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9 novembre 2011 3 09 /11 /novembre /2011 03:40

Biological reactions

H2 is a product of some types of anaerobic metabolism and is produced by several microorganisms, usually via reactions catalyzed by iron- or nickel-containingenzymes called hydrogenases. Sony VAIO VPCEB1JFX/W Battery

These enzymes catalyze the reversible redox reaction between H2 and its component two protons and two electrons. Creation of hydrogen gas occurs in the transfer of reducing equivalents produced during pyruvate fermentation to water.

Water splitting, in which water is decomposed into its component protons, electrons, Sony VAIO VPCEB1KGX Battery

and oxygen, occurs in the light reactions in all photosynthetic organisms. Some such organisms, including the alga Chlamydomonas reinhardtii and cyanobacteria, have evolved a second step in the dark reactions in which protons and electrons are reduced to form H2 gas by specialized hydrogenases in the chloroplast.Sony VAIO VPCEB1KGX/B Battery

Efforts have been undertaken to genetically modify cyanobacterial hydrogenases to efficiently synthesize H2 gas even in the presence of oxygen. Efforts have also been undertaken with genetically modified alga in a bioreactor.

Safety and precautions

Hydrogen poses a number of hazards to human safety, from potential detonations and fires when mixed with air to being an asphyxiant in its pure, oxygen-free form.Sony VAIO VPCEB1KGX/W Battery

In addition, liquid hydrogen is a cryogen and presents dangers (such as frostbite) associated with very cold liquids.[114] Hydrogen dissolves in many metals, and, in addition to leaking out, may have adverse effects on them, such as hydrogen embrittlement,[115]leading to cracks and explosions.Sony VAIO VPCEB1LFX Battery

Hydrogen gas leaking into external air may spontaneously ignite. Moreover, hydrogen fire, while being extremely hot, is almost invisible, and thus can lead to accidental burns.

Even interpreting the hydrogen data (including safety data) is confounded by a number of phenomena. Sony VAIO VPCEB1LFX/BI Battery

Many physical and chemical properties of hydrogen depend on theparahydrogen/orthohydrogen ratio (it often takes days or weeks at a given temperature to reach the equilibrium ratio, for which the data is usually given). Hydrogen detonation parameters, such as critical detonation pressure and temperature, strongly depend on the container geometry.Sony VAIO VPCEB1LFX/WI Battery

A hydrogen atom is an atom of the chemical element hydrogen. The electrically neutral atom contains a single positively-charged proton and a single negatively-charged electron bound to the nucleus by the Coulomb force. The most abundant isotope,hydrogen-1, protium, or light hydrogen, contains no neutrons;Sony VAIO VPCEB1MFX Battery

other isotopes of hydrogen, such as deuterium, contain one or more neutrons. This article primarily concerns hydrogen-1.

The hydrogen atom has special significance in quantum mechanics and quantum field theory as a simple two-body problemphysical system which has yielded many simple analytical solutions in closed-form.Sony VAIO VPCEB1MFX/BI Battery

In 1914, Niels Bohr obtained the spectral frequencies of the hydrogen atom after making a number of simplifying assumptions. These assumptions, the cornerstones of the Bohr model, were not fully correct but did yield the correct energy answers. Sony VAIO VPCEB1NFX Battery

Bohr's results for the frequencies and underlying energy values were confirmed by the full quantum-mechanical analysis which uses the Schrödinger equation, as was shown in 1925–1926. The solution to the Schrödinger equation for hydrogen isanalytical. From this, the hydrogen energy levels and thus the frequencies of the hydrogen spectral lines can be calculated.Sony VAIO VPCEB1NFX/B Battery

The solution of the Schrödinger equation goes much further than the Bohr model however, because it also yields the shape of the electron's wave function ("orbital") for the various possible quantum-mechanical states, thus explaining the anisotropiccharacter of atomic bonds.Sony VAIO VPCEB1NFX/L Battery

The Schrödinger equation also applies to more complicated atoms and molecules. However, in most such cases the solution is not analytical and either computer calculations are necessary or simplifying assumptions must be made.Sony VAIO VPCEB1NFX/P Battery

Solution of Schrödinger equation: Overview of results

The solution of the Schrödinger equation (wave equations) for the hydrogen atom uses the fact that the Coulomb potential produced by the nucleus is isotropic (it is radially symmetric in space and only depends on the distance to the nucleus). Sony VAIO VPCEB1NFX/W Battery

Although the resultingenergy eigenfunctions (the orbitals) are not necessarily isotropic themselves, their dependence on the angular coordinates follows completely generally from this isotropy of the underlying potential: The eigenstates of the Hamiltonian (that is, the energy eigenstates) can be chosen as simultaneous eigenstates of the angular momentum operator. Sony VAIO VPCEB1PFX Battery

This corresponds to the fact that angular momentum is conserved in the orbital motion of the electron around the nucleus. Therefore, the energy eigenstates may be classified by two angular momentum quantum numbers, ? and m (both are integers). The angular momentum quantum number ? = 0, 1, 2, ...determines the magnitude of the angular momentum.Sony VAIO VPCEB1PFX/B Battery

The magnetic quantum number m = ??, ..., +? determines the projection of the angular momentum on the (arbitrarily chosen) z-axis.

In addition to mathematical expressions for total angular momentum and angular momentum projection of wavefunctions, an expression for the radial dependence of the wave functions must be found.Sony VAIO VPCEB1QGX Battery

It is only here that the details of the 1/r Coulomb potential enter (leading to Laguerre polynomialsin r). This leads to a third quantum number, the principal quantum number n = 1, 2, 3, .... The principal quantum number in hydrogen is related to atom's total energy.

Note that the maximum value of the angular momentum quantum number is limited by the principal quantum number: it can run only up to n ? 1, i.e.? = 0, 1, ..., n ? 1.Sony VAIO VPCEB1QGX/BI Battery

Due to angular momentum conservation, states of the same ? but different m have the same energy (this holds for all problems with rotational symmetry). In addition, for the hydrogen atom, states of the same n but different ? are also degenerate (i.e. they have the same energy).Sony VAIO VPCEB1RGX Battery

However, this is a specific property of hydrogen and is no longer true for more complicated atoms which have a (effective) potential differing from the form 1/r (due to the presence of the inner electrons shielding the nucleus potential).

Taking into account the spin of the electron adds a last quantum number, the projection of the electron's spin angular momentum along the z-axis, which can take on two values.Sony VAIO VPCEB1RGX/BI Battery

Therefore, any eigenstate of the electron in the hydrogen atom is described fully by four quantum numbers. According to the usual rules of quantum mechanics, the actual state of the electron may be any superposition of these states.Sony VAIO VPCEB20 Battery

This explains also why the choice of z-axis for the directional quantization of the angular momentum vector is immaterial: an orbital of given ? and m? obtained for another preferred axis z? can always be represented as a suitable superposition of the various states of different m (but same l) that have been obtained for z.Sony VAIO VPCEC20 Battery

Alternatives to the Schrödinger Theory

In the language of Heisenberg's Matrix Mechanics, the hydrogen atom was first solved by Wolfgang Pauli using a rotational symmetry in four dimension [O(4)-symmetry] generated by the angular momentum and the Laplace–Runge–Lenz vector.Sony VAIO VPCEE20 Battery

By extending the symmetry group O(4) to the dynamical group O(4,2), the entire spectrum and all transitions were embedded in a single irreducible group representation.

In 1979 the (non relativistic) hydrogen atom was solved for the first time within Feynman's path integral formulation of quantum mechanics.[3][4] This work greatly extended the range of applicability of Feynman's method.Sony VAIO VPCEF20 Battery

The image to the right shows the first few hydrogen atom orbitals (energy eigenfunctions). These are cross-sections of the probability density that are color-coded (black represents zero density and white represents the highest density). The angular momentum (orbital) quantum number ? is denoted in each column, using the usual spectroscopic letter code (s means ? = 0, p means ? = 1, dmeans ? = 2). Sony VAIO VPCF112FX/B Battery

The main (principal) quantum number n (= 1, 2, 3, ...) is marked to the right of each row. For all pictures the magnetic quantum number m has been set to 0, and the cross-sectional plane is the xz-plane (z is the vertical axis). The probability density in three-dimensional space is obtained by rotating the one shown here around the z-axis.Sony VAIO VPCF115FG/B Battery

The "ground state", i.e. the state of lowest energy, in which the electron is usually found, is the first one, the 1s state (principal quantum level n = 1, ? = 0).

An image with more orbitals is also available (up to higher numbers n and ?).

Black lines occur in each but the first orbital: these are the nodes of the wavefunction, i.e. where the probability density is zero.Sony VAIO VPCF116FGBI Battery

(More precisely, the nodes are Spherical harmonics that appear as a result of solving Schrödinger's equation in polar coordinates.)

Features going beyond the Schrödinger solution

There are several important effects that are neglected by the Schrödinger equation and which are responsible for certain small but measurable deviations of the real spectral lines from the predicted ones:Sony VAIO VPCF117FJ/W Battery

  • Although the mean speed of the electron in hydrogen is only 1/137th of the speed of light, many modern experiments are sufficiently precise that a complete theoretical explanation requires a fully relativistic treatment of the problem. A relativistic treatment results in a momentum increase of about one part in 37,000 for the electron. Sony VAIO VPCF117HG/BI Battery
  • Since the electron's wavelength is determined by its momentum, orbitals containing higher speed electrons show contraction due to smaller wavelengths.
  • Even when there is no external magnetic field, in the inertial frame of the moving electron, the electromagnetic field of the nucleus has a magnetic component. Sony VAIO VPCF118FJ/W Battery
  • The spin of the electron has an associated magnetic moment which interacts with this magnetic field. This effect is also explained by special relativity, and it leads to the so-called spin-orbit coupling, i.e., an interaction between the electron's orbital motion around the nucleus, and its spin.Sony VAIO VPCF119FC Battery

Both of these features (and more) are incorporated in the relativistic Dirac equation, with predictions that come still closer to experiment. Again the Dirac equation may be solved analytically in the special case of a two-body system, such as the hydrogen atom.Sony VAIO VPCF119FC/BI Battery

The resulting solution quantum states now must be classified by the total angular momentum number j (arising through the coupling between electron spin and orbital angular momentum). States of the same j and the same n are still degenerate.

  • There are always vacuum fluctuations of the electromagnetic field, according to quantum mechanics. Sony VAIO VPCF119FJ/BI Battery
  • Due to such fluctuations degeneracy between states of the same j but different l is lifted, giving them slightly different energies. This has been demonstrated in the famous Lamb-Retherford experiment and was the starting point for the development of the theory of Quantum electrodynamics (which is able to deal with these vacuum fluctuations and employs the famous Feynman diagramsfor approximations using perturbation theory).Sony VAIO VPCF11AFJ Battery
  • This effect is now called Lamb shift.

For these developments, it was essential that the solution of the Dirac equation for the hydrogen atom could be worked out exactly, such that any experimentally observed deviation had to be taken seriously as a signal of failure of the theory.Sony VAIO VPCF11AGJ Battery

Due to the high precision of the theory also very high precision for the experiments is needed, which utilize a frequency comb.

Hydrogen ion

Hydrogen is not found without its electron in ordinary chemistry (room temperatures and pressures), as ionized hydrogen is highly chemically reactive. Sony VAIO VPCF11AHJ Battery

When ionized hydrogen is written as "H+" as in the solvation of classical acids such hydrochloric acid, the hydronium ion, H3O+, is meant, not a literal ionized single hydrogen atom. In that case, the acid transfers the proton to H2O to form H3O+.

Ionized hydrogen without its electron, or free protons, are common in the interstellar medium, and solar wind.Sony VAIO VPCF11JFX/B Battery

A hydrogen bond is the attractive interaction of a hydrogen atom with an electronegative atom, such as nitrogen,oxygen or fluorine, that comes from another molecule or chemical group. The hydrogen must be covalently bonded to another electronegative atom to create the bond. Sony VAIO VPCF11M1E Battery

These bonds can occur between molecules (intermolecularly), or within different parts of a single molecule (intramolecularly). The hydrogen bond (5 to 30 kJ/mole) is stronger than a van der Waals interaction, but weaker than covalent or ionic bonds. This type of bond occurs in both inorganic molecules such as water and organic molecules such as DNA.Sony VAIO VPCF11M1E/H Battery

Intermolecular hydrogen bonding is responsible for the high boiling point of water (100 °C) compared to the othergroup 16 hydrides that have no hydrogen bonds. Intramolecular hydrogen bonding is partly responsible for thesecondary, tertiary, and quaternary structures of proteins and nucleic acids. Sony VAIO VPCF11MFX/B Battery

It also plays an important role in the structure of polymers, both synthetic and natural.

A task group formed by IUPAC has come up with a modern definition of hydrogen bonding in 2011. This new definition can be seen in the IUPAC journal Pure and Applied Chemistry. A detailed technical report provides the rationale behind the new definition.Sony VAIO VPCF11S1E Battery

Bonding

A hydrogen atom attached to a relatively electronegative atom is a hydrogen bond donor. This electronegative atom is usually fluorine, oxygen, or nitrogen. An electronegative atom such as fluorine, oxygen, or nitrogen is a hydrogen bond acceptor, regardless of whether it is bonded to a hydrogen atom or not. Sony VAIO VPCF11S1E/B Battery

An example of a hydrogen bond donor is ethanol, which has a hydrogen bonded to oxygen; an example of a hydrogen bond acceptor which does not have a hydrogen atom bonded to it is the oxygen atom on diethyl ether.

A hydrogen attached to carbon can also participate in hydrogen bonding when the carbon atom is bound to electronegative atoms, as is the case in chloroform, CHCl3. Sony VAIO VPCF11Z1E Battery

The electronegative atom attracts the electron cloud from around the hydrogen nucleus and, by decentralizing the cloud, leaves the atom with a positive partial charge. Because of the small size of hydrogen relative to other atoms and molecules, the resulting charge, though only partial, represents a large charge density. Sony VAIO VPCF11Z1E/BI Battery

A hydrogen bond results when this strong positive charge density attracts a lone pair of electrons on another heteroatom, which becomes the hydrogen-bond Acceptor.

The hydrogen bond is often described as an electrostatic dipole-dipole interaction.  Sony VAIO VPCF11ZHJ Battery

However, it also has some features of covalent bonding: it is directional and strong, produces interatomic distances shorter than sum of van der Waals radii, and usually involves a limited number of interaction partners, which can be interpreted as a type of valence. These covalent features are more substantial when acceptors bind hydrogens from more electronegative donors.Sony VAIO VPCF127HGBI Battery

The partially covalent nature of a hydrogen bond raises the following questions: "To which molecule or atom does the hydrogen nucleus belong?" and "Which should be labeled 'donor' and which 'acceptor'?" Usually, this is simple to determine on the basis of interatomic distances in the X?H…Y system: X?H distance is typically ?110 pm, whereas H…Y distance is ?160 to 200 pm.Sony VAIO VPCF137HG/BI Battery

Liquids that display hydrogen bonding are called associated liquids.

Hydrogen bonds can vary in strength from very weak (1–2 kJ mol?1) to extremely strong (161.5 kJ mol?1 in the ionHF?
2). Typical enthalpies in vapor include:Sony VAIO VPC-P111KX/B Battery

  • 3 (18 kJ/mol or 4.3 kcal/mol; data obtained using molecular dynamics as detailed in the reference and should be compared to 7.9 kJ/mol for bulk waters, obtained using the same molecular dynamics.)

The length of hydrogen bonds depends on bond strength, temperature, and pressure. Sony VAIO VPC-P111KX/D Battery

The bond strength itself is dependent on temperature, pressure, bond angle, and environment (usually characterized by local dielectric constant). The typical length of a hydrogen bond in water is 197 pm. The ideal bond angle depends on the nature of the hydrogen bond donor.Sony VAIO VPC-P111KX/G Battery

History

In the book The Nature of the Chemical Bond, Linus Pauling credits T. S. Moore and T. F. Winmill with the first mention of the hydrogen bond, in 1912. Moore and Winmill used the hydrogen bond to account for the fact that trimethylammonium hydroxide is a weaker base than tetramethylammonium hydroxide. Sony VAIO VPC-P111KX/P Battery

The description of hydrogen bonding in its more well-known setting, water, came some years later, in 1920, from Latimer and Rodebush. In that paper, Latimer and Rodebush cite work by a fellow scientist at their laboratory, Maurice Loyal Huggins, saying,Sony VAIO VPC-P111KX/W Battery

"Mr. Huggins of this laboratory in some work as yet unpublished, has used the idea of a hydrogen kernel held between two atoms as a theory in regard to certain organic compounds."

Hydrogen bonds in water

The most ubiquitous, and perhaps simplest, example of a hydrogen bond is found betweenwater molecules. In a discrete water molecule, there are two hydrogen atoms and one oxygen atom. Sony VAIO VPC-P112KX/B Battery

Two molecules of water can form a hydrogen bond between them; the simplest case, when only two molecules are present, is called the water dimer and is often used as a model system. When more molecules are present, as is the case of liquid water, more bonds are possible because the oxygen of one water molecule has two lone pairs of electrons, Sony VAIO VPC-P112KX/D Battery

each of which can form a hydrogen bond with a hydrogen on another water molecule. This can repeat such that every water molecule is H-bonded with up to four other molecules, as shown in the figure (two through its two lone pairs, and two through its two hydrogen atoms). Hydrogen bonding strongly affects the crystal structure of ice, helping to create an open hexagonal lattice.Sony VAIO VPC-P112KX/G Battery

The density of ice is less than water at the same temperature; thus, the solid phase of water floats on the liquid, unlike most other substances.

Liquid water's high boiling point is due to the high number of hydrogen bonds each molecule can form relative to its low molecular mass. Sony VAIO VPC-P112KX/P Battery

Owing to the difficulty of breaking these bonds, water has a very high boiling point, melting point, and viscosity compared to otherwise similar liquids not conjoined by hydrogen bonds. Water is unique because its oxygen atom has two lone pairs and two hydrogen atoms, meaning that the total number of bonds of a water molecule is up to four. Sony VAIO VPC-P112KX/W Battery

For example, hydrogen fluoride—which has three lone pairs on the F atom but only one H atom—can form only two bonds; (ammonia has the opposite problem: three hydrogen atoms but only one lone pair).

The exact number of hydrogen bonds formed by a molecule of liquid water fluctuates with time and depends on the temperature.Sony VAIO VPCP113KX/B Battery

From TIP4P liquid water simulations at 25 °C, it was estimated that each water molecule participates in an average of 3.59 hydrogen bonds. At 100 °C, this number decreases to 3.24 due to the increased molecular motion and decreased density, while at 0 °C, the average number of hydrogen bonds increases to 3.69.Sony VAIO VPC-P113KX/B Battery

A more recent study found a much smaller number of hydrogen bonds: 2.357 at 25 °C. The differences may be due to the use of a different method for defining and counting the hydrogen bonds.

Where the bond strengths are more equivalent, one might instead find the atoms of two interacting water molecules partitioned into two polyatomic ions of opposite charge, specifically hydroxide (OH?) and hydronium (H3O+).Sony VAIO VPCP113KX/D Battery

(Hydronium ions are also known as 'hydroxonium' ions.)Indeed, in pure water under conditions of standard temperature and pressure, this latter formulation is applicable only rarely; on average about one in every 5.5 × 108 molecules gives up a proton to another water molecule, in accordance with the value of the dissociation constant for water under such conditions. Sony VAIO VPC-P113KX/D Battery

It is a crucial part of the uniqueness of water.

Because water forms hydrogen bonds with the donors and acceptors on solutes dissolved within it, it inhibits the formation of a hydrogen bond between two molecules of those solutes or the formation of intramolecular hydrogen bonds within those solutes through competition for their donors and acceptors. Sony VAIO VPCP113KX/G Battery

Consequently, hydrogen bonds between or within solute molecules dissolved in water are almost always unfavorable relative to hydrogen bonds between water and the donors and acceptors for hydrogen bonds on those solutes.

Bifurcated and over-coordinated hydrogen bonds in water

It can be that a single hydrogen atom participates in two hydrogen bonds, rather than one. This type of bonding is called "bifurcated" (split in two or 'two-forked'). Sony VAIO VPC-P113KX/G Battery

It can exist for instance in complex natural or synthetic organic molecules It was suggested that a bifurcated hydrogen atom is an essential step in water reorientation.
Acceptor type hydrogen bonds (terminating on an oxygen's lone pairs), are more likely to form bifurcation (it is called overcoordinated oxygen, OCO) than donor type, beginning on the same oxygen's hydrogens.Sony VAIO VPCP113KX/P Battery

Hydrogen bonds in DNA and proteins

Hydrogen bonding also plays an important role in determining the three-dimensional structures adopted by proteins and nucleic bases. In these macromolecules, bonding between parts of the same macromolecule cause it to fold into a specific shape, which helps determine the molecule's physiological or biochemical role. Sony VAIO VPC-P113KX/P Battery

The double helical structure of DNA, for example, is due largely to hydrogen bonding between the base pairs, which link one complementary strand to the other and enable replication.

In the secondary structure of proteins, hydrogen bonds form between the backbone oxygens and amide hydrogens. Sony VAIO VPCP113KX/W Battery

When the spacing of theamino acid residues participating in a hydrogen bond occurs regularly between positions i and i + 4, an alpha helix is formed. When the spacing is less, between positions i and i + 3, then a 310 helix is formed. When two strands are joined by hydrogen bonds involving alternating residues on each participating strand, a beta sheet is formed.Sony VAIO VPC-P113KX/W Battery

Hydrogen bonds also play a part in forming the tertiary structure of protein through interaction of R-groups. (See also protein folding).

Hydrogen bonds in polymers

Many polymers are strengthened by hydrogen bonds in their main chains. Among the synthetic polymers, the best known example is nylon, where hydrogen bonds occur in the repeat unit and play a major role in crystallization of the material.Sony VAIO VPC-P114KX/B Battery

The bonds occur betweencarbonyl and amine groups in the amide repeat unit. They effectively link adjacent chains to create crystals, which help reinforce the material. The effect is greatest in aramid fibre, where hydrogen bonds stabilize the linear chains laterally. The chain axes are aligned along the fibre axis, making the fibres extremely stiff and strong. Sony VAIO VPC-P114KX/D Battery

Hydrogen bonds are also important in the structure ofcellulose and derived polymers in its many different forms in nature, such as wood and natural fibres such as cotton and flax.

The hydrogen bond networks make both natural and synthetic polymers sensitive to humidity levels in the atmosphere because water molecules can diffuse into the surface and disrupt the network. Some polymers are more sensitive than others. Sony VAIO VPC-P114KX/G Battery

Thus nylons are more sensitive than aramids, and nylon 6 more sensitive than nylon-11.

Symmetric hydrogen bond

A symmetric hydrogen bond is a special type of hydrogen bond in which the proton is spaced exactly halfway between two identical atoms. The strength of the bond to each of those atoms is equal. It is an example of a 3-center 4-electron bond. Sony VAIO VPC-P114KX/P Battery

This type of bond is much stronger than "normal" hydrogen bonds. The effective bond order is 0.5, so its strength is comparable to a covalent bond. It is seen in ice at high pressure, and also in the solid phase of many anhydrous acids such as hydrofluoric acid and formic acid at high pressure. It is also seen in the bifluoride ion [F?H?F]?.Sony VAIO VPC-P114KX/W Battery

Symmetric hydrogen bonds have been observed recently spectroscopically in formic acid at high pressure (>GPa). Each hydrogen atom forms a partial covalent bond with two atoms rather than one. Symmetric hydrogen bonds have been postulated in ice at high pressure (Ice X). Sony VAIO VPCP115JC Battery

Low-barrier hydrogen bonds form when the distance between two heteroatoms is very small.

Dihydrogen bond

The hydrogen bond can be compared with the closely related dihydrogen bond, which is also anintermolecular bonding interaction involving hydrogen atoms. Sony VAIO VPCP115JC/B Battery

These structures have been known for some time, and well characterized by crystallography;however, an understanding of their relationship to the conventional hydrogen bond, ionic bond, and covalent bond remains unclear. Generally, the hydrogen bond is characterized by a proton acceptor that is a lone pair of electrons in nonmetallic atoms Sony VAIO VPCP115JC/D Battery

(most notably in the nitrogen, and chalcogen groups). In some cases, these proton acceptors may be pi-bonds ormetal complexes. In the dihydrogen bond, however, a metal hydride serves as a proton acceptor; thus forming a hydrogen-hydrogen interaction. Sony VAIO VPCP115JC/G Battery

Neutron diffraction has shown that the molecular geometry of these complexes is similar to hydrogen bonds, in that the bond length is very adaptable to the metal complex/hydrogen donor system.

Advanced theory of the hydrogen bond

In 1999, Isaacs et al. proved from interpretations of the anisotropies in the Compton profile of ordinary ice, that the hydrogen bond is partly covalent. Some NMR data on hydrogen bonds in proteins also indicate covalent bonding.Sony VAIO VPCP115JC/P Battery

Most generally, the hydrogen bond can be viewed as a metric-dependent electrostatic scalar field between two or more intermolecular bonds. This is slightly different from the intramolecular bound states of, for example, covalent or ionic bonds;Sony VAIO VPCP115JC/W Battery

however, hydrogen bonding is generally still a bound state phenomenon, since the interaction energy has a net negative sum. The initial theory of hydrogen bonding proposed by Linus Pauling suggested that the hydrogen bonds had a partial covalent nature. This remained a controversial conclusion until the late 1990s when NMR techniques were employed by F. Sony VAIO VPCP115KG Battery

Cordier et al. to transfer information between hydrogen-bonded nuclei, a feat that would only be possible if the hydrogen bond contained some covalent character. While much experimental data has been recovered for hydrogen bonds in water, Sony VAIO VPCP116KG Battery

for example, that provide good resolution on the scale of intermolecular distances and molecular thermodynamics, the kinetic anddynamical properties of the hydrogen bond in dynamic systems remains unchanged.Sony VAIO VPC-P116KX/B Battery

Hydrogen bonding phenomena

  • Dramatically higher boiling points of NH3, H2O, and HF compared to the heavier analogues PH3, H2S, and HCl.
  • Increase in the melting point, boiling point, solubility, and viscosity of many compounds can be explained by the concept of hydrogen bonding.Sony VAIO VPC-P116KX/D Battery
  • Viscosity of anhydrous phosphoric acid and of glycerol
  • Dimer formation in carboxylic acids and hexamer formation in hydrogen fluoride, which occur even in the gas phase, resulting in gross deviations from the ideal gas law.
  • Pentamer formation of water and alcohols in apolar solvents.
  • High water solubility of many compounds such as ammonia is explained by hydrogen bonding with water molecules.
  • Negative azeotropy of mixtures of HF and water.Sony VAIO VPC-P116KX/G Battery
  • Deliquescence of NaOH is caused in part by reaction of OH? with moisture to form hydrogen-bonded H3O?
    2 species. An analogous process happens between NaNH2 and NH3, and between NaF and HF.
  • The fact that ice is less dense than liquid water is due to a crystal structure stabilized by hydrogen bonds.Sony VAIO VPC-P116KX/P Battery
  • The presence of hydrogen bonds can cause an anomaly in the normal succession of states of matter for certain mixtures of chemical compounds as temperature increases or decreases. These compounds can be liquid until a certain temperature, then solid even as the temperature increases, and finally liquid again as the temperature rises over the "anomaly interval".Sony VAIO VPC-P116KX/W Battery
  • Smart rubber utilizes hydrogen bonding as its sole means of bonding, so that it can "heal" when torn, because hydrogen bonding can occur on the fly between two surfaces of the same polymer.
  • Strength of nylon and cellulose fibres. Sony VAIO VPCP118JC Battery
  • Wool, being a protein fibre is held together by hydrogen bonds, causing wool to recoil when stretched. However, washing at high temperatures can permanently break the hydrogen bonds and a garment may permanently lose its shape.Sony VAIO VPCP118JC/B Battery

Hydrogen production is the industrial method for generating hydrogen. Currently the dominant technology for direct production is steam reforming fromhydrocarbons. Hydrogen is also produced as a byproduct of other processes and managed with hydrogen pinch.Sony VAIO VPCP118JC/P Battery

Many other methods are known including electrolysis andthermolysis. The discovery and development of less expensive methods of production of bulk hydrogen is relevant to the establishment of a hydrogen economy.

Hydrogen waste stream

Hydrogen is used for the following.

  • Ammonia production via Haber process in fertilizer industries, Sony VAIO VPCP118JC/W Battery
  • Conversion of heavy petroleum fractions into lighter ones via hydrocracking and other petroleum fractions (dehydrocyclization and the aromatization process).

Earlier it was common to vent the surplus hydrogen off, nowadays the process systems are balanced with hydrogen pinch to collect hydrogen for further use.Sony VAIO VPC-P118KX/B Battery

Hydrogen is also produced as a by-product of industrial chlorine production by electrolysis. It can be cooled, compressed and purified for use in other processes on site or sold to a customer via pipeline, cylinders or trucks.

Steam reforming

Fossil fuel currently is the main source of hydrogen production.Sony VAIO VPC-P118KX/D Battery

Hydrogen can be generated from natural gas with approximately 80% efficiency, or from otherhydrocarbons to a varying degree of efficiency. Specifically, bulk hydrogen is usually produced by the steam reforming of methane or natural gas[4] At high temperatures (700–1100 °C), steam (H2O) reacts with methane (CH4) to yield syngas.Sony VAIO VPC-P118KX/G Battery

Essentially, the oxygen (O) atom is stripped from the additional water (steam) to oxidize CO to CO2. This oxidation also provides energy to maintain the reaction. Additional heat required to drive the process is generally supplied by burning some portion of the methane.Sony VAIO VPC-P118KX/P Battery

Steam reforming generates carbon dioxide (CO2). Since the production is concentrated in one facility, it is possible to separate the CO2 and dispose of it properly, for example by injecting it in an oil or gas reservoir (see carbon capture), although this is not currently done in most cases. Sony VAIO VPC-P118KX/W Battery

A carbon dioxide injection project has been started by a Norwegian company StatoilHydro in the North Sea, at the Sleipner field. This is disputed in The Hype about Hydrogen: Fact and Fiction in the Race to Save the Climate, a book byJoseph J. Romm, published in 2004 by Island Press and updated in 2005. Sony VAIO VPCP119JC Battery

Romm says that directly burning fossil fuels generates less CO2 than hydrogen production.

Integrated steam reforming / co-generation - It is possible to combine steam reforming and co-generation of steam and power into a single plant. This can deliver benefits for an oil refinery because it is more efficient than separate hydrogen, steam and power plants.Sony VAIO VPCP119JC/BI Battery

Air Products recently built an integrated steam reforming /co-generation plant in Port Arthur, Texas.

Plasma reforming

The Kværner-process or Kvaerner carbon black & hydrogen process (CB&H) is a plasma reforming method, developed in the 1980s by a Norwegian company of the same name, for the production of hydrogen and carbon black from liquid hydrocarbons (CnHm).Sony VAIO VPCP11S1E/B Battery

Of the available energy of the feed, approximately 48% is contained in the hydrogen, 40% is contained in activated carbon and 10% in superheated steam. CO2 is not produced in the process.

A variation of this process is presented in 2009 using plasma arc waste disposal technology for the production of hydrogen, heat and carbon from methane and natural gas in a plasma converter.Sony VAIO VPC-P11S1E/B Battery

Coal

Coal can be converted into syngas and methane, also known as town gas, via coal gasification. Syngas consists of hydrogen and carbon monoxide.[9] Another method for conversion is low temperature and high

temperature coal carbonization.Sony VAIO VPCP11S1E/D Battery

Electrolysis and thermolysis

Hydrogen is produced on an industrial scale by the electrolysis of water. While this can be done with a few volts in a simple apparatus like a Hofmann voltameter, larger scale production usually relies on high-pressure and high-temperature systems to improve the energy efficiency of electrolysis. Sony VAIO VPC-P11S1E/D Battery

Experimental processes include electrolysis at very high temperatures (800 C), so that much of the energy required to release hydrogen is supplied as heat instead of electricity. Various catalytic agents are being studied to improve the efficiency of high-temperature electrolysis.Sony VAIO VPCP11S1E/G Battery

Water spontaneously dissociates at around 2500 C, but this thermolysis occurs at temperatures too high for usual process piping and equipment. Catalysts are required to reduce the dissociation temperature.

Photocatalytic water splitting

The conversion of solar energy to hydrogen by means of water splitting process is one of the most interesting ways to achieve clean and renewable energy systems. Sony VAIO VPC-P11S1E/G Battery

However if this process is assisted by photocatalysts suspended directly in water instead of using photovoltaic and an electrolytic system the reaction is in just one step, therefore it can be more efficient.

Sulfur-iodine cycle

The sulfur-iodine cycle (S-I cycle) is a thermochemical process which generates hydrogen from water, but at a much higher efficiency than water splitting. Sony VAIO VPCP11S1E/P Battery

The sulfur and iodine used in the process are recovered and reused, and not consumed by the process. It is well suited to production of hydrogen by high-temperature nuclear reactors or by concentrating solar power systems (CSP).

From urine

Hydrogen can also be made from urine. Using urine, hydrogen production is 332% more energy efficient than using water.Sony VAIO VPCP11S1E/W Battery

The research was conducted by Geraldine Botte from the Ohio University. Recently, Dr. Shanwen Tao of the Heriot-Watt University has invented a Carbamide Power System Fuel Cell that can immediately convert urine into electricity.Sony VAIO VPC-P11S1E/W Battery

Biohydrogen routes

Biomass and waste streams can be converted into biohydrogen with biomass gasification, steam reforming or biological conversion like biocatalysed electrolysis or fermentative hydrogen production:Sony VAIO VPCP11Z9E/B Battery

Fermentative hydrogen production

Fermentative hydrogen production is the fermentative conversion of organic substrate to biohydrogen manifested by a diverse group of bacteria using multi enzymesystems involving three steps similar to anaerobic conversion.Sony VAIO VPCS111FM/S Battery

Dark fermentation reactions do not require light energy, so they are capable of constantly producinghydrogen from organic compounds throughout the day and night. Photofermentation differs from dark fermentation because it only proceeds in the presence of light.Sony VAIO VPCS115EC Battery

For example photo-fermentation with Rhodobacter sphaeroides SH2C can be employed to convert small molecular fatty acids into hydrogen.

Fermentative hydrogen production can be done using direct biophotolysis by green algae, indirect biophotolysis by cyanobacteria, photo-fermentation by anaerobic photosynthetic bacteria and dark fermentation by anaerobic fermentative bacteria.Sony VAIO VPCS115FG Battery

For example studies on hydrogen production using H. salinarium, an anaerobic photosynthetic bacteria, coupled to a hydrogenase donor like E. coli, are reported in literature .

Biohydrogen can be produced in bioreactors that utilize feedstocks, the most common feedstock being waste streams. Sony VAIO VPCS117GG Battery

The process involves bacteria feeding on hydrocarbons and exhaling hydrogen and CO2. The CO2 can be sequestered successfully by several methods, leaving hydrogen gas. A prototype hydrogen bioreactor using waste as a feedstock is in operation at Welch's grape juice factory in North East, Pennsylvania (U.S.).Sony VAIO VPCS117GGB Battery

Enzymatic hydrogen generation

Due to the Thauer limit (four H2/glucose) for dark fermentation, a non-natural enzymatic pathway was designed that can generate 12 moles of hydrogen per mole of glucose units of polysaccharides and water in 2007. The stoichiometric reaction is C6H10O5+ 7 H2O --> 12 H2 + 6 CO2.Sony VAIO VPCS118EC Battery

The key technology is cell-free synthetic enzymatic pathway biotransformation (SyPaB). A biochemist can understand it as "glucose oxidation by using water as oxidant". A chemist can describe it as "water splitting by energy in carbohydrate". Sony VAIO VPCS119FJ/B Battery

A thermodynamics scientist can describe it as the first entropy-driving chemical reaction that can produce hydrogen by absorbing waste heat. In 2009, cellulosic materials were first used to generate high-yield hydrogen.Sony VAIO VPCS119GC Battery

Furthermore, the use of carbohydrate as a high-density hydrogen carrier was proposed so to solve the largest obstacle to the hydrogen economy and propose the concept of sugar fuel cell vehicles.Sony VAIO VPCS11AFJ Battery,Sony VAIO VPCS11AGJ Battery,Sony VAIO VPCS11AHJ Battery,Sony VAIO VPCS11AVJ Battery,Sony VAIO VPCS11J7E/B Battery

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