A chemical reaction is a process that leads to the transformation of a set of chemicals into another. Classically, a chemical reaction involves a change involving only the position of electrons in forming and breaking the chemical bonds between atoms, with no change to the nucleus (no change in the elements), and often can be described by chemical equations. Nuclear chemistry is a chemical sub-discipline that involves chemical reactions of unstable and radioactive elements in which electronic and nuclear changes can occur.
The substance (or substance) initially involved in a chemical reaction is called a reactant or reagent. Chemical reactions are usually characterized by chemical changes, and they produce one or more products, which usually have different properties than reactants. Reactions often consist of individual sub-step sequences, called elementary reactions, and information about appropriate action is part of the reaction mechanism. Chemical reactions are explained by chemical equations, which symbolically present the starting materials, the final product, and sometimes the intermediate product and reaction conditions.
Chemical reactions occur at the level of reaction characteristics at a given temperature and chemical concentration. Typically, the reaction rate increases with increasing temperature as there is more heat energy available to achieve the activation energy required to break the bonds between atoms.
Reactions can take place in a forward or backward direction until they reach a settlement or reach equilibrium. The reaction that goes in the forward direction to approximate equilibrium is often described as spontaneous, requiring no free energy input to go forward. Non-spontaneous reactions require free-for-forward energy input (for example including charging by applying an external power source, or photosynthesis driven by the absorption of electromagnetic radiation in the form of sunlight).
Different chemical reactions are used in combination during chemical synthesis to get the desired product. In biochemistry, a series of successive chemical reactions (in which the product of one reaction is the reactant of the next reaction) forms a metabolic pathway. These reactions are often catalyzed by protein enzymes. Enzymes increase the rate of biochemical reactions, so that metabolic synthesis and decomposition are not possible under ordinary conditions can occur at the temperature and concentrations present in the cell.
The general concept of chemical reactions has been extended to the reaction between smaller entities of atoms, including nuclear reactions, radioactive decay, and the reactions between elementary particles, as described by quantum field theory.
Video Chemical reaction
History
Chemical reactions such as burning in flames, fermentation and reduction of metal ores have been known since antiquity. The early theories of material transformation were developed by Greek philosophers, such as the Empedocles Four-Element Theory which states that any substance consists of four basic elements - fire, water, air and earth. In the Middle Ages, chemical transformations were studied by the alchemists. They are trying, in particular, to turn lead into gold, whose goal is to use lead and lead copper alloys with sulfur.
The production of chemicals that normally do not occur in nature has long been tried, such as the synthesis of sulfuric acid and nitric acid which is associated with the controversial alchemy of Jó bir ibn Hayy? N. This process involves heating of sulfate and nitrate minerals such as copper sulfate, alum and belching. In the 17th century, Johann Rudolph Glauber produced hydrochloric acid and sodium sulfate by reacting sulfuric acid and sodium chloride. With the development of the main room process in 1746 and the Leblanc process, enabling the production of large sulfuric acid and sodium carbonate, respectively, chemical reactions are being implemented into the industry. Further optimization of sulfuric acid technology resulted in a contact process in the 1880s, and the Haber process was developed in 1909-1910 for the synthesis of ammonia.
From the 16th century, researchers including Jan Baptist van Helmont, Robert Boyle, and Isaac Newton tried to develop experimentally observed theories of chemical transformation. The phlogiston theory was proposed in 1667 by Johann Joachim Becher. It postulates the existence of a fire-like element called "phlogiston", which is contained in a body that is flammable and released during combustion. This was proved wrong in 1785 by Antoine Lavoisier who discovered the correct explanation of combustion as a reaction with oxygen from the air.
Joseph Louis Gay-Lussac admitted in 1808 that gas always reacts in certain relationships with one another. Based on this idea and the atomic theory of John Dalton, Joseph Proust has developed a law of definite proportions, which then produced the concept of stoichiometry and chemical equations.
Regarding organic chemistry, it has long been believed that compounds obtained from living organisms are too complex to be obtained synthetically. According to the concept of vitalism, organic matter is endowed with a "vital force" and is distinguished from inorganic materials. This separation was terminated by the synthesis of urea from inorganic precursors by Friedrich WÃÆ'¶hler in 1828. Other chemists who brought great contributions to organic chemistry included Alexander William Williamson with the synthesis of ether and Christopher Kelk Ingold, who, among many discoveries, substitution.
Maps Chemical reaction
Equation
Chemical equations are used to describe graphical chemical reactions. They consist of the chemical or structural formula of the reactant on the left and the products on the right. They are separated by arrows (->) indicating the direction and type of reaction; arrows are read as "yield". The arrow tip points in the direction where the reaction takes place. Double arrows (?) Which point in opposite directions are used for equilibrium reactions. The equation must be balanced according to stoichiometry, the number of atoms of each species must be equal on both sides of the equation. This is achieved by the scale of the number of molecules involved ( and in the schematic example below with the exact integers a, b, c and d .
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More complicated reactions are represented by the reaction scheme, which in addition to the starting materials and products indicate important intermediates or transition states. Also, some relatively small additions to the reaction can be shown above the reaction arrow; Examples of such additions are water, heat, lighting, catalysts, etc. Similarly, some small products can be placed under arrows, often with a minus sign.
Retrosynthetic analysis can be applied to design complex synthesis reactions. Here the analysis begins with the product, for example by separating the selected chemical bonds, to arrive at a reasonable initial reagent. Special arrows (=>) are used in retro reactions.
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Basic reactions
The basic reaction is the smallest division in which a chemical reaction can be decomposed, it has no intermediate product. Most experimentally observed reactions are constructed from many elementary reactions that occur in parallel or in sequence. The actual sequence of individual basic reactions is known as the reaction mechanism. The basic reaction involves several molecules, usually one or two, because of the low probability for some molecules to meet at a given time.
The most important basic reaction is the reaction of unimolecules and bimolecules. Only one molecule is involved in the unimolecular reaction; it is transformed by isomerization or dissociation into one or more other molecules. Such a reaction requires the addition of energy in the form of heat or light. A typical example of an unimolecular reaction is cis-trans isomerization, in which the cis form of a compound converts to a trans form or vice versa.
In typical dissociative reactions, the bond in a split molecule ( shatter ) produces two molecular fragments. Separation can be homolytic or heterolytic. In the first case, the bonds are divided so that each product retains the electron and becomes a neutral radical. In the second case, the two electron bonds remain with one chemical product, producing charged ions. Dissociation plays an important role in triggering chain reactions, such as hydrogen-oxygen or polymerization reactions.
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- Disociation of AB molecules into fragments A and B
For bimolecular reactions, two molecules collide and react with each other. Their merging is called chemical synthesis or adduct reaction.
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Another possibility is that only a portion of one molecule is transferred to another molecule. This type of reaction occurs, for example, in redox and acid-base reactions. In redox reactions, the particles transferred are electrons, while in acid-base reaction is a proton. This type of reaction is also called metathesis.
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as an example
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Chemical equilibrium
Most chemical reactions are reversible, ie they can and run in both directions. The forward and backward reactions compete with each other and differ in reaction rates. This level depends on the concentration and therefore changes with the reaction time: the reverse rate gradually increases and becomes equal to the rate of forward reaction, forming so-called chemical equilibrium. The time to achieve balance depends on the parameters such as the temperature, pressure and the material involved, and is determined by the minimum free energy. In equilibrium, Gibbs free energy must be zero. Dependency of pressure can be explained by Le Chatelier principle. For example, an increase in pressure due to a decrease in volume causes the reaction to shift sideways with fewer moles of gas.
The reaction yield is stable at equilibrium, but can be increased by removing the product from the reaction mixture or modified by increasing the temperature or pressure. The change in the concentration of the reactant does not affect the equilibrium constant, but affects the equilibrium position.
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Thermodynamics
The chemical reaction is determined by the laws of thermodynamics. Reactions can run on their own if they are exergonic, ie if they release energy. The corresponding reaction-free energy comprises two different thermodynamic, enthalpy and entropy quantities:
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- G : free energy, H : enthalpy, T : temperature, S : entropy , ? : difference (changed between original and product)
The reaction can be exothermic, where H is negative and energy is released. Typical examples of exothermic reactions are precipitation and crystallization, in which solid solids are formed from phases of irregular gases or liquids. Conversely, in endothermic reactions, heat is consumed from the environment. This can occur by increasing the entropy of the system, often through the formation of a gas reaction product, which has high entropy. Because entropy increases with temperature, many preferred endothermic reactions occur at elevated temperatures. In contrast, many exothermic reactions such as crystallization occur at low temperatures. Temperature changes can sometimes reverse the reaction enthalpy signals, such as carbon monoxide reduction from molybdenum dioxide:
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This reaction to form carbon dioxide and molybdenum is endothermic at low temperatures, becoming less so with increasing temperature. ? HÃ, Â ° is zero at 1855Ã, K , and the reaction becomes exothermic above that temperature.
Temperature changes can also reverse the trend direction of the reaction. For example, the reaction of water gas shifts
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preferably by a low temperature, but otherwise favored by high temperatures. The trend direction shift occurs in 1100Ã, K .
Reactions can also be characterized by internal energy that takes into account changes in entropy, volume and chemical potential. The latter depends, inter alia, on the activity of the substances involved.
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- U : internal energy, S : entropy, p : pressure, ? : chemical potential, n : number of molecules, d : small change sign
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Kinetics
The speed at which the reaction takes place is studied by the reaction kinetics. Rates depend on various parameters, such as:
- The concentrations of reactants, which usually make the reaction happen at a faster rate if lifted by increasing collisions per unit time. Some reactions, however, have an independent level of reactant concentration. This is called a zero-order reaction.
- The surface area is available for contact between reactants, especially those that are solid in heterogeneous systems. Larger surface areas lead to higher reaction rates.
- Pressure - increases the pressure of reducing the volume between molecules and therefore increases the frequency of collisions between molecules.
- The activation energy, which is defined as the amount of energy needed to make the reaction begin and proceed spontaneously. Higher activation energy implies that the reactants require more energy to start than the reaction with lower activation energy.
- Temperature, which speeds up the reaction if raised, as higher temperatures increase the molecular energy, creating more collisions per unit of time,
- The presence or absence of a catalyst. The catalyst is a substance that alters the path (mechanism) of a reaction which in turn increases the reaction rate by decreasing the activation energy required for the reaction to take place. The catalyst is not destroyed or altered during the reaction, so it can be used again.
- For some reactions, the presence of electromagnetic radiation, especially ultraviolet light, is required to induce breaking of the bond to initiate the reaction. This is especially true for reactions involving radicals.
Some theories allow calculating the reaction rates at the molecular level. This field is referred to as the dynamics of the reaction. The v value of the first-order reaction, which can be a disintegration of a substance A, is given by:
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Results of integration:
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Here k is the first order-level constant having dimension 1/time, [A] (t) is the concentration at a time t and [A] 0 is the initial concentration. The rate of first-order reactions depends only on the concentration and nature of the substances involved, and the reaction itself can be described by the characteristics of part-time. More than one constant is necessary when explaining reactions from higher orders. The temperature dependence of the rate constants usually follows the Arrhenius equation:
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where E a is the activation energy and k B is Boltzmann's constant. One of the simplest reaction rate models is the collision theory. More realistic models are tailored to specific problems and include transition state theory, potential energy level calculations, Marcus theory and Rice-Ramsperger-Kassel-Marcus (RRKM) theory.
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Types of reactions
Four basic types
Synthesis
In the synthesis reaction, two or more simple substances combine to form more complex substances. These reactions are in a general form:
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Two or more reactants producing one product are another way of identifying the synthesis reaction. One example of a synthesis reaction is a combination of iron and sulfur to form iron (II) sulfide:
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Another example is simple hydrogen gas combined with simple oxygen gas to produce more complex substances, such as water.
Decomposition
The decomposition reaction is when more complex substances decompose into simpler parts. Thus the opposite of the synthesis reaction, and can be written as
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One example of a decomposition reaction is the electrolysis of water to make oxygen and hydrogen gas:
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Single replacement
In a single replacement reaction, one unincorporated element displaces another in a compound; in other words, one element trades its place with other elements in a compound. This reaction appears in a general form:
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One example of a single displacement reaction is when magnesium replaces hydrogen in water to make magnesium hydroxide and hydrogen gas:
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Double replacement
In a double-replacement reaction, the anions and cations of the two compounds exchange places and form two very different compounds. These reactions are in a general form:
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For example, when barium chloride (BaCl 2 ) and magnesium sulfate (MgSO 4 ) react, SO 4 2 - anions alternated with anion 2Cl - , giving the compound BaSO 4 and MgCl 2 .
Another example of a dual displacement reaction is the reaction of lead (II) nitrate with potassium iodide to form lead (II) iodide and potassium nitrate:
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Oxidation and reduction
The redox reaction can be understood in terms of transfer of electrons from one species involved (reductant) to another (oxidizer). In this process, the previous species is oxidized and the last is reduced . Although it is sufficient for many purposes, this description is not entirely correct. Oxidation is better defined as an increase in the oxidation state, and a reduction as a decrease in the oxidation state. In practice, electron transfer will always change the oxidation state, but there are many reactions that are classified as "redox" even though no electron transfer occurs (such as those involving covalent bonds).
In the following redox reaction, harmful sodium metals react with toxic chlorine gas to form ionic compounds of sodium chloride, or common table salt:
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In reaction, the sodium metal goes from the oxidation state 0 (since it is a pure element) to 1: in other words, sodium loses one electron and is said to have oxidized. On the other hand, chlorine gas goes from oxidation 0 (it's also a pure element) to -1: chlorine gets an electron and is said to have decreased. Sin
Source of the article : Wikipedia
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