A chemical reaction is a transformation of the Matière. During a chemical reaction, the chemical species (atomic, ionic or molecular) which constitute the matter are modified: the species which are consumed are called Réactif S. the species formed during the reaction are called produced (of reaction). Since work of Lavoisier (1777), one knows that the chemical reaction is done without measurable variation of the Masse: it is the famous " Nothing is lost, nothing is not created, all transforme" who translates the Conservation of the mass.

The chemical reactions cause a change of the chemical nature of the matter, one thus excludes the purely physical transformations, like the changes of state (fusion, Solidification, evaporation, boiling…), wear and erosion, rupture…

One also excludes the transformation from the cores of the atoms, therefore the nuclear reactions.

A reaction can release from energy (in general in the form of heat, but also of the light), it is then known as exothermic . It can require a contribution of energy, in the form of heat (thus “to produce cold”) or of light, it is then known as endothermic . Generally, a reaction can only take place so certain conditions are joined together (presence of all the reagents, conditions of temperature, pressure, light). Certain reactions require, or are facilitated by the presence of a chemical substance called Catalyseur.

What occurs on the level from the atoms

The matter made up of Atome S is gathered in chemical compounds, during a chemical reaction, the compounds exchange their atoms; by doing this, the nature of the compounds changes. The chemical reactions relate to only the changes of connections between the atoms (covalent bonds, ionic connections, metal connections).

To represent the phenomena which take place during a chemical reaction, one writes a chemical equation.

Chemical reaction and energy

In Thermodynamic, one considers the total energy of a system. The transformations taking place during the chemical reaction involve a reduction in total energy. Indeed, in a Molecule or a Crystal, the “fixing” of the atoms between them requires energy, called energy binding. When a connection is broken, one “breaks” the molecule or the crystal in “scattering” his atoms. Energy should be provided. When the atoms recombine, they recover energy to form the connection. At the end of a reaction, the energy stored in the connections of the products of reaction is weaker than that which was stored in the connections of réactant.

But during the reaction, one moment ago when the old connections are broken and the news is not created yet. It is a state where the energy of the system is raised, a transitory state, which makes a genuine barrier with the reaction. The starting of the reaction quite simply consists in making cross this energy barrier with the system. This energy barrier is called energy of activation .

In order to simplify the study, one isolates the purely chemical part of energy by considering the other constant parts of energy, in particular the heat and the energy of compression. This chemical energy is called enthalpy and is noted H . The chemical profit of energy by the reorganization of the connections is the enthalpy of reaction $\backslash \; Delta\_\; \{R\}\; H$.

The study of the energy aspect of the chemical reactions is the Thermochimie.

Reaction speed

See also: Kinetic chemical

The study of the energy of the system (Thermochemistry) makes it possible to know if a reaction can occur or not, which initial energy it is necessary to provide to cross the barrier. But there is another important parameter: the reaction speed.

Examples of chemical reactions

Among the most current chemical reactions, let us quote:

• the Breathing, the lactic Fermentation and the alcoholic fermentation which make it possible the organizations to produce energy
• the secretion of products by the bodies (tears, sweats, saliva, gastric juices, Hormone S…), the action of these secretions
• the Combustion (inter alia in the spark-ignition engines and the boilers), the Fire
• the cooking of food, the burns
• the Corrosion of the matter (for example rust)
• the chlorophyllian Photosynthesis which makes it possible the plants to regenerate the Dioxygène air by recovering the Carbon dioxide
• the dissolution of metals by the acid
• the revelation of the Photographie S
• the manufacture of electricity by the piles, the storage and the release of electricity by the batteries and accumulating
• development of the metals starting from the Ore S (Metallurgy)
• the manufacture of the gasoline, oils and the plastic starting from the Oil
• the manufacture of the maintenance products: Soap (reaction of saponification), Bleach, Hydrochloric acid, Soda caustic, ammonium
• the manufacture of Manures, Pesticide S, produced plant health…
• the manufacture of the drugs
• wine making, the transformation alcohol → ethanoic acid (vinegar)
• the “turn with the green” of the breathalyser test
• pollution with the Ozone starting from the atmospheric pollutants
• destruction of ozone by the compounds chlorofluorocarbons (CFC, freons)

See too

chemical reaction|chemical reaction

Related articles

• chemical Chemistry

• Kinetic
• chemical Equation
• Chemical formula
• List of the chemical reactions

Simple: Chemical reaction Zh-yue: 化學反應

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