Activation energy

Chemical reactions can occur at different rates. Some of them end in a few seconds, others can drag on for hours, days and even decades. In order to determine the productivity and size of the necessary equipment, as well as the amount of the product produced, it is important to know the rate at which chemical reactions occur. It can have different values, depending on:
-concentration of reacting substances;
-temperature of the system.

The Swedish scientist S. Arrhenius in the late nineteenth century derived an equation showing the dependence of the rate of the chemical reaction on such an indicator as the activation energy. This indicator is a constant value and is determined by the nature of the chemical interaction of substances.
According to the scientist's suggestion, only molecules that are formed from ordinary molecules and are in motion can enter into a reaction between themselves. Such particles were called active. The activation energy is the force necessary for the transition of ordinary molecules to a state in which their movement and reaction become the fastest.

During the course of chemical interactions, some particles of matter are destroyed, while others arise. In this case, the connections between them change, that is, the electron density is redistributed. The rate of the chemical reaction, in which the old interactions would be completely destroyed, would have a very low value. At the same time, the amount of energy supplied must be high. Scientific studies have shown that during the interaction of substances, any system forms an activated complex, which is its transition state. At the same time, old ties are weakened, and new ones are only being outlined. This period is very small. It is a fraction of a second. The result of the decay of this complex is the formation of initial substances, or products of chemical interaction.

In order for the transient component to arise, it is necessary to impart activity to the system. For this, the activation energy of the chemical reaction is also needed. The formation of the transition complex is determined by the strength that the molecules possess. The amount of such particles in the system depends on the temperature regime. If it is high enough, the fraction of active molecules is large. In this case, the magnitude of the force of their interaction is higher or equal to the index, called the "activation energy". Thus, at sufficiently high temperatures, the number of molecules capable of forming a transition complex is high. As a result, the rate of chemical reaction increases. On the contrary, if the activation energy is of great importance, the fraction of particles capable of interaction is small.
The presence of a high energy barrier is an obstacle to the occurrence of chemical reactions at low temperatures, although their probability exists. Exothermic and endothermic interactions have different characteristics. The first of them proceed with the lowest activation energy, and the second ones with the greater activation energy.

This concept is also used in physics. The activation energy of a semiconductor is the minimum force that should give acceleration to electrons for transition to the conduction band. During this process, the bonds between atoms are broken. In addition, the electron must move from the valence band to the conduction region. The increase in temperature is the reason for the enhancement of the thermal motion of the particles. In this case, some of the electrons go over into the state of free charge carriers. Internal connections can also be broken by an electric field, light, etc. The activation energy is much larger in intrinsic semiconductors than in impurity ones.