Many people know about the fact of the existence of the Le Chatelier principle from the school bench. But few understand and can explain exactly what this well-known principle is.
The French scientist told the world about the law of dynamic equilibrium in 1884. For the end of the nineteenth century, the discovery was very significant and immediately attracted the attention of the scientific community. But due to the lack of international scientific cooperation a century and a half ago, only his compatriots knew about Le Chatelle's scientific breakthrough. In 1887, the German scientist Karl Ferdinand Brown, who discovered the same scientific law, being unaware of the discovery of the Frenchman, said about a shift in chemical equilibrium with changing external conditions. It is no accident that this principle is often called the Le Chatelier - Brown principle.
So what is the principle of Le Chatelier?
Equilibrium systems always strive to maintain their equilibrium and counteract external forces, factors, and conditions. This rule is valid for any systems and for any processes: chemical, electrical, mechanical, thermal. The Le Chatelier principle is of particular practical importance for reversible chemical reactions.
The effect of temperature on the reaction rate is directly dependent on the type of reaction in terms of thermal effect. With increasing temperature, an equilibrium shift towards the endothermic reaction is observed. Lowering the temperature, respectively, leads to a shift in chemical equilibrium towards an exothermic reaction. The reason for this seems to be that when the system is brought out of equilibrium by external forces, it goes into a state of less dependence on factors from outside. The dependence of endothermic and exothermic processes on the equilibrium state is expressed by the Van Goff equation:
V2 = V1 * y (T2-T1) / 10,
where V2 is the rate of a chemical reaction at a changed temperature, V1 is the initial reaction rate, and y is an indicator of the temperature difference.
The Swedish scientist Arrhenius derived a formula for the exponential dependence of the reaction rate on the temperature regime.
K = A • e (-E (RT)), where E is the activation energy, R is the universal gas constant, and T is the temperature in the system. The value of A is a constant.
With increasing pressure, a shift in chemical equilibrium is observed in the direction where substances occupy a smaller volume. If the volume of the starting materials is greater than the volume of the reaction products, then the equilibrium shifts toward the starting components. Accordingly, if the volume of reaction products exceeds the volume of reagents, then the equilibrium shifts toward the resulting chemical compounds. It is assumed that each mole of gas occupies the same volume under normal conditions. But a change in pressure in the system does not always affect chemical equilibrium. The Le Chatelier principle shows that adding inert gas to the reaction changes the pressure, but does not bring the system out of balance. Moreover, only the pressure that is associated with reacting substances is significant for the reaction (helium does not have free electrons, it does not interact with substances in the system).
Adding a certain amount of a substance to the reaction leads to a shift in equilibrium towards the process where this substance becomes smaller.
Equilibrium is dynamic. It is “broken” and “leveled” in a natural way during the course of the reaction. Let us explain this position through an example. Hydrogenation of a bromine solution produces hydrobromic acid. There comes a time when the final product is formed too much, its volume exceeds the total volume of hydrogen and bromine monomolecules, and the reaction rate slows down. If you add hydrogen or bromine to the system, the reaction will go in the opposite direction.