Hydrogen

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Hydrogen (Symbol H.) is a colourless odourless gaseous chemical element; a.n.is 1; r.a.m. is 1.008; d. is 0.0899 g dm^-3; m.p. is -259.14°C; b.p. is -252.87°C. It is the lightest element and the most abundant in the universe. It is present in water and in all organic compounds. There are three isotopes: naturally occurring hydrogen consists of the two stable isotopes hydrogen-1 (99.985%) and deuterium. The radioactive tritium is made artificially. The gas is diatomic and has two forms: orthohydrogen, in which the nuclear spins are parallel, and parahydrogen, in which they are antiparallel. At normal temperatures the gas is 25% of parahydrogen. In the liquid it is 99.8% of parahydrogen. The main source of hydrogen is steam reforming of natural gas. It can also be made by the Bosch process and by electrolysis of water. The main use is in the Haber process for making ammonia. Hydrogen is also used in various other industrial processes, such as the reduction of oxide ores, the refining of petroleum, the production of hydrocarbons from coal, and the hydrogenation of vegetable oils. Considerable interest has also been shown in its potential use in a 'hydrogen fuel economy' in which primary energy sources not based on fossil fuels (e.g. nuclear, solar or geothermal energy) are used to produce electricity, which is employed in electrolysing water. The hydrogen formed is stored as liquid hydrogen or as metal hydrides. Chemically, hydrogen reacts with many elements. It was discovered by Henry Cavendish in 1776.

Hammett equation

Hammett equation is an equation relating the structure to the reactivity of side-chain derivatives of aromatic compounds. It arises from a comparison between rate constants for various reactions with the rate of hydrolysis of benzyl chloride on the one hand and a comparison between equilibrium constants (such as the dissociation constant of benzoic acid) on the other hand. The Hammett equation can be written in the form log(k/k₀) = plog (K/Ko), where log (K/K₀) refers to comparing dissociation constants to the dissociation constant, K_o, of benzoic acid in water at 25°C, and log (k/k₀) refers to comparing rates of reaction to the rate, k₀, of hydrolysis of benzyl chloride. The term log (K/K₀) = a is called the substituent constant, since the nature of the substituent affects the strength of the benzoic acid. If  is positive, the substituent is electron attracting, while if  is negative the substituent is electron donating, p is a reaction constant, which is determined for a given reaction by the slope of a graph of log (k/k₀) against . The numerical value of p depends on temperature and the type of solvent.

The Hammett equation applies to meta- and parasubstituents (provided that resonance interaction from the substituents does not occur) but not to ortho-substituents.

Albert Einstein

Albert Einstein (1879-1955) is a German-born US physicist, who took Swiss nationality in 1901. A year later he went to work in the Bern patent office. In 1905 he published five enormously influential papers, one on Brownian movement, one on the photoelectric effect, one on the special theory of relativity and one on energy and inertia (which included the famous expression E = mc²). In 1915 he published the general theory of relativity, concerned mainly with gravitation. In 1921 he was awarded the Nobel Prize for Physics. In 1933 as a Jew, Einstein decided to remain in the USA (where he was lecturing), as Hitler had come to power. For all his life he sought a unified field theory. In 1939 he informed President Roosevelt that an atom bomb was feasible and that Germany might be able to make one.

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Einstein coefficients are coefficients used in the quantum theory of radiation, related to the probability of a transition occurring between the ground state and an excited state (or vice versa) in the processes of induced emission and spontaneous emission. For an atom exposed to electromagnetic radiation, the rate of absorption R_a is given by R_a = Bp, where p is the density of electromagnetic radiation and В is the Einstein В coefficient associated with absorption. The rate of induced emission is also given by Bp, with the coefficient В of induced emission being equal to the coefficient of absorption. The rate of spontaneous emission is given by A, where A is the Einstein A coefficient of spontaneous emission. The A and В coefficients are related by A = 8πhv³B/c³, where h is the Planck constant, v is the frequency of electromagnetic radiation, and с is the speed of light. The coefficients were put forward by Albert Einstein in 1916-17 in his analysis of the quantum theory of radiation.

Einstein equation is the mass-energy relationship announced by Albert Einstein in 1905 in the form E = mc², where E is a quantity of energy, m its mass, and с is the speed of light. It presents the concept that energy possesses mass. The relationship E_max = hf - W, where E_max is the maximum kinetic energy of electrons emitted in the photoemissive effect, h is the Planck constant,  is the frequency of the incident radiation, and W is the work function of the emitter. This is also written E_max = hf - φе, where e is the electronic charge and φ is a potential difference, also called the work function. (Sometimes W and φ are distinguished as work function energy and work function potential.) The equation can also be applied to photoemission from gases, when it has the form: E = hf - I, where I is the ionization potential of the gas.

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