Valence shell electron pair repulsion theory is a model used in chemistry to predict the geometry of individual molecules from the number of electron pairs surrounding their central atoms. It is named the Gillespie-Nyholm theory after its two main developers, Ronald Gillespie and Ronald Nyholm; the acronym "VSEPR" is pronounced either "ves-pur" or "vuh-seh-per". The premise of VSEPR is that the valence electron pairs surrounding an atom tend to repel each other and will, adopt an arrangement that minimizes this repulsion, thus determining the molecule's geometry. Gillespie has emphasized that the electron-electron repulsion due to the Pauli exclusion principle is more important in determining molecular geometry than the electrostatic repulsion. VSEPR theory is based on observable electron density rather than mathematical wave functions and hence unrelated to orbital hybridisation, although both address molecular shape. While it is qualitative, VSEPR has a quantitative basis in quantum chemical topology methods such as the electron localization function and the quantum theory of atoms in molecules.
The idea of a correlation between molecular geometry and number of valence electron pairs was proposed in 1939 by Ryutaro Tsuchida in Japan, was independently presented in a Bakerian Lecture in 1940 by Nevil Sidgwick and Herbert Powell of the University of Oxford. In 1957, Ronald Gillespie and Ronald Sydney Nyholm of University College London refined this concept into a more detailed theory, capable of choosing between various alternative geometries. In recent years, VSEPR theory has been criticized as an outdated model from the standpoint of both scientific accuracy and pedagogical value. In particular, the equivalent lone pairs of water and carbonyl compounds in VSEPR theory neglect fundamental differences in the symmetries of molecular orbitals and natural bond orbitals that correspond to them, a difference, sometimes chemically significant. Furthermore, there is little evidence, computational or experimental, proposing that lone pairs are "bigger" than bonding pairs, it has been suggested that Bent's rule is capable of replacing VSEPR as a simple model for explaining molecular structure.
VSEPR theory captures many of the essential features of the structure and electron distribution of simple molecules, most undergraduate general chemistry courses continue to teach it. VSEPR theory is used to predict the arrangement of electron pairs around non-hydrogen atoms in molecules simple and symmetric molecules, where these key, central atoms participate in bonding to two or more other atoms; the number of electron pairs in the valence shell of a central atom is determined after drawing the Lewis structure of the molecule, expanding it to show all bonding groups and lone pairs of electrons. In VSEPR theory, a double bond or triple bond are treated as a single bonding group; the sum of the number of atoms bonded to a central atom and the number of lone pairs formed by its nonbonding valence electrons is known as the central atom's steric number. The electron pairs are assumed to lie on the surface of a sphere centered on the central atom and tend to occupy positions that minimize their mutual repulsions by maximizing the distance between them.
The number of electron pairs, determines the overall geometry that they will adopt. For example, when there are two electron pairs surrounding the central atom, their mutual repulsion is minimal when they lie at opposite poles of the sphere. Therefore, the central atom is predicted to adopt a linear geometry. If there are 3 electron pairs surrounding the central atom, their repulsion is minimized by placing them at the vertices of an equilateral triangle centered on the atom. Therefore, the predicted geometry is trigonal. For 4 electron pairs, the optimal arrangement is tetrahedral; the overall geometry is further refined by distinguishing between bonding and nonbonding electron pairs. The bonding electron pair shared in a sigma bond with an adjacent atom lies further from the central atom than a nonbonding pair of that atom, held close to its positively charged nucleus. VSEPR theory therefore views repulsion by the lone pair to be greater than the repulsion by a bonding pair; as such, when a molecule has 2 interactions with different degrees of repulsion, VSEPR theory predicts the structure where lone pairs occupy positions that allow them to experience less repulsion.
Lone pair–lone pair repulsions are considered stronger than lone pair–bonding pair repulsions, which in turn are considered stronger than bonding pair–bonding pair repulsions, distinctions that guide decisions about overall geometry when 2 or more non-equivalent positions are possible. For instance, when 5 valence electron pairs surround a central atom, they adopt a trigonal bipyramidal molecular geometry with two collinear axial positions and three equatorial positions. An electron pair in an axial position has three close equatorial neighbors only 90° away and a fourth much farther at 180°, while an equatorial electron pair has only two adjacent pairs at 90° and two at 120°; the repulsion from the close neighbors at 90° is more important, so that the axial positions experience more repulsion than the equatorial positions. The difference between lone pairs and bonding pairs may be used to rationalize deviations from idealized geometries. For example, the H2O molecu
Water vapor, water vapour or aqueous vapor is the gaseous phase of water. It is one state of water within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from the sublimation of ice. Unlike other forms of water, water vapor is invisible. Under typical atmospheric conditions, water vapor is continuously generated by evaporation and removed by condensation, it triggers convection currents that can lead to clouds. Being a component of Earth's hydrosphere and hydrologic cycle, it is abundant in Earth's atmosphere where it is a potent greenhouse gas along with other gases such as carbon dioxide and methane. Use of water vapor, as steam, has been important to humans for cooking and as a major component in energy production and transport systems since the industrial revolution. Water vapor is a common atmospheric constituent, present in the solar atmosphere as well as every planet in the Solar System and many astronomical objects including natural satellites and large asteroids.
The detection of extrasolar water vapor would indicate a similar distribution in other planetary systems. Water vapor is significant in that it can be indirect evidence supporting the presence of extraterrestrial liquid water in the case of some planetary mass objects. Whenever a water molecule leaves a surface and diffuses into a surrounding gas, it is said to have evaporated; each individual water molecule which transitions between a more associated and a less associated state does so through the absorption or release of kinetic energy. The aggregate measurement of this kinetic energy transfer is defined as thermal energy and occurs only when there is differential in the temperature of the water molecules. Liquid water that becomes water vapor takes a parcel of heat with it, in a process called evaporative cooling; the amount of water vapor in the air determines how molecules will return to the surface. When a net evaporation occurs, the body of water will undergo a net cooling directly related to the loss of water.
In the US, the National Weather Service measures the actual rate of evaporation from a standardized "pan" open water surface outdoors, at various locations nationwide. Others do around the world; the US data is compiled into an annual evaporation map. The measurements range from under 30 to over 120 inches per year. Formulas can be used for calculating the rate of evaporation from a water surface such as a swimming pool. In some countries, the evaporation rate far exceeds the precipitation rate. Evaporative cooling is restricted by atmospheric conditions. Humidity is the amount of water vapor in the air; the vapor content of air is measured with devices known as hygrometers. The measurements are expressed as specific humidity or percent relative humidity; the temperatures of the atmosphere and the water surface determine the equilibrium vapor pressure. This condition is referred to as complete saturation. Humidity ranges from 0 gram per cubic metre in dry air to 30 grams per cubic metre when the vapor is saturated at 30 °C.
Sublimation is when water molecules directly leave the surface of ice without first becoming liquid water. Sublimation accounts for the slow mid-winter disappearance of ice and snow at temperatures too low to cause melting. Antarctica shows this effect to a unique degree because it is by far the continent with the lowest rate of precipitation on Earth; as a result, there are large areas where millennial layers of snow have sublimed, leaving behind whatever non-volatile materials they had contained. This is valuable to certain scientific disciplines, a dramatic example being the collection of meteorites that are left exposed in unparalleled numbers and excellent states of preservation. Sublimation is important in the preparation of certain classes of biological specimens for scanning electron microscopy; the specimens are prepared by cryofixation and freeze-fracture, after which the broken surface is freeze-etched, being eroded by exposure to vacuum till it shows the required level of detail.
This technique can display protein molecules, organelle structures and lipid bilayers with low degrees of distortion. Water vapour will only condense onto another surface when that surface is cooler than the dew point temperature, or when the water vapour equilibrium in air has been exceeded; when water vapour condenses onto a surface, a net warming occurs on that surface. The water molecule brings heat energy with it. In turn, the temperature of the atmosphere drops slightly. In the atmosphere, condensation produces clouds and precipitation; the dew point of an air parcel is the temperature to which it must cool before water vapour in the air begins to condense concluding water vapour is a type of water or rain. A net condensation of water vapour occurs on surfaces when the temperature of the surface is at or below the dew point temperature of the atmosphere. Deposition is a phase transition separate from condensation which leads to the direct formation of ice from water vapour. Frost and snow are examples of deposition.
There are many mechanism of cooling by which condensation occurs 1.direct loss of heat known as radiation cooling. 2.cooling with upliftment of air known as adiabatic cooling. There are 4 types of upliftment of air: a. orographic upliftment- mountains work as barrier for upliftment of air b. convectional upliftment- upliftment of air due to pressure difference c. frontal upliftment- upliftment of air due to temp difference d. cyclonic upliftment bu
Fractional distillation is the separation of a mixture into its component parts, or fractions. Chemical compounds are separated by heating them to a temperature at which one or more fractions of the mixture will vaporize, it uses distillation to fractionate. The component parts have boiling points that differ by less than 25 °C from each other under a pressure of one atmosphere. If the difference in boiling points is greater than 25 °C, a simple distillation is used. Fractional distillation in a laboratory makes use of common laboratory glassware and apparatuses including a Bunsen burner, a round-bottomed flask and a condenser, as well as the single-purpose fractionating column. Heat source, such as a hot plate with a bath distilling flask a round-bottom flask receiving flask also a round-bottom flask fractionating column distillation head thermometer and adapter if needed condenser, such as a Liebig condenser or Allihn condenser vacuum adapter Standard laboratory glassware with ground glass joints, e.g. quickfit apparatus.
As an example consider the distillation of a mixture of water and ethanol. Ethanol boils at 78.4 °C while water boils at 100 °C. So, by heating the mixture, the most volatile component will concentrate to a greater degree in the vapor leaving the liquid; some mixtures form azeotropes. In this example, a mixture of 96% ethanol and 4% water boils at 78.2 °C. For this reason, ethanol cannot be purified by direct fractional distillation of ethanol-water mixtures; the apparatus is assembled as in the diagram. The mixture is put into the round bottomed flask along with a few anti-bumping granules, the fractionating column is fitted into the top; the fractional distillation column is set up with the heat source at the bottom on the still pot. As the distance from the stillpot increases, a temperature gradient is formed in the column; as the mixed vapor ascends the temperature gradient, some of the vapor condenses and revaporizes along the temperature gradient. Each time the vapor condenses and vaporizes, the composition of the more volatile component in the vapor increases.
This distills the vapor along the length of the column, the vapor is composed of the more volatile component. The vapor condenses on the glass platforms, known as trays, inside the column, runs back down into the liquid below, refluxing distillate; the efficiency in terms of the amount of heating and time required to get fractionation can be improved by insulating the outside of the column in an insulator such as wool, aluminium foil or preferably a vacuum jacket. The hottest tray is at the bottom and the coolest is at the top. At steady state conditions, the vapor and liquid on each tray are at equilibrium; the most volatile component of the mixture exits as a gas at the top of the column. The vapor at the top of the column passes into the condenser, which cools it down until it liquefies; the separation is more pure with the addition of more trays Initially, the condensate will be close to the azeotropic composition, but when much of the ethanol has been drawn off, the condensate becomes richer in water.
The process continues. This point can be recognized by the sharp rise in temperature shown on the thermometer; the above explanation reflects. Normal laboratory fractionation columns will be simple glass tubes filled with a packing small glass helices of 4 to 7 millimetres diameter; such a column can be calibrated by the distillation of a known mixture system to quantify the column in terms of number of theoretical trays. To improve fractionation the apparatus is set up to return condensate to the column by the use of some sort of reflux splitter - a typical careful fractionation would employ a reflux ratio of around 4:1. In laboratory distillation, several types of condensers are found; the Liebig condenser is a straight tube within a water jacket, is the simplest form of condenser. The Graham condenser is a spiral tube within a water jacket, the Allihn condenser has a series of large and small constrictions on the inside tube, each increasing the surface area upon which the vapor constituents may condense.
Alternate set-ups may use a multi–outlet distillation receiver flask to connect three or four receiving flasks to the condenser. By turning the cow or pig, the distillates can be channeled into any chosen receiver; because the receiver does not have to be removed and replaced during the distillation process, this type of apparatus is useful when distilling under an inert atmosphere for air-sensitive chemicals or at reduced pressure. A Perkin triangle is an alternative apparatus used in these situations because it allows isolation of the receiver from the rest of the system, but does require removing and reattaching a single receiver for each fraction. Vacuum distillation systems operate at reduced pressure, thereby lowering the boiling points of the materials. Anti-bumping granules, become ineffectiv
Aluminium oxide or aluminum oxide is a chemical compound of aluminium and oxygen with the chemical formula Al2O3. It is the most occurring of several aluminium oxides, identified as aluminium oxide, it is called alumina and may be called aloxide, aloxite, or alundum depending on particular forms or applications. It occurs in its crystalline polymorphic phase α-Al2O3 as the mineral corundum, varieties of which form the precious gemstones ruby and sapphire. Al2O3 is significant in its use to produce aluminium metal, as an abrasive owing to its hardness, as a refractory material owing to its high melting point. Corundum is the most common occurring crystalline form of aluminium oxide. Rubies and sapphires are gem-quality forms of corundum, which owe their characteristic colors to trace impurities. Rubies are given their characteristic deep red color and their laser qualities by traces of chromium. Sapphires come in different colors given by various other impurities, such as titanium. Al2O3 is an electrical insulator but has a high thermal conductivity for a ceramic material.
Aluminium oxide is insoluble in water. In its most occurring crystalline form, called corundum or α-aluminium oxide, its hardness makes it suitable for use as an abrasive and as a component in cutting tools. Aluminium oxide is responsible for the resistance of metallic aluminium to weathering. Metallic aluminium is reactive with atmospheric oxygen, a thin passivation layer of aluminium oxide forms on any exposed aluminium surface; this layer protects the metal from further oxidation. The thickness and properties of this oxide layer can be enhanced using a process called anodising. A number of alloys, such as aluminium bronzes, exploit this property by including a proportion of aluminium in the alloy to enhance corrosion resistance; the aluminium oxide generated by anodising is amorphous, but discharge assisted oxidation processes such as plasma electrolytic oxidation result in a significant proportion of crystalline aluminium oxide in the coating, enhancing its hardness. Aluminium oxide was taken off the United States Environmental Protection Agency's chemicals lists in 1988.
Aluminium oxide is on the EPA's Toxics Release Inventory list. Aluminium oxide is an amphoteric substance, meaning it can react with both acids and bases, such as hydrofluoric acid and sodium hydroxide, acting as an acid with a base and a base with an acid, neutralising the other and producing a salt. Al2O3 + 6 HF → 2 AlF3 + 3 H2O Al2O3 + 2 NaOH + 3 H2O → 2 NaAl4 The most common form of crystalline aluminium oxide is known as corundum, the thermodynamically stable form; the oxygen ions form a nearly hexagonal close-packed structure with the aluminium ions filling two-thirds of the octahedral interstices. Each Al3+ center is octahedral. In terms of its crystallography, corundum adopts a trigonal Bravais lattice with a space group of R3c; the primitive cell contains two formula units of aluminium oxide. Aluminium oxide exists in other, phases, including the cubic γ and η phases, the monoclinic θ phase, the hexagonal χ phase, the orthorhombic κ phase and the δ phase that can be tetragonal or orthorhombic.
Each has properties. Cubic γ-Al2O3 has important technical applications; the so-called β-Al2O3 proved to be NaAl11O17. Molten aluminium oxide near the melting temperature is 2/3 tetrahedral, 1/3 5-coordinated, with little octahedral Al-O present. Around 80% of the oxygen atoms are shared among three or more Al-O polyhedra, the majority of inter-polyhedral connections are corner-sharing, with the remaining 10–20% being edge-sharing; the breakdown of octahedra upon melting is accompanied by a large volume increase, the density of the liquid close to its melting point is 2.93 g/cm3. The structure of molten alumina is temperature dependent and the fraction of 5- and 6-fold aluminium increases during cooling, at the expense of tetrahedral AlO4 units, approaching the local structural arrangements found in amorphous alumina. Aluminium hydroxide minerals are the main component of the principal ore of aluminium. A mixture of the minerals comprise bauxite ore, including gibbsite and diaspore, along with impurities of iron oxides and hydroxides and clay minerals.
Bauxites are found in laterites. Bauxite is purified by the Bayer process: Al2O3 + H2O + NaOH → NaAl4 Al3 + NaOH → NaAl4Except for SiO2, the other components of bauxite do not dissolve in base. Upon filtering the basic mixture, Fe2O3 is removed; when the Bayer liquor is cooled, Al3 precipitates. NaAl4 → NaOH + Al3The solid Al3 Gibbsite is calcined to give aluminium oxide: 2 Al3 → Al2O3 + 3 H2OThe product aluminium oxide tends to be multi-phase, i.e. consisting of several phases of aluminium oxide rather than corundum. The production process can therefore be optimized to produce a tailored product; the type of phases present affects, for example, the solubility and pore structure of the aluminium oxide product which, in turn, affects the cost of aluminium production and pollution control. Known as alundum or aloxite in the mining and materials science communities, aluminium oxide finds wide use. Annual world production of aluminium oxide in 2015 was 115 million tonnes, over 90% of, used in the manufacture of aluminium metal.
The major uses of speciali
Molecular geometry is the three-dimensional arrangement of the atoms that constitute a molecule. It includes the general shape of the molecule as well as bond lengths, bond angles, torsional angles and any other geometrical parameters that determine the position of each atom. Molecular geometry influences several properties of a substance including its reactivity, phase of matter, color and biological activity; the angles between bonds that an atom forms depend only weakly on the rest of molecule, i.e. they can be understood as local and hence transferable properties. The molecular geometry can be determined by diffraction methods. IR, microwave and Raman spectroscopy can give information about the molecule geometry from the details of the vibrational and rotational absorbance detected by these techniques. X-ray crystallography, neutron diffraction and electron diffraction can give molecular structure for crystalline solids based on the distance between nuclei and concentration of electron density.
Gas electron diffraction can be used for small molecules in the gas phase. NMR and FRET methods can be used to determine complementary information including relative distances, dihedral angles and connectivity. Molecular geometries are best determined at low temperature because at higher temperatures the molecular structure is averaged over more accessible geometries. Larger molecules exist in multiple stable geometries that are close in energy on the potential energy surface. Geometries can be computed by ab initio quantum chemistry methods to high accuracy; the molecular geometry can be different as a solid, in solution, as a gas. The position of each atom is determined by the nature of the chemical bonds by which it is connected to its neighboring atoms; the molecular geometry can be described by the positions of these atoms in space, evoking bond lengths of two joined atoms, bond angles of three connected atoms, torsion angles of three consecutive bonds. Since the motions of the atoms in a molecule are determined by quantum mechanics, one must define "motion" in a quantum mechanical way.
The overall quantum mechanical motions translation and rotation hardly change the geometry of the molecule. In addition to translation and rotation, a third type of motion is molecular vibration, which corresponds to internal motions of the atoms such as bond stretching and bond angle variation; the molecular vibrations are harmonic, the atoms oscillate about their equilibrium positions at the absolute zero of temperature. At absolute zero all atoms are in their vibrational ground state and show zero point quantum mechanical motion, so that the wavefunction of a single vibrational mode is not a sharp peak, but an exponential of finite width. At higher temperatures the vibrational modes may be thermally excited, but they oscillate still around the recognizable geometry of the molecule. To get a feeling for the probability that the vibration of molecule may be thermally excited, we inspect the Boltzmann factor β ≡ exp , where Δ E is the excitation energy of the vibrational mode, k the Boltzmann constant and T the absolute temperature.
At 298 K, typical values for the Boltzmann factor β are: β = 0.089 for ΔE = 500 cm−1. When an excitation energy is 500 cm−1 about 8.9 percent of the molecules are thermally excited at room temperature. To put this in perspective: the lowest excitation vibrational energy in water is the bending mode. Thus, at room temperature less than 0.07 percent of all the molecules of a given amount of water will vibrate faster than at absolute zero. As stated above, rotation hardly influences the molecular geometry. But, as a quantum mechanical motion, it is thermally excited at low temperatures. From a classical point of view it can be stated that at higher temperatures more molecules will rotate faster, which implies that they have higher angular velocity and angular momentum. In quantum mechanical language: more eigenstates of higher angular momentum become thermally populated with rising temperatures. Typical rotational excitation energies are on the order of a few cm−1; the results of many spectroscopic experiments are broadened because they involve an averaging over rotational states.
It is difficult to extract geometries from spectra at high temperatures, because the number of rotational states probed in the experimental averaging increases with increasing temperature. Thus, many spectroscopic observations can only be expected to yield reliable molecular geometries at temperatures close to absolute zero, because at higher temperatures too many higher rotational states are thermally populated. Molecules, by definition, are most held together with covalent bonds involving single, and/or triple bonds, where a "bond
Halogenation is a chemical reaction that involves the addition of one or more halogens to a compound or material. The pathway and stoichiometry of halogenation depends on the structural features and functional groups of the organic substrate, as well as on the specific halogen. Inorganic compounds such as metals undergo halogenation. Several pathways exist for the halogenation of organic compounds, including free radical halogenation, ketone halogenation, electrophilic halogenation, halogen addition reaction; the structure of the substrate is one factor. Saturated hydrocarbons do not add halogens but undergo free radical halogenation, involving substitution of hydrogen atoms by halogen; the regiochemistry of the halogenation of alkanes is determined by the relative weakness of the available C–H bonds. The preference for reaction at tertiary and secondary positions results from greater stability of the corresponding free radicals and the transition state leading to them. Free radical halogenation is used for the industrial production of chlorinated methanes: CH4 + Cl2 → CH3Cl + HClRearrangement accompany such free radical reactions.
Unsaturated compounds alkenes and alkynes, add halogens: RCH=CHR′ + X2 → RCHX–CHXR′The addition of halogens to alkenes proceeds via intermediate halonium ions. In special cases, such intermediates have been isolated. Aromatic compounds are subject to electrophilic halogenation: RC6H5 + X2 → HX + RC6H4XThis reaction works only for chlorine and bromine and is carried in the presence of a Lewis acid such as FeX3; the role of the Lewis acid is to polarize the halogen-halogen bond, making the halogen molecule more electrophilic. Industrially, this is done by treating the aromatic compound with X2 in the presence of iron metal; when the halogen is pumped into the reaction vessel, it reacts with iron, generating FeX3 in catalytic amounts. The reaction mechanism can be represented as follows: Because fluorine is reactive, the protocol described above would not be efficient as the aromatic molecule would react destructively with F2. Therefore, other methods, such as the Balz–Schiemann reaction, must be used to prepare fluorinated aromatic compounds.
For iodine, oxidising conditions must be used in order to perform iodination. Because iodination is a reversible process, the products have to be removed from the reaction medium in order to drive the reaction forward, see Le Chatelier's principle; this can be done by conducting the reaction in the presence of an oxidising agent that oxidises HI to I2, thus removing HI from the reaction and generating more iodine that can further react. The reaction steps involved in iodination are the following: Another method to obtain aromatic iodides is the Sandmeyer reaction. In the Hunsdiecker reaction, from carboxylic acids are converted to the chain-shortened halide; the carboxylic acid is first converted to its silver salt, oxidized with halogen: RCO2Ag + Br2 → RBr + CO2 + AgBrThe Sandmeyer reaction is used to give aryl halides from diazonium salts, which are obtained from anilines. In the Hell–Volhard–Zelinsky halogenation, carboxylic acids are alpha-halogenated. In oxychlorination, the combination of hydrogen chloride and oxygen serves as the equivalent of chlorine, as illustrated by this route to dichloroethane: 2 HCl + CH2=CH2 + 1⁄2 O2 → ClCH2CH2Cl + H2O The facility of halogenation is influenced by the halogen.
Fluorine and chlorine are more aggressive halogenating agents. Bromine is a weaker halogenating agent than both fluorine and chlorine, while iodine is the least reactive of them all; the facility of dehydrohalogenation follows the reverse trend: iodine is most removed from organic compounds, organofluorine compounds are stable. Organic compounds and unsaturated alike, react usually explosively, with fluorine. Fluorination with elemental fluorine requires specialised conditions and apparatus. Many commercially important organic compounds are fluorinated electrochemically using hydrogen fluoride as the source of fluorine; the method is called electrochemical fluorination. Aside from F2 and its electrochemically generated equivalent, a variety of fluorinating reagents are known such as xenon difluoride and cobalt fluoride. See also: PhotochlorinationChlorination is highly exothermic. Both saturated and unsaturated compounds react directly with chlorine, the former requiring UV light to initiate homolysis of chlorine.
Chlorination is conducted on a large scale industrially. Bromination is more selective than chlorination. Most bromination is conducted by the addition of Br2 to alkenes. An example of bromination is the organic synthesis of the anesthetic halothane from trichloroethylene: Organobromine compounds are the most common organohalides in nature, their formation is catalyzed by the enzyme bromoperoxidase which utilizes bromide in combination with oxygen as an oxidant. The oceans are estimated to release 1–2 million tons of bromoform and 56,000 tons of bromomethane annually. Iodine is reluctant to react with most organic compounds; the addition of iodine to alkenes is the basis of the analytical method called the iodine number, a measure of the degree of unsaturation for fats. The iodoform reaction involves degradation of methyl ketones. All elements aside from argon and helium form fluorides by direct reaction with fluorine. Chlorine is more selective, but still reacts with most metals and heavier nonmetals.
Following the usual trend, bromine is less reactive and iodine leas
Vapor pressure or equilibrium vapor pressure is defined as the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases at a given temperature in a closed system. The equilibrium vapor pressure is an indication of a liquid's evaporation rate, it relates to the tendency of particles to escape from the liquid. A substance with a high vapor pressure at normal temperatures is referred to as volatile; the pressure exhibited by vapor present above a liquid surface is known as vapor pressure. As the temperature of a liquid increases, the kinetic energy of its molecules increases; as the kinetic energy of the molecules increases, the number of molecules transitioning into a vapor increases, thereby increasing the vapor pressure. The vapor pressure of any substance increases non-linearly with temperature according to the Clausius–Clapeyron relation; the atmospheric pressure boiling point of a liquid is the temperature at which the vapor pressure equals the ambient atmospheric pressure.
With any incremental increase in that temperature, the vapor pressure becomes sufficient to overcome atmospheric pressure and lift the liquid to form vapor bubbles inside the bulk of the substance. Bubble formation deeper in the liquid requires a higher temperature due to the higher fluid pressure, because fluid pressure increases above the atmospheric pressure as the depth increases. More important at shallow depths is the higher temperature required to start bubble formation; the surface tension of the bubble wall leads to an overpressure in the small, initial bubbles. Thus, thermometer calibration should not rely on the temperature in boiling water; the vapor pressure that a single component in a mixture contributes to the total pressure in the system is called partial pressure. For example, air at sea level, saturated with water vapor at 20 °C, has partial pressures of about 2.3 kPa of water, 78 kPa of nitrogen, 21 kPa of oxygen and 0.9 kPa of argon, totaling 102.2 kPa, making the basis for standard atmospheric pressure.
Vapor pressure is measured in the standard units of pressure. The International System of Units recognizes pressure as a derived unit with the dimension of force per area and designates the pascal as its standard unit. One pascal is one newton per square meter. Experimental measurement of vapor pressure is a simple procedure for common pressures between 1 and 200 kPa. Most accurate results are obtained near the boiling point of substances and large errors result for measurements smaller than 1kPa. Procedures consist of purifying the test substance, isolating it in a container, evacuating any foreign gas measuring the equilibrium pressure of the gaseous phase of the substance in the container at different temperatures. Better accuracy is achieved when care is taken to ensure that the entire substance and its vapor are at the prescribed temperature; this is done, as with the use of an isoteniscope, by submerging the containment area in a liquid bath. Low vapor pressures of solids can be measured using the Knudsen effusion cell method.
In a medical context, vapor pressure is sometimes expressed in other units millimeters of mercury. This is important for volatile anesthetics, most of which are liquids at body temperature, but with a high vapor pressure. Anesthetics with a higher vapor pressure at body temperature will be excreted more as they are exhaled from the lungs; the Antoine equation is a mathematical expression of the relation between the vapor pressure and the temperature of pure liquid or solid substances. The basic form of the equation is: log P = A − B C + T and it can be transformed into this temperature-explicit form: T = B A − log P − C where: P is the absolute vapor pressure of a substance T is the temperature of the substance A, B and C are substance-specific coefficients log is either log 10 or log e A simpler form of the equation with only two coefficients is sometimes used: log P = A − B T which can be transformed to: T = B A − log P Sublimations and vaporizations of the same substance have separate sets of Antoine coefficients, as do components in mixtures.
Each parameter set for a specific compound is only applicable over a specified temperature range. Temperature ranges are chosen to maintain the equation's accuracy of a few up to 8–10 percent. For many volatile substances, several different sets of parameters are available and used for different temperature ranges; the Antoine equation has poor accuracy with any single parameter set when used from a compound's melting point to its critical temperature. Accuracy is usually poor when vapor pressure is under 10 Torr because of the limitations of the apparatus used to establish the Antoine parameter values; the Wagner equation gives "o