Before discovering that the Masse is one of the forms of the energy, the laws of the Physique and the Chimie had like principle the conservation of the mass .

Law of Lavoisier

The doctor Jean Rey published in 1630 a work under the title Essais on the investigation into the cause for which tin and lead increase weight when they are calcined . This work remained a long time been unaware of, until, in 1777, Antoine Lavoisier states the law which bears today its name in front of the Academy of Science: “Nothing is lost, nothing is not created, all changes. ”

This principle for example is illustrated by the following reaction: Ag⁺ + Cl^- - > AgCl

Are an ion Ag⁺ and an ion Cl^-, at the end of the reaction, the two ions did not disappear, it did not appear about it again, they simply joined chemically. The quantities of matter thus remain the same ones with the wire of the reaction. If the sum of the mass of the reagents at the beginning of the reaction is of 10 G, then the sum of the mass of the products at the end of the reaction is also of 10 G.

Energy of reaction

It is known today that what is called the energy of reaction in a chemical reaction is due to nature Quantique of the electronic structure of the matter.

This quantum nature limit the possible electronic configurations and defines the energy which corresponds to them. A chemical reaction corresponds to the transition from a configuration to another (passage of an energy level with another).

If the energy level of arrival is higher than that of departure, energy should be provided. The reaction is known as endo-energetics or endothermic (energy being provided by the medium, the temperature of this one decreases).

If the energy level of arrival is lower than that of departure, there is release of energy. The reaction is known as exo-energetics or exothermic (released energy is captured by the medium whose temperature increases).

In the case of a nuclear reaction (Fission or fusion), the mass of the cores of arrival and the starting components is different. This difference is translated in the form of energy by the famous relation of Einstein $E\; =\; mc\; ^2$, where $E$ is the energy released during the reaction and $m$ the variation of mass between the initial state and the final state and where " c" is the celerity which is speed of light in the vacuum of 300.000 km/s.

The energy released during a nuclear reaction is much more important than that implied during a chemical reaction. For example, the fusion of a proton and a neutron to form a core of Deutérium releases 3,36 J whereas the combustion of hydrogen and oxygen releases only 4,75 J per water molecule, that is to say approximately 700 thousand times less energy.

See too

• energy
• Constant physics

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