The Le Châtelet principle, also known as the principle of balanced movement, is expressed in various versions of the same principle in textbooks. The Le Châtelier Principle in the high school chemistry edition of Principles of Chemical Reactions (February 3, 2007) describes the Le Châtelier principle as follows: "If one of the conditions affecting equilibrium is changed (such as temperature, pressure, and the concentration of the chemicals participating in the reaction), the equilibrium will move in a direction that will attenuate this change."
Le Châtrelle's principle is an empirical law that makes it easy to analyze the problem of equilibrium movement direction, and in most cases can also be semi-quantitatively judged the result of equilibrium movement, that is, "weakening". Apply the Les châtre principle to determine the direction of equilibrium movement, because it is easy to operate and is often used in high school chemistry, but when using it, we must pay attention to the scope of application and accurate understanding, otherwise it is easy to draw wrong conclusions.
1. The Le Châtelier principle points to the spontaneous tendency of chemical equilibrium, which is the internal mechanism of the chemical equilibrium system without the help of external forces. The chemical equilibrium referred to here refers not only to chemical reactions, but also to phase equilibrium (such as the equilibrium of water vapor and ice), dissolution equilibrium (such as the equilibrium of carbon dioxide gas in a beer bottle and the carbon dioxide dissolved in beer or the balance between the sodium chloride crystal and its saturated solution). All these equilibriums are the equilibrium of the mutual transformation of the states of matter. For example, using the Le Châtelet principle, it is explained why the carbonated drinks drunk in life will burst into a lot of foam when the bottle cap is opened and poured into the glass. There is an equilibrium between the undissolved carbon dioxide and the dissolved carbon dioxide in carbonated beverages: when the cap is opened, the pressure decreases, and according to the Le Chateau principle, the equilibrium moves in the direction of releasing carbon dioxide to weaken the effect of the pressure reduction on the equilibrium. Therefore, carbonated drinks will burst into a lot of foam when they open the bottle cap and pour it into the glass.
2. The LeChatre principle is effective for systems that have reached a chemical equilibrium state, and if the system does not achieve a chemical equilibrium state, it is ineffective. Therefore, before using this principle, it is necessary to determine whether the system has reached a balanced state.
3. The Le Châtree principle is effective for changing the factors that maintain the state of chemical equilibrium, and if the change is not the factor that maintains the state of chemical equilibrium, it is invalid. For example, for a chemical equilibrium system, if the amount or composition of the catalyst is changed, it will not cause the equilibrium to move, because the catalyst is not a factor in maintaining the chemical equilibrium.
4. The Le Châtelett principle does not involve dynamic factors and cannot predict the speed or slowness of equilibrium movement. For example, the Le Château-Trale principle can be used to predict that low temperatures can increase the theoretical yield of synthetic ammonia, but it does not judge how long it will take to have such a high yield. The reason why the industrial synthesis conditions of ammonia did not adopt low temperature and chose high temperature is to reduce the theoretical yield of thermodynamics in exchange for higher production time efficiency, because for industrial production, "time is money".
5. The pressure change only refers to the change of the equal proportion of the concentration of each gaseous substance in the equilibrium system, that is, the change of the volume of the container or the equal proportion of the change of the concentration of each gas component. Although the concentration of each gas component changes at the same time, because it is an equal proportion of the change, it can only be regarded as a change in the pressure of this one condition, and cannot be regarded as several concentration conditions that are changed separately. In the constant capacity equilibrium system, the refilling of a pure gaseous substance only increases the concentration of the substance, and cannot be used as an increase in pressure to apply the le Châtelet principle.
6. The Le Châtelet principle applies to the relatively simple case of a change in external conditions (other conditions unchanged).
For example, the following reaction is performed in a container with a removable piston:
N2(g)+3H2(g)＝2NH3(g)。 After the reaction reaches equilibrium, add N2 to the reaction mixture, keep the temperature and container volume unchanged, in which direction will the equilibrium move? If the temperature and pressure are kept constant, in which direction will the balance move?
Under the condition of constant capacity, N2 is charged (only the concentration of N2 is increased), that is, the concentration of only one substance is changing. It is fairly easy to judge the direction of equilibrium movement using the Le Châtelet principle (the equilibrium is definitely moving in a positive direction).
However, under the condition of constant pressure, when N2 is charged, the partial pressure of N2 increases, and the partial pressure of H2 and NH3 is reduced, and the concentration of the three substances changes. This is more troublesome, because the increase in the partial pressure of N2 and the decrease in the partial pressure of NH3 will cause the balance to move forward. The reduction of the partial pressure of H2 will cause the balance to move in the opposite direction. In which direction will the balance move? For this problem, which involves multiple conditional changes and causes the equilibrium to move in exactly the opposite direction, it is beyond the scope of application of the Le Châte column principle and should be solved by the method of calculating the equilibrium constant. The equilibrium constant is a major achievement of thermodynamic theory in solving the problem of chemical reactions, which is based on rigorous theoretical foundations. It has played a huge role in both theory and practice and has stood the test of time.
7. For heterogeneous systems, sometimes the Le Châtelet principle cannot be used to judge the result of equilibrium movement, and the correlation equilibrium constant should be applied to analyze the judgment. For example, in a closed container at a certain temperature,
CaCO3(s)
CaO(s) + CO2(g) to reduce the container volume after reaching equilibrium, according to the LesChatre principle can be judged that the equilibrium reversal, but the concentration of CO2 after reaching equilibrium again is still unchanged rather than increasing, because the temperature does not change, K = c (CO2) unchanged. Another example, when AgCl(s)
After Ag+ (aq)+Cl-(aq) reaches equilibrium, the equilibrium is constantly shifted by adding water (when there is still an AgCl solid), but the concentrations of Ag+ and Cl- are unchanged because Ksp(AgCl) does not change at a certain temperature.
Although the Le Châtree principle has certain limitations, this does not affect its application and ideological value. Some argue that Le Châtrechtle's principle is by no means applicable only to chemical equilibrium, but to universal principles between the universes, from biology to physiology, from economics to sociology, all of which obey the "generalized Le Châtreet principle"—that systems subject to additional constraints have the ability to resist changes in additional constraints. In electromagnetism, the famous Lenz's law is expressed in almost the same way as Le Château's principle. The seventy-seventh chapter of Lao Tzu's Tao Te Ching has this to say, "The way of heaven, its bow and bow!" The high one suppresses it, the lower one lifts it, the surplus loses it, the insufficient one loses it, and the Heavenly Way has more than enough to make up for it. This passage highly summarizes the "way of heaven", and the principle of Le Châtel is also in line with the "way of heaven". It has even been said that human emotions conform to le Châtre's principle – you have a system of happiness, and if you make it unhappy, it will find happiness on its own! Pauling, a famous American chemist who has won two Nobel Prizes, said in his chemistry textbook: "If after graduating from college, students no longer engage in chemical research, they may forget all the mathematical formulas about chemical equilibrium, but I hope that students will not forget the Le Chateau principle."