Principle Le Chatelier: a scientific breakthrough of the 18th century

Many people know about the existence of the Le Chatelier principle from the school bench. But few understand and can explain what this famous principle is.

The French scientist told the world about the law of dynamic equilibrium in 1884. For the late 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 the scientific breakthrough of Le Chatelier. In 1887, the German scientist Karl Ferdinand Brown, who independently discovered the same scientific law, being ignorant of the discovery of the Frenchman, said about the shift of the chemical equilibrium under 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?

The systems that are in equilibrium always try to maintain their equilibrium and counteract external forces, factors and conditions. This rule applies to all systems and for any process: chemical, electrical, mechanical, thermal. The Le Chatelier principle has a special practical significance for reversible chemical reactions.

The effect of temperature on the reaction rate is directly dependent on the type of reaction for the thermal effect. With an increase in temperature, a shift in the equilibrium toward the endothermic reaction is observed. Lowering the temperature, respectively, leads to a shift in the chemical equilibrium toward the exothermic reaction. The reason for this is seen in the fact that when the system is removed from equilibrium by external forces, it turns into a state of less dependence on external factors. The dependence of endothermic and exothermic processes on the equilibrium state is expressed by the Van't Hoff equation:

V2 = V1 * y (T2-T1) / 10,

Where V2 is the rate of the chemical reaction at a changed temperature, V1 is the initial reaction rate, and y is the temperature difference parameter.

Swedish scientist Arrhenius deduced the 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.

As the pressure increases, a shift in chemical equilibrium is observed in the direction where the substances occupy a smaller volume. If the volume of initial substances is greater than the volume of the reaction products, then the equilibrium shifts towards the original 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 the change in pressure in the system does not always affect the chemical equilibrium. The Le Chatelier principle shows that the addition of an inert gas to the reaction changes pressure, but does not remove the system from equilibrium. In this case, only the pressure that is associated with the reacting substances is significant for the reaction (helium does not have free electrons, it does not interact with substances in the system).

The addition of a certain amount of a substance to the reaction results in a shift in the equilibrium towards the process where this substance becomes smaller.

Equilibrium has a dynamic character. It is "disturbed" and "leveled" in a natural way during the course of the reaction. Let us explain this situation through an example. Hydrogenation of the bromine solution produces hydrobromic acid. There comes a time when the end product is formed too much, its volume exceeds the total volume of monomolecules of hydrogen and bromine, the reaction rate slows down. If you add hydrogen or bromine to the system, the reaction will go in the opposite direction.