A chemical reaction is a process in which new substances that the reaction proceeds, the original form of some substances, called reactants. Usually a chemical reaction accompanied by physical events, such as discoloration, sludge formation, or the onset of gas. Chemical reactions are usually characterized by a chemical change, and will produce one or more products that typically have characteristics that are different from the reactants. Classically, chemical reactions involve changes involving the movement of electrons in the forming and breaking of chemical bonds, although the general concept is basically a chemical reaction can also be applied to the transformation of elementary particles such as the nuclear reaction.
Reactions with different chemical used in chemical synthesis to produce the desired compound. In biochemistry, series of chemical reactions catalyzed by enzymes form metabolic pathways, in which the synthesis and decomposition is usually not possible in the cell do.

Equation
Equation used to describe chemical reactions. Equation consists of a chemical formula or structural formula of the reactants on the left and the right products. Between products and reactants are separated by arrows (→) which indicates the direction and type of reaction. The tip of the arrow shows which way the reaction moves. Double arrow (), which has two different ends direction arrows, used in the equilibrium reaction. Chemical equations must be balanced according to the stoichiometry, the number of atoms of each element on the left must equal the number of atoms of each element on the right. Balancing is done by adding the numbers in front of each molecule compounds (denoted by A, B, C and D in the schematic diagram below) with a small number (a, b, c and d) in the future.

aA + bB —> cC + dD

More complicated reactions represented by reaction schemes, the goal is to find compounds early or late, or also to indicate the phase transition. Some chemical reactions can also be added to the writing on the sig
n bow; eg the addition of water, heat, illumination, catalysis, etc.. Also, some minor products can be placed below the arrow.

Retrosintetik analysis can be used to design a complex synthesis reaction. The analysis starts from the products, for example by breaking chemical bonds are chosen to be the new reagents. Special arrows (⇒) is used in retro reactions.

Elementary reaction

Elementary reaction is the reaction of the simplest solution and the result of this reaction has no byproducts. Most reactions have been found at this time is the development of the emergence of elementary reactions in parallel or sequentially. An elementary reaction usually consists of only a few molecules, usually only one or two, because it is unlikely for many molecules join together.

Azobenzena isomerization induced by light (hν) or heat (Δ)
The most important reaction is an elementary reaction and the reaction unimolekuler bimolekuler. Reaction unimolekuler consists of only one molecule formed from the transformation or diasosiasi one or more other molecules. Some of these reactions require energy from light or heat. An example of unimolekuler reaction is cis-trans isomerization, in which a compound cis form will turn into a form of trance.
In the dissociation reaction, a bond in a molecule is split into two fragments of molecules. This solution can be homolitik or heterolitik. In solving homolitik, the bond will be split so that each product still has one electron to become neutral radicals. In solving heterolitik, both electrons of the chemical bond will remain one of its products, so it will produce charged ions. Dissociation reactions play an important role in the chain reaction, such as for example hydrogen-oxygen or polymerization reactions.

Disoasi of AB molecules into fragments A and B
In bimolecular reactions, two molecules will react with each other and bertabreakan. The results of the reaction is called chemical synthesis or an addition reaction.

Another possible reaction is part of a molecular switch to other molecules. The reaction of this type, for example, is a redox and acid-base reactions. In the redox reaction that moving particles are electrons, whereas the acid-base reactions are proton moving. The reaction is also called metathesis reactions.

example
NaCl (aq) + AgNO3 (aq) → NaNO3 (aq) + AgCl (s)

thermodynamics

Chemical reactions can be determined by the laws of thermodynamics. The reaction can occur by itself when the compound is exergonic or energy release. The resulting free energy reaction consists of two massive thermodynamic enthalpy and entropy are

G: free energy, H: enthalpy, T: temperature, S: entropy, Δ: difference
Exothermic reaction occurs when ΔH is negative and energy is released. Examples of exothermic reactions are precipitation and crystallization, in which the solids are formed from gases or liquids. In contrast, in endothermic reactions, heat is taken from the environment. This can be done by increasing the entropy of the system. Because of entropy increase is directly proportional to its temperature, then most endothermic reaction carried out at high temperatures. In contrast, most of the exothermic reaction carried out at low temperature. Changes in temperature can sometimes change the course of the reaction, such as for example the Boudouard reaction:

The reaction between carbon dioxide and carbon to form carbon monoxide is endothermic at temperatures above 800 ° C and becomes exothermic reaction if the temperature is below this temperature
The reaction can also be determined by the energy that causes a change in entropy, volume, and chemical potential.

U: energy, S: entropy, p: pressure, μ: chemical potential, n: number of molecules, d: small change sign means

Grouping chemical reaction

The diversity of chemical reactions and approaches taken in the study resulted in many ways to classify these reactions, which often overlap. Below are examples of classification of chemical reactions that are normally used.

Four basic reaction

Synthesis
In direct combination or synthesis reaction, two or more simple compounds combine to form a new, more complex compound. Two or more reactants react to produce a product that is also one way to find out if it is the synthesis reaction. An example of this is the reaction of hydrogen gas with oxygen gas combine the result is water.
Another example is nitrogen gas combine with hydrogen gas to form ammonia, by the equation:

N2 + 3 H2 → 2 NH3

Dekomposisisi

Decomposition reaction is the opposite of analysis or synthesis reactions. A more complex compounds are broken down into simpler compounds.
Examples are water molecules are split into hydrogen gas and oxygen gas, with the equation:

2 H2O → 2 H2 + O2

Single replacement

In a single replacement or substitution reaction, a single element of a single element replaces another in a compound. Examples are sodium metal reacts with hydrochloric acid will produce sodium chloride or table salt, the reaction persamaaan:
2 Na (s) + 2 HCl (aq) → 2 NaCl (aq) + H2 (g)

Replacement double

In a double replacement reaction, two ionic compounds or bond mutual change to a different form new compounds. [15] This occurs when the cations and anions of two different compounds each move, and form two new compounds. The general formula of this reaction is:

AB + CD → AD + CB

An example of a double replacement reaction is lead (II) nitrate reacts with potassium iodide to form lead (II) iodide and potassium nitrate, with the equation:
Pb (NO3) 2 + 2 KI → PbI2 + 2 KNO3

Another example is sodium chloride (table salt) reacts with silver nitrate to form sodium nitrate and silver chloride, the reaction equation:
NaCl (aq) + AgNO3 (aq) → NaNO3 (aq) + AgCl (s)

Oxidation and reduction

Illustration of a redox reaction (oxidation reduction)

The two parts of a redox reaction
Redox reactions can be understood as the transfer of electrons from one compound (called a reducing agent) to another compound (called oxidants). In this process, one will be oxidized compounds and other compounds are reduced, therefore called redox. Oxidation itself is understood as an increase in oxidation and reduction is a decrease in oxidation. In practice, the transfer of electrons will always change the oxidation number, but many of the reactions were classified as redox reactions despite the fact that no electrons are moved (such as those involving covalent bonds).
Examples of redox reactions are:

2 S2O32-(aq) + I2 (aq) → S4O62-(aq) + 2 I-(aq)

Which I2 is reduced to I-and S2O32-(thiosulfate anion) is oxidized into S4O62-.
To determine which reactant will be a reducing agent and which will be oxidized to unknown agents of electronegativity elements. Elements that have a low electronegativity value, like most metals, it will easily give their electrons and oxidize – this element into the reductant. Conversely, many ions have a high oxidation state, such as H2O2, MnO4-, CrO3, Cr2O72-, OsO4) can have one or more extra electrons, so-called oxidants.
The number of electrons that are given or received in the redox reaction can be determined from the configuration of the reactant element elektronn. Each element will try to make the same electron configuration as the noble gas configuration element. Alkali metals and halogens will give and accept one electron. Elements of the natural gas itself is not chemically active.

One important part of redox reactions are the electrochemical reactions, where the electrons from the power source is used as the reductant. This reaction is important for the manufacture of chemical elements, such as chlorine or aluminum. The reverse process where the redox reaction is used to generate electricity and also there is the principle used in the battery.

Acid-base reactions

Acid-base reaction is the reaction that donates a proton from a molecule of acid to alkaline molecules. Here, acids act as proton donors and bases act as a proton acceptor.

Acid-base reaction, HA: acid, B: Bases, A-: conjugated base, HB +: conjugated acid
The results of this proton transfer is the conjugate acid and conjugate base. Reaction equilibrium (back and forth) also exist, and therefore acid / alkaline and acid / conjugate base is always in equilibrium. The reaction equilibrium is characterized by the presence of acids and bases diasosiasi constants (Ka and Kb) of each substance. A special reaction of the acid-base reaction is the neutralization of acids and bases in which the same amount of salt will form a neutral nature.
Acid-base reactions have different definitions depending on the acid-base concepts used. Some of the most common definitions are:
Definition Arrhenius: acid dissociates in water releasing H3O + ions; bases dissociate in water releasing OH-ions.
Brønsted-Lowry definition: Acids are proton donors (H +) donors; bases is the recipient (acceptor) proton. Arrhenius definition covers
Lewis definition: Acids are electron pair acceptors; bases are electron pair donors. This definition covers the Brønsted-Lowry definition.

Precipitation

Precipitation
Precipitation is the formation reaction of solids (sediment) in a solution as a result of chemical reactions. Precipitation is usually formed when the concentration of the dissolved ions have reached the solubility limit and the result is a form of salt. This reaction can be accelerated by adding a reducing agent or solvent precipitation. Rapid precipitation reaction will produce microcrystalline residue and slow process that will result in a single crystal. Single crystals can also be obtained from recrystallization from microcrystalline salts.

Reactions on solid
The reaction can take place between two solids. However, since the rate of diffusion in solids is very low, the chemical reactions that take place happen very slowly. The reaction can be accelerated by increasing the temperature so it will break the reactants, so that the contact surface area becomes larger.

Photochemical Reaction

In the Paterno-Buchi reaction, an excited carbonyl group into olefin will diamahkan not excited, and produce oksetan.
In photochemical reactions, atoms and molecules absorb energy (photons) from the light and convert it into excitation. Atoms and molecules and can release energy by breaking chemical bonds, the yield radicals. Reaction ang included in photochemical reactions include hydrogen-oxygen reactions, radical polymerization, chain reactions and rearrangement reactions.
Many important processes using photochemical. The most common example is photosynthesis, which plants use solar energy to convert carbon dioxide and water into glucose and oxygen as a byproduct. Humans rely on photochemistry in the formation of vitamin D, and visual perception resulting from the photochemical reaction in rhodopsin. In fireflies, an enzyme in the abdomen catalyzes a reaction that produces bioluminesensi. Many photochemical reactions, such as ozone formation, occur in the Earth’s atmosphere, which is part of the atmospheric chemistry.

Catalysis

Energy schematic diagram showing the effect of giving a catalyst in an endothermic chemical reaction. Presence of a catalyst will speed up the reaction by lowering the activation energy. The end result will be the same as the reaction without catalyst.
In catalysis, the reaction does not take place spontaneously, but through a third substance known as catalyst. Unlike other reagents that participate in the chemical reaction, the catalyst does not take part in the reaction itself, but to inhibit, shut down, or destroyed by secondary processes. The catalyst can be used in different phases (heterogeneous catalysts) and in the same phase (homogeneous catalyst) as the reactants. The function of the catalyst is accelerating the reaction – chemicals that slow the reaction are called inhibitors. Substances that increase the activity of catalysts are called promoters, and substances called catalysts deadly poison catalysts. A chemical reaction that should not take place because the activation energy is too high, it could be underway by the presence of this catalyst.
Heterogeneous catalysts are usually solids and powder form in order to maximize the surface area that reacted. Substances that are important in heterogeneous catalysis include platinum group metals and other transition metals. These substances are commonly used in hydrogenation, catalytic formation and synthesis of chemical compounds such as nitric acid and ammonia. Acid is an example of a homogeneous catalyst, they increase nukleofilitas of carbonyl. The advantages of homogeneous catalysts is easy to be mixed with the reactant, but the drawback is difficult to be separated from the final product. Therefore, heterogeneous catalysts are preferred in many industrial processes.

Reactions in organic chemistry

In organic chemistry, many reactions that can occur involving covalent bonds between carbon atoms and heteroatoms such as oxygen, nitrogen, or other halogen atoms. Some more specific reactions are described below.

Substitution
In a substitution reaction, a functional group in a chemical compound is replaced by other functional groups. This reaction can be divided into several subtypes, namely nucleophilic, electrophilic substitution, or radical substitution.

SN1 mechanism

SN2 mechanism
In the first type, a nucleophile, an atom or molecule that has an excess of electrons that are negatively charged, will replace another atom or other parts of the molecule “substrate”. Nucleophile electron pair will unite to form a new bond with the substrate, while the group will take off along with a pair of electrons. Nucleophile itself can be neutral or positively charged, whereas the substrate is usually positively charged or neutral. Examples of nucleophiles are hydroxide ion, alkoxides, amines and halides. Such reactions are usually found in aliphatic hydrocarbons, and rarely found in aromatic hydrocarbons. Aromatic hydrocarbons having a steeper electron density and could only hold a nucleophilic aromatic substitution only with towing force powerful electron. Nucleophilic substitution can take place through two mechanisms, SN1 and SN2 reactions. According to its name, the S stands for substitution, N stands for nucleophilic dai, and, and the numbers indicate the order kinetic reaction, unimolekuler or bimolekuler.

3 stages in SN2 reactions. Nucleophile green and red loose clusters

SN2 reaction causes inversion stereo (Walden inversion)
SN1 reaction takes place in two stages. The first phase, the group will take off and form a carbocation. This stage will be followed by a very rapid reaction with the nucleophile.
In the SN2 mechanism, a nucleophile to form a transition phase with molecules that are terlekang off. These two mechanisms differ in stereochemistry results. SN1 reaction produces non-stereospecific addition and does not produce a chiral center, but rather in the form of geometrical isomers (cis / trans). In contrast, inversion Warden who observed the SN2 mechanism.
Electrophilic substitution is the opposite of nucleophilic substitution in which the atoms or molecules of a release, or elektrofilnya, has a low electron density thus positively charged. Usually this is the electrophile carbon atom of the carbonyl groups, carbocations or sulfur or nitronium cations. This reaction takes place in the course of aromatic hydrocarbons, so-called electrophilic aromatic substitution. Electrophile attack would create a complex called σ-compleks, a phase transition in which the aromatic system is lost. Then, loose clusters (typically protons), will separate and the nature kearomatikannya back. Another alternative to aromatic substitution is electrophilic aliphatic substitution. Substitution is similar to electrophilic aromatic substitution and also has two major types, namely SE1 and SE2

Addition and elimination

Addition and elimination partner is a reaction that converts the number of substituents on the carbon atoms, and form a covalent bond. Double and triple bonds can be produced by eliminating a suitable loose clusters. As the nucleophilic substitution, there are several possible reaction mechanisms. In the E1 mechanism, first remove loose clusters and form a carbocation. Furthermore, the formation of a double bond occurs through the elimination of a proton (deprotonation). In E1cb mechanism, reverse the order of release: proton eliminated first. In this mechanism there should be the involvement of a base. [35] in the elimination reaction E1 and E1cb always competes with SN1 substitution reaction conditions because they have the same condition.

E2 mechanism also requires a base. However, the change of position of the group off the bases and elimination take place simultaneously and produce no ionic intermediates. In contrast to the E1 eliminations, different stereochemical configurations can be generated in the reaction with E2 mechanism bases will be more favored for elimination of protons that are in a position to force the anti-loose. Therefore, the reaction conditions and reagents were similar, E2 elimination always compete with SN2 substitution.

Electrophilic addition of hydrogen bromide
The opposite of elimination reaction is an addition reaction. In addition reactions, double bonds or triple converted to a single bond. Similar to the substitution reactions, there are several types of adducts were indistinguishable from particles mengadisi. For example, the electrophilic addition of hydrogen bromide, an electrophile (proton) will replace the double bond to form carbocation, and then reacts with the nucleophile (bromine). Carbocation can form a bond depends on the group attached at the end. Configurations can be predicted more accurately with the Markovnikov rule. Markovnikov rule says: “In addition sebuuah heterolitik of polar molecules in the alkene or alkyne, atoms that have a large electronegativity, it will be bound to the carbon atom with fewer hydrogen atoms.”

Other organic chemical reaction

Orbital overlap in a Diels-Alder reaction
In the rearrangement reaction, the carbon skeleton of a molecule rearranged to form isomeric structure of the original molecule. The reaction was included with [reaction sigmatropik]] such as the Wagner-Meerwein rearrangement, where hydrogen, alkyl, or aryl place moving from one carbon atom to another carbon atom. Most of the rearrangement reaction is breaking and formation of new carbon-carbon bonds. Another example of this reaction is the rearrangement cope.

another reaction

Isomerization, which undergo chemical rearrangement of the structure without a change in the composition of the atom
Burning, is a kind of redox reaction in which materials can ignite join elements oxidant, usually oxygen, to generate heat and form oxidized products. Combustion term usually used to refer only to the large-scale oxidation of whole molecules. Controlled oxidation of only one single functional group is not included in the combustion process.
C10H8 + 12 O2 → 10 CO2 + 4 H2O
CH2S + 6 F2 → CF4 + 2 HF + SF6
Disproportionation, with one reactant form two different types of products only in the state of oxidation.
SN2 → Sn 2 + + + SN4