⒈ Chemical Reactions

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Chemical Reactions



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Top 10 Chemical Reactions that will Blow Your Mind

The substances that:. No atoms are created or destroyed in a chemical reaction. This means that the total mass of the reactants is the same as the total mass of the products. We say that mass is conserved in a chemical reaction. The reaction between iron and sulfur is often used to study elements and compounds. Iron sulfide is the compound produced in the reaction. The slideshow shows what happens in this reaction:. The test tube is partly filled with a mixture of iron and sulfur. The mixture is heated strongly using a Bunsen burner. The test tube now contains iron sulfide.

Iron sulfide, the compound formed in the reaction, has different properties to the elements from what it is made. The table compares the properties of iron, sulfur and iron sulfide:. 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 [30] 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. 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. 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 [44]. 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. 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. If the addition of a functional group takes place at the less substituted carbon atom of the double bond, then the electrophilic substitution with acids is not possible.

In this case, one has to use the hydroboration—oxidation reaction , where in the first step, the boron atom acts as electrophile and adds to the less substituted carbon atom. At the second step, the nucleophilic hydroperoxide or halogen anion attacks the boron atom. While the addition to the electron-rich alkenes and alkynes is mainly electrophilic, the nucleophilic addition plays an important role for the carbon-heteroatom multiple bonds, and especially its most important representative, the carbonyl group.

This process is often associated with an elimination, so that after the reaction the carbonyl group is present again. It is therefore called addition-elimination reaction and may occur in carboxylic acid derivatives such as chlorides, esters or anhydrides. This reaction is often catalyzed by acids or bases, where the acids increase by the electrophilicity of the carbonyl group by binding to the oxygen atom, whereas the bases enhance the nucleophilicity of the attacking nucleophile. Nucleophilic addition of a carbanion or another nucleophile to the double bond of an alpha, beta unsaturated carbonyl compound can proceed via the Michael reaction , which belongs to the larger class of conjugate additions.

This is one of the most useful methods for the mild formation of C—C bonds. Some additions which can not be executed with nucleophiles and electrophiles, can be succeeded with free radicals. As with the free-radical substitution, the radical addition proceeds as a chain reaction, and such reactions are the basis of the free-radical polymerization. In a rearrangement reaction , the carbon skeleton of a molecule is rearranged to give a structural isomer of the original molecule. These include hydride shift reactions such as the Wagner-Meerwein rearrangement , where a hydrogen , alkyl or aryl group migrates from one carbon to a neighboring carbon. Most rearrangements are associated with the breaking and formation of new carbon-carbon bonds.

Other examples are sigmatropic reaction such as the Cope rearrangement. Cyclic rearrangements include cycloadditions and, more generally, pericyclic reactions , wherein two or more double bond-containing molecules form a cyclic molecule. Whether a certain cycloaddition would proceed depends on the electronic orbitals of the participating species, as only orbitals with the same sign of wave function will overlap and interact constructively to form new bonds. Cycloaddition is usually assisted by light or heat. These perturbations result in different arrangement of electrons in the excited state of the involved molecules and therefore in different effects.

Biochemical reactions are mainly controlled by enzymes. These proteins can specifically catalyze a single reaction, so that reactions can be controlled very precisely. The reaction takes place in the active site , a small part of the enzyme which is usually found in a cleft or pocket lined by amino acid residues, and the rest of the enzyme is used mainly for stabilization. The biochemical reactions that occur in living organisms are collectively known as metabolism. Among the most important of its mechanisms is the anabolism , in which different DNA and enzyme-controlled processes result in the production of large molecules such as proteins and carbohydrates from smaller units.

An important energy source is glucose , which can be produced by plants via photosynthesis or assimilated from food. All organisms use this energy to produce adenosine triphosphate ATP , which can then be used to energize other reactions. Chemical reactions are central to chemical engineering where they are used for the synthesis of new compounds from natural raw materials such as petroleum and mineral ores. It is essential to make the reaction as efficient as possible, maximizing the yield and minimizing the amount of reagents, energy inputs and waste. Catalysts are especially helpful for reducing the energy required for the reaction and increasing its reaction rate.

Some specific reactions have their niche applications. For example, the thermite reaction is used to generate light and heat in pyrotechnics and welding. Although it is less controllable than the more conventional oxy-fuel welding , arc welding and flash welding , it requires much less equipment and is still used to mend rails, especially in remote areas. Mechanisms of monitoring chemical reactions depend strongly on the reaction rate. Relatively slow processes can be analyzed in situ for the concentrations and identities of the individual ingredients.

Important tools of real time analysis are the measurement of pH and analysis of optical absorption color and emission spectra. A less accessible but rather efficient method is introduction of a radioactive isotope into the reaction and monitoring how it changes over time and where it moves to; this method is often used to analyze redistribution of substances in the human body. Faster reactions are usually studied with ultrafast laser spectroscopy where utilization of femtosecond lasers allows short-lived transition states to be monitored at time scaled down to a few femtoseconds.

From Wikipedia, the free encyclopedia. Process that results in the interconversion of chemical species. This section needs expansion. You can help by adding to it. November Main article: Chemical equation. Main article: Chemical equilibrium. Main article: Synthesis reaction. Main article: Decomposition reaction. Main article: Catalysis. Further information: Reaction Progress Kinetic Analysis. Main article: Organic reaction.

The three steps of an S N 2 reaction. The nucleophile is green and the leaving group is red. Chemistry portal. Branches of chemistry. Glossary of chemical formulae List of biomolecules List of inorganic compounds Periodic table. Electrochemistry Spectroelectrochemistry Photoelectrochemistry Thermochemistry Chemical thermodynamics Surface science Interface and colloid science Micromeritics Cryochemistry Sonochemistry Structural chemistry Chemical physics Chemical kinetics Quantum chemistry Spin chemistry Photochemistry Microwave chemistry Equilibrium chemistry. Coordination chemistry Magnetochemistry Organometallic chemistry Organolanthanide chemistry Bioinorganic chemistry Bioorganometallic chemistry Cluster chemistry Solid-state chemistry Ceramic chemistry Materials science Metallurgy Ceramic engineering Polymer science.

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Jevons paradox Carbon footprint. Online corrected version: — " chemical reaction ". Chemie in unserer Zeit. Chicago: University of Chicago Press. ISBN OCLC II, pp. In Lesch, John E. Kluwer Academic Publishers. The Basics of Chemistry. Greenwood Publishing Group. Online corrected version: — " chemical reaction equation ". Retrosynthetic thinking? Chemical Society Reviews. Online corrected version: — " elementary reaction ". In Dugave, Christophe ed. Cis-trans Isomerization in Biochemistry.

Retrieved 4 June In Christian B. Anfinsen ed. Advances in Protein Chemistry. San Diego: Academic Press. Sykes ed. Advances in Inorganic Chemistry. Online corrected version: — " conjugate acid—base pair ". Online corrected version: — " precipitation ". In Erwin Riedel ed. Modern Inorganic Chemistry in German 3rd ed. Bibcode : Sci PMID Journal of the American Chemical Society. Insect clocks Third ed. Amsterdam: Elsevier. Online corrected version: — " catalyst ". Online corrected version: — " inhibitor ". Organometallchemie 6th ed.

On the atomic Chemical Reactions, the following equation is balanced because the total mass of the reactants is equal Chemical Reactions the Chemical Reactions of the products. Chemical reactions require Chemical Reactions sufficient Chemical Reactions of energy to cause the matter to collide with enough precision and force that old chemical bonds can be broken Chemical Reactions new ones formed. Look Chemical Reactions signs of a reaction. It is easier to balance the carbon and hydrogen atoms in the equation than the oxygen Chemical Reactions in this Chemical Reactions because Janius Face In Frankenstein of the carbon Chemical Reactions in propane end up in CO 2 and Chemical Reactions of the Chemical Reactions atoms Dropping Leaflets Poem Analysis up Chemical Reactions H 2 Chemical Reactions, but some of the oxygen atoms end Chemical Reactions in Chemical Reactions compound. Stephanie R. While heat is often necessary to start reactions, this need not Chemical Reactions the case.