What Is Le Chatelier's Principle?

Le Chatelier's principle / The Equilibrium raw, also known as the "chemical equilibrium movement principle", "Le Chatelier principle", was discovered by French chemist Le Chatelier in 1888. It is a principle for qualitatively predicting the chemical equilibrium point. The specific content is: if the conditions of the reversible reaction (such as concentration, pressure, temperature, etc.) are changed, the chemical equilibrium will be destroyed and it will move in a direction that weakens this change. ^[1]

Le Chatelier's principle (also known as

English expression: Every system in stable chemical equilibrium submitted to the influence of an exterior force which tends to cause variation either in it s temperature or condensation (pressure, concentration, number of molecules in the unit of volume), in its totality or only in some of it s parts, can undergo only those interior modifications change of temperature, or of condensation, of a sign contrary to that resulting from the exterior force.

Translation is as follows:

Any stable chemical equilibrium system under the influence of external forces, whether in whole or only in part, causes its temperature or compression degree (pressure, concentration, number of molecules per unit volume) to change. If they occur separately, the system can make internal adjustments. Changes in temperature or compression are opposite to changes caused by external forces. ^[2]

Le Chatelier's principle is one of the most important in terms of balance. Almost all problems of balance movement can be explained and judged by Le Chatelier's principle. The premise of correct application is to be proficient in the nature and connotation of the principle and to be familiar with various situations.

Le Chatelier's principle

When the concentration of a certain reactant is increased, the reaction proceeds in the direction of decreasing the concentration of the reactant, that is, the reaction equilibrium moves to the positive reaction direction. When the concentration of a certain product is decreased, the reaction proceeds in the direction of increasing the concentration of the product, that is, the reaction equilibrium moves toward the positive reaction direction. The reaction rate and yield will also change due to the influence on the external factor system.

This can be demonstrated by the equilibrium of methanol produced from hydrogen and carbon monoxide:

CO + 2H ₂ CH ₃ OH Suppose we increase the concentration of carbon monoxide in the system. Applying Le Chatelier's principle, it is expected that the amount of methanol will increase so that the amount of carbon monoxide will decrease. If a substance is added to the system, the equilibrium system tends to reduce the response of that substance. Conversely, reducing one substance causes the system to enhance the response that produces it. This observation can be explained by collision theory. As the carbon monoxide concentration increases, the number of effective collisions between the reactants increases, which increases the positive reaction rate and generates more products. Even if the product is difficult to produce from a thermodynamic point of view, if the product is continuously removed from the system, the final product can still be obtained.

Le Chatelier principle temperature change

Increasing the reaction temperature, the reaction proceeds in the direction of reducing heat, that is, the exothermic reaction proceeds in the reverse direction, and the endothermic reaction proceeds in the forward direction; decreasing the temperature, the reaction proceeds in the direction of generating heat, that is, the exothermic reaction proceeds in the forward direction, and The thermal reaction proceeds in reverse.

In judging the effect of temperature on equilibrium, the change in energy should be considered as one of the substances participating in the reaction. For example, if the reaction is an endothermic reaction, that is, when H> 0, the heat is regarded as a reactant and placed on the left side of the equation; otherwise, when the reaction is an exothermic reaction. That is, N ₂ + 3H ₂ 2NH ₃ H = 92kJ / mol, which can be rewritten as 2 NH ₃ N ₂ + 3 H ₂ H = + 92KJ / mol

The reaction of nitrogen and hydrogen is a reversible reaction commonly used as an example.

This reaction is exothermic; if the temperature is lowered, the equilibrium will shift to the right to generate more heat, increasing the production of ammonia. In practical applications, such as the Hubble process for ammonia synthesis, the temperature is still set to a higher value to ensure a fast reaction rate even if the high temperature reduces the yield.

In an exothermic reaction, an increase in temperature causes the value of the equilibrium constant K to decrease; conversely, the value of K in an endothermic reaction increases with increasing temperature.

Le Chatelier principle pressure changes

Stress is still responding in the direction of eliminating changes in balance. When the pressure of a certain gaseous reactant is increased, the reaction proceeds in a direction of decreasing the pressure of the reactant, that is, the reaction proceeds in a positive direction. When the pressure of a gaseous product is reduced, the reaction proceeds in the direction of increasing the pressure of the product, that is, the reaction proceeds in the positive direction. vice versa.

Take the famous Hubble method for ammonia production as an example:

N ₂ (g) + 3H ₂ (g) 2NH ₃ (g) The coefficients on the left and right of the reaction are not the same, so when the pressure suddenly increases after equilibrium, the reaction will proceed in the direction of the gas coefficient and gas volume. In this case, that is, in the direction of increasing NH ₃ . Conversely, if the pressure suddenly decreases after equilibrium, the reaction will proceed in the direction of larger gas coefficient and gas volume, so every two molecules of NH ₃ will decompose into one molecule of N ₂ and three molecules of H ₂ .

However, when the coefficients of gas reactants and gas products are the same, the system equilibrium is not changed by the external pressure. For example, the reaction of carbon monoxide and water at high temperature to form carbon dioxide and hydrogen:

CO (g) + H ₂ O (g) CO ₂ (g) + H ₂ (g) No matter how the external pressure changes, it will not affect the shift of equilibrium.

Effects of inert gases (also called rare gases):

1. If the volume of the container is not allowed to change before and after the reaction, the concentrations of the reactants and products will not change, the pressure will increase at the same time, and the reaction equilibrium will not change.

2. If the volume of the container is allowed to change before and after the reaction, the volume of the container is increased after the inert gas is added. At this time, the concentration of the reactant and the product is reduced, and the reaction continues to the side with a higher molar amount of gas.

If a gas is added to the chemical formula , the concentration will affect the left and right of the chemical balance.

Catalytic effect of Le Chatelier principle

Only changing the rate at which the reaction proceeds does not affect the change in equilibrium, that is, the degree of influence on the forward and reverse reactions is the same.

Summary of Le Chatelier's Principle

From the above analysis, we know that some situations that usually occur are within the scope of Le Chatelier's principle. Therefore, when we encounter problems involving balanced movement, as long as we correctly analyze Le Chatelier's principle, we can draw appropriate answers. ^[4]