent with the expression E = -μ·H for the energy of a dipole in a magnetic field.

If we now apply an oscillating magnetic field at right angles to the main field, it can be resolved into circularly polarized components that rotate in opposite directions. If the frequency of this rotation is different from the frequency of precession, this field will have no permanent effect. If the frequency is equal to the frequency of precession, a constant torque tending to rotate the moment from the direction of the field will be exerted, and the moments will exchange energy with the oscillating field. This energy exchange can be detected in several ways. By varying the frequency of the oscillating field (or, more easily, by varying the steady magnetic field), the condition of resonance can easily be detected, and the precessional frequency determined.

The equality ω = γH can be written 2&pif = H(M/J) = H(4πμ/h), where we have used J = (1/2)(h/2π), correct for the proton. Then, μ is the proton magnetic moment, 1.410 x 10 erg/gauss. Now, f = (2μ/h)H. For the proton, 2μ/h = 4257.52 Hz/gauss. Therefore, with a steady field of 10,000 gauss (1 T), f = 42.5752 MHz. Resonance will be observed at this frequency for a free proton. In the 21-cm line, we found that the orbital electron produced a field at the proton of 334,000 gauss. In this case, the electron and the nuclear moment are very strongly coupled, and the proton moment will certainly not precess independently. Any practical laboratory field will have very little effect in this case. The expression for the frequency can also be obtained from the transition energy: hf = 2μH.

In compounds containing hydrogen, the electrons either form closed shells, or occur in pairs in the bonds. In either case, the magnetic field will be closer to zero than to the large value existing in the hydrogen atom. Differences in the field that does exist at different protons will cause differences in the resonance frequency, and the result will be a nuclear resonance spectrum, from which much information can be derived.

Ortho- and Parahydrogen

Hydrogen has offered many illustrations of surprising quantum-mechanical effects, always confirming them precisely. There are so many that there is no space here to even mention most of them, but one was the discovery of two apparently different kinds of hydrogen gas that could interconvert slowly, called orthohydrogen and parahydrogen. The normal gas was a mixture of 3 parts of the ortho to 1 part of the para. The two forms were chemically and physically identical, except that they differed slightly in spectra and specific heats. This was verified by Bonhöffer and Harteck, and independently by Eucken and Miller, in 1929. If hydrogen was cooled, it eventually became pure parahydrogen (more rapidly in the presence of an activated charcoal catalyst). Alternate lines were missing in the rotational spectrum of parahydrogen, as if only states of even total angular momentum J were present. In normal hydrogen, the lines alternated in intensity with J, those of odd J three times more intense than those of even J.

This classically incomprehensible behavior was easily explained quantum-mechanically. In a molecule like hydrogen, the two nuclei are identical particles, so the molecule goes over into itself under an (imagined) inversion. Therefore, all its states must be either symmetric or antisymmetric under this transformation, and transitions between symmetric and antisymmetric states are forbidden. Since the two protons have spin 1/2, the total nuclear angular momentum I must be either 1 or 0. For I = 1, we have three possibilities, +1, 0, and -1. For I = 0, we have only the one. Therefore, in a random mixture, there are three times as many molecules in the nuclear triplet (I = 1) state than in the singlet (I = 0).

The overall wave function on inversion must be symmetric, since there are four spin-1/2 particles involved. This wave function is the product of the nuclear wave function and the spatial wave functions, including electronic, vibrational and rotational. It turns out that in the ground state, a symmetric nuclear state demands a symmetrical rotational state, and an antisymmetric nuclear state demands an antisymmetrical rotational state. The rotational states are even or odd depending on whether J is even or odd. Therefore, the singlet nuclear state, parahydrogen, (antisymmetric) corresponds to J even, and the triplet nuclear state, orthohydrogen, corresponds to J odd. In cases like this, "ortho" refers to states of greater statistical weight, here 3, and "para" refers to the states of lesser statistical weight.

Water and Hydrogen Ions

The oxygen atom has space for two additional electrons that are easily supplied by two hydrogen atoms. The leftover protons stick to the surface of the oxygen at approximately right angles, though their electrostatic repulsion pushes them farther apart, so that they make an angle of 104° 31'. The O-H bond length is 0.965 Å. This is, of course, the remarkable water molecule, of which a great deal can be said. It has a permanent electric dipole moment, the side with the protons positive, the other end negative. Water molecules tend to stick together when the positive and negative ends approach, forming hydrogen bonds. This makes water liquid at ordinary temperatures, and solid below 0°C, when other molecules of similar weight without hydrogen bonds are gaseous (such as NH3). The polar water molecules, like busy little hands, can pick apart ionic crystals like NaCl by clustering their negative ends around the Na+ and their positive ends around the Cl-, freeing enough energy to dissolve the crystal. It is always a balance between the energy released by hydration and the energy released by forming a crystal that determines whether ions dissolve or precipitate.

46 water molecules can form a structure with icosahedral symmetry that possesses six "cages" in which molecules can be trapped. For a discussion of such clathrates or gas hydrates, see Gas Hydrates.

Liquid water has a density close to 1.000 g/cc, and an index of refraction 1.333. Solid water (ice) has a density close to 0.915, and an index of refraction of 1.309. Water is very important in meteorology, where water vapor is the most effective greenhouse gas.

A molecule like HCl dissolves readily in water, since the hydrations of H+ and Cl- are very profitable. Because the concentration of H+ is large, HCl is called a strong acid, and dissociates nearly completely in water solution. What makes it react like an acid is the hydrogen ion H+. If there is one mole (6.02 x 1023 ions, or about 1 gram) in one liter of solution, the concentration is called 1M. H+ concentration, written [H+], is usually expressed in pH, which is -log[H+]. For 1M, this gives pH = 0 for a completely dissociated acid. The hydrogen ion is sometimes written H3O+ to make the hydration explicit, but this is not really correct, and is irrelevant. One should also then write H2ClO-.

Water is busy even with itself, picking molecules apart, hydrating the H+ and OH-. The latter is called the hydroxyl ion, and is characteristic of bases the way the hydrogen ion is characteristic of acids. This dissociation does not go very far, but in pure water the product of the concentrations [H+][OH-] = 10-14. This product is constant in equilibrium whatever the concentrations of one or the other ion is by considering reaction kinetics. Since the concentrations of hydrogen ion and hydroxyl ion are equal in this case, [H+] = 10-7M and the pH = 7. If we add one mole of NaOH to make a litre of solution, then [OH-] = 1M (neglecting the small contribution of the water), so [H+] is depressed to 10-14M, and the pH = 14. When equal amounts of a 1M acid (pH = 0), contributing [H+], and a 1M base (pH = 14), contributing [OH-], are mixed, the ions combine to form water, leaving only enough to make the pH = 7.

Water solutions are a way to achieve chemical reactions easily. The water picks apart the reactants forming hydrated ions, which can then move about freely to find partners. Sodium carbonate and calcium chloride can be mixed as solids, but will not react. When dissolved in water, calcium carbonate is rapidly precipitated because water cannot pick it apart, while sodium and chlorine ions remain as a salt solution. So much chemistry occurs in water solution that other possibilities are often neglected, such as gas reactions or reactions in the fused state, or even reactions on colloidal surfaces, in which atoms can also get together to try different arrangements.

Heavy water, D2O, has a density of 1.1076 g/cc, melts at 3.813°C, boils at 101.431°C at 1 atm, and has a refractive index of 1.32844. It is said to be harmful to life in large concentrations. Its importance with respect to fission reactors is that unlike ordinary water, D has a very low absorption cross section for thermal neutrons, only 0.00057 barn, compared to 0.33 barn for H. Collisions with D are almost as effective as collisions with H to slow down fast neutrons, so heavy water is still an effective moderator. The O16 in both light and heavy water has a very small absorption cross section, less than 20 μbarn. Heavy water is probably the most effective neutron moderator. Using it, reactors can be made that go critical with natural uranium.

Preparing Hydrogen

Hydrogen does not occur uncombined on Earth. The only practical source of hydrogen is water. Ultimately, even the hydrogen in hydrocarbons and organic matter came from water. Energy is required to separate the hydrogen from water, and when the hydrogen is subsequently burned, less energy is obtained than was consumed to produce the hydrogen. On the Earth, hydrogen is not an energy source, only a means of storing and transporting energy.

The cheapest way to produce hydrogen is as water gas. Coke is burned in air to bring it to red heat. Then the air is shut off and steam is blown into the reactor. The reaction is C + H2O → CO + H2. The water gas, enriched somewhat with hydrocarbons, was supplied as town gas for heating purposes in most cities until natural gas became available. This gas burned with a blue flame, and made putting one's head in the oven an easy suicide. It also was explosive, so that every winter one building or another blew up disastrously. With natural gas, this no longer happens as much, fortunately, but natural gas can also explode, simply not as readily. To make hydrogen, the CO is oxidized to CO2 catalytically, and then is easily removed by scrubbing with water. This is one of the cheapest ways to make hydrogen, and it will be noticed that not only does is require a good deal more energy than will be recovered by burning the hydrogen, but also produces CO2.

These days, water gas is little used, and most hydrogen comes from reforming natural gas, usually to make plastics. A typical reaction is 2CH4 → C2H4 (ethylene) + 2H2, promoted by a catalyst. The ethylene is a very useful chemical primary, which polymerizes to polyethylene, the plastic PE, among other things. The hydrogen can then be used for hydrogenating unsaturated vegetable oils to turn them into more desirable margarine and Crisco. This is the most probable source of hydrogen for a "hydrogen economy." Again, more energy must be used in making the hydrogen than is recovered by burning it. Incidentally, natural gas is only the most convenient raw material. Hydrogen can be made from other natural organic sources, but much less efficiently, and at the expense of much more energy.

Soon after Volta's discovery of the electric cell, Nicholson and Carlisle electrolyzed water to form O2 and H2, then recombined the gases to show that the water was reconstituted. This is an excellent demonstration, showing that two volumes of hydrogen combines with one volume of oxygen to make one volume of water vapor. Many people think of this as a way to manufacture hydrogen, but it is relatively inefficient and requires much more electrical energy than is recovered by burning the product. Nevertheless, photovoltaic cells and wind turbines can produce electricity without any expensive inputs, so this may be a good way to store the energy generated when the sun shines and the wind blows for when it is dark and calm.

The usual laboratory source of hydrogen is the reaction of strong acids with granulated impure zinc. This is an interesting reaction with many applications. The reactions that occur are the discharging of H+ ions, 2H+ + 2e- → H2, and the oxidation of zinc atoms, Zn → 2e- + Zn++. This is really the corrosion of zinc, and similar reactions are responsible for the corrosion of any active metal, such as iron. Adding these two reactions, the net effect is 2H+ + Zn → Zn++ + H2, where charge and mass balance. How strongly each reaction goes depends logarithmically on the concentration (activity) of the reactants. It is easier to discharge hydrogen from a concentrated solution of H+ than from a weak one, for example. The tendency of the reactions to proceed is measured by their electrode potentials, probably familar to the reader, quoted for standard concentrations. We won't go into the activity dependence here, but it should be kept in mind.

The discharging of hydrogen consumes electrons, so regions where this reaction occurs are sources of electric current (remember that electrons are negative, so an electron moving to the right is a current moving to the left), and have a positive potential. We can allow this to occur at a platinum electrode, which will remain inert. As we draw current from the platinum electrode, bubbles of hydrogen gas will appear on its immersed surface. Whatever voltage is required with respect to the solution to make this happen is conventionally taken as zero. The zinc reaction occurs at a zinc electrode in the solution, and current flows into the electrode as the reaction proceeds. If we connect the platinum and the zinc electrodes, a current flows so long as there is zinc and hydrogen ions. If we apply a reverse voltage between the electrodes, we can eventually make the current stop, and this voltage is the electrode potential of the zinc. At standard concentrations, this voltage turns out to be 0.761 V. In the tables of electrode potentials, the zinc reaction is written as Zn++ + 2e- → Zn, as a reduction rather than an oxidation, and this makes the tabulated voltage -0.761 V.

The platinum and zinc electrodes in an acid can be used as a battery, of course, and if the H+ concentration is 1 M, its voltage will be 0.761 V. The platinum electrode will be (+), the zinc (-). At an H+ concentration of 10-7 M, as in pure water, the hydrogen electrode potential is -0.414 V, so the voltage will drop to only 0.347 V. In general, any metal with a more positive reduction potential than hydrogen will displace it from solutions containing H+. This includes quite a number of metals, notably iron, cobalt, nickel, magnesium, aluminum, titanium, and even lead. However, there are complicating factors, such as the formation of protective surface layers, that may make the release of hydrogen very slow, as in the case of lead, which can be used to hold concentrated sulphuric acid. Aluminium resists acids because of the surface layer of oxide.

Even pure zinc reacts very slowly when immersed in acid. When used in an electrolytic cell, however, pure zinc reacts in proportion to the current flow. Impure zinc immersed in acid, however, reacts rapidly. The reason for this is that the impurities act like the platinum electrode in a cell, becoming positive relative to the zinc, so small cells are formed, and as current flows, hydrogen is evolved at the site of the impurity. In this way, the impurities can serve to corrode all of the zinc, so long as acid is left. In electrical cells, this is called local action, and wastes zinc unprofitably. It was traditionally overcome by amalgamating the zinc, which put pure zinc on the surface and short-circuited the parasitic cells. Amalgamated aluminium also makes a good electrode, overcoming the effects of the oxide layer.

Many other reactions produce hydrogen. Sodium reacts rather violently with water to form sodium hydroxide and hydrogen: Na + H2O → NaOH + (1/2)H2. Sodium hydride reacts similarly: NaH + H2O → NaOH + H2. Zinc also dissolves in strong alkalis to release hydrogen: Zn + 2NaOH → Na2ZnO2 + H2. Methanol can be catalytically oxidized to give hydrogen: CH3OH + H2O → CO2 + 3H2. This reaction is specially useful for portable hydrogen generators.

Using Hydrogen

The principal use of hydrogen today is not as a fuel, but as a chemical feedstock. It is produced as a by-product of plastics manufacture, and is then used in a variety of ways. One is for the hydrogenation of vegetable oils, as mentioned above. Another is the production of methanol by the Patart synthesis, 2H2 + CO → CH3OH, with the aid of a catalyst. This is the inverse of the hydrogen-producing reaction of the preceding paragraph.

Most of the proposed uses of hydrogen, however, produce heat by burning it in oxygen. This may be done in an open flame, in an internal-combustion engine, in a gas turbine, or fuel cell. The basic reaction is H2 + (1/2)O2 → H2O + Q. If the water is condensed to liquid at a standard state, Q = 68.32 kcal (per mol). If the water is exhausted as a vapor, then Q = 57.80 kcal. The difference is the heat of vaporization of water. These heats may also be expressed as 34.18 kcal/g or 29.15 kcal/g. Multiplying kcal/g by 1800 gives Btu/lb. This is a very high energy content on a weight basis, but unfortunately hydrogen is light. On a volume basis, it gives only 325 Btu/cuft, while methane gives 1008 Btu/cuft.

Liquid hydrogen and liquid oxygen make a compact, high-energy rocket fuel, but handling and storage of the liquid gases is very difficult, and is not practical for other vehicles. Because of the energy required to liquefy the hydrogen, it is rather inefficient as well. A commercial steel cylinder of compressed hydrogen contains 176 cuft at STP, less than a pound, which will give only 57,200 Btu in all. The pressure in such a cylinder is 120-150 atm (1800-2200 psi). The problem of hydrogen storage is a severe one. Unlike for natural gas, liquefaction is not an option for widespread use.

Iron, gold, platinum and palladium adsorb hydrogen strongly. Although the hydrogen diffuses rapidly through these metals, the adsorption is a surface effect and increases when the metals are in colloidal form. One volume of colloidal Pd will adsorb 870 volumes of hydrogen, releasing it when heated. Apparently, this is in terms of hydrogen at STP, and the amount adsorbed does not increase proportionally to the pressure. Relative to steel tanks, Pd only decreases the volume required by a factor of about 5.8. A 13-gallon fuel tank filled with gasoline (87 lb) will yield about 1.8 x 106 Btu. 30 lb of hydrogen would give the same yield, so 32 steel tanks of compressed gas would be required for the same range. These tanks could be replaced by 5.5 times the volume of one tank in palladium, which would be very costly, not to mention heavy. The use of hydrogen in vehicles will be severely restricted by these considerations, which are similar to those of vehicles using storage batteries, but worse. Fuel cells have no bearing on the problem. Hydrogen filling stations had better be closely spaced.

Hydrogen diffuses more rapidly than any other gas. It will even diffuse through metals, especially those that adsorb it, as well as through quartz, and probably also glass. It will search out the smallest leak, especially in valves and similar devices, which must be carefully designed to hold hydrogen. This property may well result in occasional surprises for hydrogen users.

Hydrogen can be burned in a torch with air or oxygen. An air-hydrogen torch flame reaches 2045°C, while an oxyhydrogen flame reaches 2660°C. Flame temperatures are subject to considerable uncertainty, and depend on the mixture used. The hydrogen flame contains no carbon, and so is invisible. A mixture of 45% 1025 Btu/cuft natural gas and oxygen produces a flame temperature of 2930°C, it is reported. Oxyhydrogen flames playing on CaO refractories produced the "limelight" used in theatrical productions before electrical lighting.

Hydrogen can be dissociated in an electrical arc in an atomic hydrogen torch. The recombination of the atoms produces a very high temperature, 4000°C to 5000°C, that can be used for welding. In addition, the atmosphere surrounding the weld is nonoxidizing, so flux is not required. The reaction is 2H → H2 + 102.6 kcal. It is not necessary to burn the hydrogen, but this adds extra heat and eliminates an explosion hazard.

I have recently heard of a microorganism, carboxydothermus hydrogenoformous, that consumes carbon monoxide and water, releasing hydrogen and carbon dioxide. This could be another source of hydrogen, but note that carbon dioxide is also produced. This is just the water gas reaction, but mediated by life.

Lighter-than-Air Craft

Hydrogen was famously used to fill lighter-than-air craft, since it is the lightest gas available. At 0°C and 1 atm, air weighs 1.293 kg/m3, a far from negligible weight, though not apparent to us living immersed in it. This is the buoyant force available for lifting. The essential thing is making the weight of a craft less than the available buoyancy; that is, the average density of the craft must be less than that of air. This is achieved by filling most of the volume with something lighter than air. Air has a molecular weight of 28.966 g/mol. Any gas with a smaller molecular weight will provide lift. The possibilities are hot air, water vapor (M = 18), ammonia (M = 17), methane (M = 16), helium (M = 4), and of course, hydrogen (M = 2). Since one gram mol of gas at 0°C and 1 atm occupies about 22.4 l, it is easy to calculate the net buoyant force in any case.

Hydrogen weighs 89.3 g/m3, so the maximum lifting force is 1293 - 89 = 1204 g/m3. Helium weighs twice as much, so the lifting force with helium is 1114 g/3, only 7.4% less. Hydrogen and helium are, therefore, practically equivalent in lifting force. At the other end of the scale, air heated to 100°C weighs 946 g/m3, providing only 258 g/m3 lifting force. The gas container must be as light as possible, so it is generally a thin-skinned bag. A number of bags may be used so that all the buoyancy is not lost if a bag is ruptured.

Lighter-than-air craft may be classified as tethered balloons, free balloons, and dirigibles or airships. The dirigibles may be classified as nonrigid or rigid. Nonrigid airships, with no internal frame defining its shape, are often called blimps, from the British code name for such (limp) ships under development, which were termed A-limp, B-limp and so on. Rigid airships are often called after their inventor, Zeppelin. The Montgolfier brothers arranged the first balloon ascents in 1783, using hot air and a fabric envelope. In the same year, Charles demonstrated hydrogen-filled balloons. In 1836, the Great Balloon of Nassau, 85,000 cuft and hydrogen-filled (total lift 2890 kg) made a flight from England to Germany. Balloons were used for observation in the Franco-Prussian War of 1870, during which Léon Gambetta made his celebrated escape from beseiged Paris by balloon. Manned balloon ascents reached 113,700 ft (34,700 m) in 1961, and an unmanned balloon reached 168,000 ft (51,200 m). The largest scientific balloon had a capacity of 26,000,000 cuft. Sport ballooning is still practiced, with hydrogen in Europe and hot air in the United States, provided by a butane or propane burner. Long-distance balloon flights are made as stunts. These free balloons are at the mercy of the winds, so they generally travel from west to east, and cannot make return trips.

The first dirigibles awaited the availability of engines of low weight but sufficient power. Graf Ferdinand von Zeppelin (1838-1917) brought out the first dirigible in 1900, hydrogen-filled with an aluminium frame. Its volume was 399,000 cuft (11,300 m3), so its gross lift was 13,600 kg. The application of Zeppelins as terror weapons was quickly perceived, and they were used for rather ineffective bombing of civilians. The first Zeppelin raid against Britian was on 19 January 1915. Of the 67 Zeppelins made during the War, only 16 survived, since they were very susceptible to attack. After the War, military authorities in Britain, the United States and Italy became interested in the possibilities of dirigibles, especially in view of the possibility of helium filling, due to the anticipated availability of helium from the United States. (See Helium for this interesting story.)

The British R34 of 1919, 1,980,000 cuft, made the first transatlantic crossing, but was lost in 1921. The Roma, supplied by Italy to the United States in 1921, 1,200,000 cuft, was lost in 1922. The U.S. dirigible Shenandoah (originally ZR1), 2,115,000 cuft, built in 1923, was the first filled with helium. It was lost in 1925 at the cost of 14 lives. The Los Angeles, 2,475,000 cu. ft., followed in 1924. The Graf Zeppelin, 3,710,000 cu. ft., of 1928, entered regular long-distance passenger service. These ships were powered mostly by multiple marine diesel engines. In 1929, Britain built the large R100 and R101, of 5,000,000 cu. ft.. When R101 was lost at Beauvais in 1930 with 46 killed, Britain ceased the development of airships. In 1931 and 1933, the United States constructed the Akron and Macon (they were named like navy cruisers), 6,500,000 cu. ft.. They were wrecked in 1933 and 1935, respectively, and the United States terminated the development of rigid airships. Blimps, for submarine warfare and air defense, were used by the U.S. Navy until 1983. A few blimps are still maintained for advertising purposes.

It is clear that all these large ships had very short lives, being unable to cope with the winds. This is a general characteristic, and every new generation of airship enthusiasts has to learn it anew, it appears. No more dangerous and unreliable method of transportation than the airship was ever devised, but its charm is undeniable. The von Hindenburg, a hydrogen-filled airship of 6,710,000 cu. ft., was built in 1936 for transatlantic service. It very famously burned at Lakehurst, NJ on 6 May 1937 with the loss of 36 passengers out of a total of 92. As disasters go, this was pretty tame compared with later ones, in which hundreds die in numerous air crashes. Nevertheless, it was a very impressive disaster. The flames in the images are from the fabric bags; the hydrogen flame is invisible. The accident was probably due to a small spark combined with leakage of air into the hydrogen. Although much is made of the danger of hydrogen, this was the only significant accident due to this cause. Consideration of the fates of the previous dirigibles show that hydrogen fires were unimportant. In military connections, however, they are more significant.

Any large balloon or airship must carry ballast, either water or sand, to adjust the buoyancy as the lifting gas diffuses away, or is heated or cooled. Discarding ballast causes the ship to rise, venting lifting gas causes it to descend. The excess of the lift over the weight of the craft is called the free lift. For a small balloon, it can be measured by a scale before the balloon is released. The amount of free lift determines how fast the balloon will rise, and how high it will ascend.

Free balloons are of three general types: (1) rubber or neoprene elastic balloons; (2) polyethylene or similar inelastic balloons; and (3) superpressure balloons of constant lift. The neoprene balloon, typically used for radiosondes, expands as the balloon rises until the balloon bursts, and the payload descends on a parachute. A polyethylene balloon, for lifting heavier loads, is only partially filled at the surface, becoming spherical at altitude. A valve dumps lifting gas to maintain the altitude constant. At nightfall, the gas cools and the balloon descends to the surface. The superpressure balloon adjusts the tension in its skin to suit changing conditions, and remains at its equilibrium altitude until the lifting gas diffuses away. Barrage balloons, tethered balloons looking like blimps, but unpowered, were used in air defense in World War II. Low-flying aircraft would fly into wires suspended below them. Higher-flying aircraft were a better target. Of course, they are now completely obsolete.

Filling of such balloons with helium, as is done in the United States, is an arrogant waste of a scarce and valuable resource. Only the hazards of the hydrogen supply for filling are avoided by the use of helium, as the loss of a released balloon by fire (which does not happen anyway) is inconsequential. Hydrogen for filling balloons requires a source of a size between that of an industrial supply, and and a laboratory supply. Some suitable methods were mentioned above. The apparatus can be put easily into a road trailer, and moved to wherever it is needed. These hydrogen sources have been confused with weapons of mass destruction in some cases by the desperate and unintelligent.

References

H. E. White, Introduction to Atomic Spectra (New York: McGraw-Hill, 1934). Chapter II gives a good explanation of the Bohr atom. The spectrum of atomic hydrogen is, of course, also described in this text.

G. Herzberg, Molecular Spectra and Molecular Structure, I. Spectra of Diatomic Molecules, 2nd ed. (New York: D. Van Nostrand Co., 1950). The symmetry of rotational levels, and ortho- and para-hydrogen, are explained on pp. 128-141.

H. Kopfermann, Nuclear Moments (New York: Academic Press, 1958). See Table 4 on p. 72, and Sec. 25, pp. 115-123 for hyperfine structure with s electrons.

I thank Robin van Spaandonk, who pointed out an error in my description of the proton cycle in stars, which I have corrected.


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Composed by J. B. Calvert
Created 14 March 2004
Last revised 21 April 2008