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In a typical dissociation reaction, a bond in a molecule splits ruptures resulting in two molecular fragments. The splitting can be homolytic or heterolytic. In the first case, the bond is divided so that each product retains an electron and becomes a neutral radical. In the second case, both electrons of the chemical bond remain with one of the products, resulting in charged ions.

Dissociation plays an important role in triggering chain reactions , such as hydrogen—oxygen or polymerization reactions. For bimolecular reactions, two molecules collide and react with each other. Their merger is called chemical synthesis or an addition reaction. Another possibility is that only a portion of one molecule is transferred to the other molecule. This type of reaction occurs, for example, in redox and acid-base reactions. In redox reactions, the transferred particle is an electron, whereas in acid-base reactions it is a proton.

This type of reaction is also called metathesis. Most chemical reactions are reversible, that is they can and do run in both directions. The forward and reverse reactions are competing with each other and differ in reaction rates. These rates depend on the concentration and therefore change with time of the reaction: the reverse rate gradually increases and becomes equal to the rate of the forward reaction, establishing the so-called chemical equilibrium.

The time to reach equilibrium depends on such parameters as temperature, pressure and the materials involved, and is determined by the minimum free energy. In equilibrium, the Gibbs free energy must be zero. The pressure dependence can be explained with the Le Chatelier's principle. For example, an increase in pressure due to decreasing volume causes the reaction to shift to the side with the fewer moles of gas. The reaction yield stabilizes at equilibrium, but can be increased by removing the product from the reaction mixture or changed by increasing the temperature or pressure.

A change in the concentrations of the reactants does not affect the equilibrium constant, but does affect the equilibrium position. Chemical reactions are determined by the laws of thermodynamics. Reactions can proceed by themselves if they are exergonic , that is if they release energy. The associated free energy of the reaction is composed of two different thermodynamic quantities, enthalpy and entropy : [14]. Typical examples of exothermic reactions are precipitation and crystallization , in which ordered solids are formed from disordered gaseous or liquid phases.

In contrast, in endothermic reactions, heat is consumed from the environment. This can occur by increasing the entropy of the system, often through the formation of gaseous reaction products, which have high entropy.

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Since the entropy increases with temperature, many endothermic reactions preferably take place at high temperatures. On the contrary, many exothermic reactions such as crystallization occur at low temperatures. Changes in temperature can sometimes reverse the sign of the enthalpy of a reaction, as for the carbon monoxide reduction of molybdenum dioxide :. This reaction to form carbon dioxide and molybdenum is endothermic at low temperatures, becoming less so with increasing temperature.

Changes in temperature can also reverse the direction tendency of a reaction. For example, the water gas shift reaction. Reactions can also be characterized by the internal energy which takes into account changes in the entropy, volume and chemical potential. The latter depends, among other things, on the activities of the involved substances. The speed at which reactions takes place is studied by reaction kinetics. The rate depends on various parameters, such as:. Several theories allow calculating the reaction rates at the molecular level.

This field is referred to as reaction dynamics. The rate v of a first-order reaction , which could be disintegration of a substance A, is given by:. The rate of a first-order reaction depends only on the concentration and the properties of the involved substance, and the reaction itself can be described with the characteristic half-life.

More than one time constant is needed when describing reactions of higher order. The temperature dependence of the rate constant usually follows the Arrhenius equation :. One of the simplest models of reaction rate is the collision theory. More realistic models are tailored to a specific problem and include the transition state theory , the calculation of the potential energy surface , the Marcus theory and the Rice—Ramsperger—Kassel—Marcus RRKM theory.

In a synthesis reaction, two or more simple substances combine to form a more complex substance. These reactions are in the general form:. Two or more reactants yielding one product is another way to identify a synthesis reaction. One example of a synthesis reaction is the combination of iron and sulfur to form iron II sulfide :. Another example is simple hydrogen gas combined with simple oxygen gas to produce a more complex substance, such as water. A decomposition reaction is when a more complex substance breaks down into its more simple parts.

It is thus the opposite of a synthesis reaction, and can be written as [18] [19]. One example of a decomposition reaction is the electrolysis of water to make oxygen and hydrogen gas:. In a single replacement reaction , a single uncombined element replaces another in a compound; in other words, one element trades places with another element in a compound [18] These reactions come in the general form of:.

One example of a single displacement reaction is when magnesium replaces hydrogen in water to make magnesium hydroxide and hydrogen gas:. In a double replacement reaction , the anions and cations of two compounds switch places and form two entirely different compounds. Another example of a double displacement reaction is the reaction of lead II nitrate with potassium iodide to form lead II iodide and potassium nitrate :. Redox reactions can be understood in terms of transfer of electrons from one involved species reducing agent to another oxidizing agent.

In this process, the former species is oxidized and the latter is reduced. Though sufficient for many purposes, these descriptions are not precisely correct. Oxidation is better defined as an increase in oxidation state , and reduction as a decrease in oxidation state. In practice, the transfer of electrons will always change the oxidation state, but there are many reactions that are classed as "redox" even though no electron transfer occurs such as those involving covalent bonds.

In the following redox reaction, hazardous sodium metal reacts with toxic chlorine gas to form the ionic compound sodium chloride , or common table salt:. Because the chlorine is the one reduced, it is considered the electron acceptor, or in other words, induces oxidation in the sodium — thus the chlorine gas is considered the oxidizing agent.

Conversely, the sodium is oxidized or is the electron donor, and thus induces reduction in the other species and is considered the reducing agent. Which of the involved reactants would be reducing or oxidizing agent can be predicted from the electronegativity of their elements. Elements with low electronegativity, such as most metals, easily donate electrons and oxidize — they are reducing agents. The number of electrons donated or accepted in a redox reaction can be predicted from the electron configuration of the reactant element.

Elements try to reach the low-energy noble gas configuration, and therefore alkali metals and halogens will donate and accept one electron respectively. Noble gases themselves are chemically inactive. An important class of redox reactions are the electrochemical reactions, where electrons from the power supply are used as the reducing agent. These reactions are particularly important for the production of chemical elements, such as chlorine [23] or aluminium. The reverse process in which electrons are released in redox reactions and can be used as electrical energy is possible and used in batteries.

In complexation reactions, several ligands react with a metal atom to form a coordination complex.

Chlorine-hydrocarbon photochemistry in the marine troposphere and lower stratosphere

This is achieved by providing lone pairs of the ligand into empty orbitals of the metal atom and forming dipolar bonds. The ligands are Lewis bases , they can be both ions and neutral molecules, such as carbon monoxide, ammonia or water. The number of ligands that react with a central metal atom can be found using the electron rule , saying that the valence shells of a transition metal will collectively accommodate 18 electrons , whereas the symmetry of the resulting complex can be predicted with the crystal field theory and ligand field theory.

Complexation reactions also include ligand exchange , in which one or more ligands are replaced by another, and redox processes which change the oxidation state of the central metal atom. When a proton is removed from an acid, the resulting species is termed that acid's conjugate base. When the proton is accepted by a base, the resulting species is termed that base's conjugate acid.

The equilibrium is determined by the acid and base dissociation constants K a and K b of the involved substances. A special case of the acid-base reaction is the neutralization where an acid and a base, taken at exactly same amounts, form a neutral salt. Acid-base reactions can have different definitions depending on the acid-base concept employed.

Some of the most common are:. Precipitation is the formation of a solid in a solution or inside another solid during a chemical reaction. It usually takes place when the concentration of dissolved ions exceeds the solubility limit [26] and forms an insoluble salt. This process can be assisted by adding a precipitating agent or by removal of the solvent.

Rapid precipitation results in an amorphous or microcrystalline residue and slow process can yield single crystals.

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A Brief Introduction to the History of Chemical Kinetics

The latter can also be obtained by recrystallization from microcrystalline salts. Reactions can take place between two solids. However, because of the relatively small diffusion rates in solids, the corresponding chemical reactions are very slow in comparison to liquid and gas phase reactions. They are accelerated by increasing the reaction temperature and finely dividing the reactant to increase the contacting surface area. Reaction can take place at the solid gas interface, surfaces at very low pressure such as ultra-high vacuum.

Via scanning tunneling microscopy , it is possible to observe reactions at the solid gas interface in real space, if the time scale of the reaction is in the correct range. In photochemical reactions , atoms and molecules absorb energy photons of the illumination light and convert into an excited state.

They can then release this energy by breaking chemical bonds, thereby producing radicals. Photochemical reactions include hydrogen—oxygen reactions, radical polymerization , chain reactions and rearrangement reactions. Many important processes involve photochemistry. The premier example is photosynthesis , in which most plants use solar energy to convert carbon dioxide and water into glucose , disposing of oxygen as a side-product. Humans rely on photochemistry for the formation of vitamin D, and vision is initiated by a photochemical reaction of rhodopsin.

In catalysis , the reaction does not proceed directly, but through reaction with a third substance known as catalyst. Although the catalyst takes part in the reaction, it is returned to its original state by the end of the reaction and so is not consumed. However, it can be inhibited, deactivated or destroyed by secondary processes.

Catalysts can be used in a different phase heterogeneous or in the same phase homogeneous as the reactants.

In heterogeneous catalysis, typical secondary processes include coking where the catalyst becomes covered by polymeric side products. Additionally, heterogeneous catalysts can dissolve into the solution in a solid—liquid system or evaporate in a solid—gas system. Catalysts can only speed up the reaction — chemicals that slow down the reaction are called inhibitors. With a catalyst, a reaction which is kinetically inhibited by a high activation energy can take place in circumvention of this activation energy.

Heterogeneous catalysts are usually solids, powdered in order to maximize their surface area. Of particular importance in heterogeneous catalysis are the platinum group metals and other transition metals, which are used in hydrogenations , catalytic reforming and in the synthesis of commodity chemicals such as nitric acid and ammonia.

Acids are an example of a homogeneous catalyst, they increase the nucleophilicity of carbonyls , allowing a reaction that would not otherwise proceed with electrophiles. The advantage of homogeneous catalysts is the ease of mixing them with the reactants, but they may also be difficult to separate from the products. Therefore, heterogeneous catalysts are preferred in many industrial processes. In organic chemistry, in addition to oxidation, reduction or acid-base reactions, a number of other reactions can take place which involve covalent bonds between carbon atoms or carbon and heteroatoms such as oxygen, nitrogen, halogens , etc.

Many specific reactions in organic chemistry are name reactions designated after their discoverers. In a substitution reaction , a functional group in a particular chemical compound is replaced by another group. In the first type, a nucleophile , an atom or molecule with an excess of electrons and thus a negative charge or partial charge , replaces another atom or part of the "substrate" molecule. The electron pair from the nucleophile attacks the substrate forming a new bond, while the leaving group departs with an electron pair.

The nucleophile may be electrically neutral or negatively charged, whereas the substrate is typically neutral or positively charged. Examples of nucleophiles are hydroxide ion, alkoxides , amines and halides. This type of reaction is found mainly in aliphatic hydrocarbons , and rarely in aromatic hydrocarbon. The latter have high electron density and enter nucleophilic aromatic substitution only with very strong electron withdrawing groups.

Nucleophilic substitution can take place by two different mechanisms, S N 1 and S N 2. In their names, S stands for substitution, N for nucleophilic, and the number represents the kinetic order of the reaction, unimolecular or bimolecular. The S N 1 reaction proceeds in two steps. First, the leaving group is eliminated creating a carbocation.

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This is followed by a rapid reaction with the nucleophile. In the S N 2 mechanism, the nucleophile forms a transition state with the attacked molecule, and only then the leaving group is cleaved. These two mechanisms differ in the stereochemistry of the products. In contrast, a reversal Walden inversion of the previously existing stereochemistry is observed in the S N 2 mechanism.

Electrophilic substitution is the counterpart of the nucleophilic substitution in that the attacking atom or molecule, an electrophile , has low electron density and thus a positive charge. Typical electrophiles are the carbon atom of carbonyl groups , carbocations or sulfur or nitronium cations. This reaction takes place almost exclusively in aromatic hydrocarbons, where it is called electrophilic aromatic substitution.

Then, the leaving group, usually a proton, is split off and the aromaticity is restored. An alternative to aromatic substitution is electrophilic aliphatic substitution. It is similar to the nucleophilic aliphatic substitution and also has two major types, S E 1 and S E 2 [40]. In the third type of substitution reaction, radical substitution, the attacking particle is a radical. In the first step, light or heat disintegrates the halogen-containing molecules producing the radicals.

Then the reaction proceeds as an avalanche until two radicals meet and recombine. The addition and its counterpart, the elimination , are reactions which change the number of substituents on the carbon atom, and form or cleave multiple bonds. Double and triple bonds can be produced by eliminating a suitable leaving group. Similar to the nucleophilic substitution, there are several possible reaction mechanisms which are named after the respective reaction order.

In the E1 mechanism, the leaving group is ejected first, forming a carbocation. The next step, formation of the double bond, takes place with elimination of a proton deprotonation. The leaving order is reversed in the E1cb mechanism, that is the proton is split off first. This mechanism requires participation of a base.

The E2 mechanism also requires a base, but there the attack of the base and the elimination of the leaving group proceed simultaneously and produce no ionic intermediate.


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In contrast to the E1 eliminations, different stereochemical configurations are possible for the reaction product in the E2 mechanism, because the attack of the base preferentially occurs in the anti-position with respect to the leaving group. Because of the similar conditions and reagents, the E2 elimination is always in competition with the S N 2-substitution. The counterpart of elimination is the addition where double or triple bonds are converted into single bonds. Similar to the substitution reactions, there are several types of additions distinguished by the type of the attacking particle.

For example, in the electrophilic addition of hydrogen bromide, an electrophile proton attacks the double bond forming a carbocation , which then reacts with the nucleophile bromine. The carbocation can be formed on either side of the double bond depending on the groups attached to its ends, and the preferred configuration can be predicted with the Markovnikov's rule.

Hg, and the pressure of the mercury vapor is 1,, mm.

PHOTOCHEMICAL REACTIONS LEADING TO UNSTABLE SPECIES

A method according to claim 1 wherein the pressure of the thallium iodide vapor is mm. Hg and the pressure of the mercury vapor is mm. Much, 3.

HCL - Hydrochloric Acid -- ICSE CLASS 10 CHEMISTRY --

The following examples are given for a better understanding of the invention. What is claimed is: 1. USA true USA en. Adams et al. On the mechanism of the radiation-induced inactivation of lysozyme in dilute aqueous solution. Rabe et al. Lukes et al. Generation of ozone by pulsed corona discharge over water surface in hybrid gas—liquid electrical discharge reactor. GBA en. Forrestal et al.

Chemical reaction - Wikipedia

Burton et al. Williams et al. Shinzawa et al. JPHA en. Flynn et al. Photochemical preparation of o-xylylene from 1, 3-dihydrophthalazine in rigid glass. Kubodera et al. ESR evidence for the cation radicals of tetrahydrofurans and dimethyl ether produced in a. TWB en. A process for the production of methylbenzofuran MBF impurities in phenol obtained from decomposition product of cumene hydroperoxide. Bancroft et al. Stevens et al. Photoperoxidation of unsaturated organic molecules. Sensitizer yields of O