Le Chatelier was deduced by the French physicist and chemist in 1885, and the law of chemical equilibrium and the chemical equilibrium constant were substantiated by the German physicist Brown in 1887, and their dependence on the influence of various external factors was studied.
The essence of chemical equilibrium
Balance is a dynamic state, meaning that things always move. Products are decomposed into reagents, and reagents are combined into products. Things move, but concentrations remain unchanged. The reaction is written with a double arrow instead of an equal sign to show that it is reversible.
Classic patterns
Back in the last century, chemists discovered certain patterns that provide for the likelihood of a change in the direction of the reaction in the same tank. Knowledge of how chemical reactions proceed is incredibly important for both laboratory research and industrial production. Moreover, the ability to control all these phenomena is of great importance. It is human nature to intervene in many natural processes, especially for reversible ones, in order to use them later for their own benefit. Knowledge of chemical reactions will be more beneficial if one is fluent in leverage to control them.
The law of the acting masses in chemistry uses chemists to correctly calculate reaction rates. It gives a clear idea that not a single chemical process will be brought to an end if it takes place in a closed-type system. The molecules of the substances formed are in constant and irregular motion, and the reverse reaction may soon occur, in which the molecules of the starting material will be restored.
In industry, open systems are most often used. Vessels, apparatuses and other containers where chemical reactions take place remain unlocked. This is necessary so that during these processes it is possible to extract the desired product and get rid of useless reaction products. For example, coal is burned in open furnaces, cement is produced in open furnaces, blast furnaces operate with a constant supply of air, and ammonia is synthesized by the continuous removal of ammonia itself.
Reversible and irreversible chemical reactions
Based on the name, appropriate definitions can be made: reactions that are completed to the end, do not change their direction and proceed along a given path regardless of pressure drops and temperature fluctuations are considered irreversible. Their distinctive feature is that some products may leave the reaction sphere. Thus, for example, it is possible to obtain a gas (CaCO 3 = CaO + CO 2), a precipitate (Cu (NO 3 ) 2 + H 2 S = CuS + 2HNO 3 ) or other compounds. The reaction will also be considered irreversible, if a large amount of heat is released during the process, for example: 4P + 5O 2 = 2P 2 O 5 + Q.
Almost all reactions that occur in nature are reversible. Regardless of external conditions such as pressure and temperature, almost all processes can proceed simultaneously in different directions. As the law of masses in chemistry says, the amount of absorbed heat will be equal to the amount of heat released, which means that if one reaction was exothermic, then the second (reverse) would be endothermic.
Chemical equilibrium: chemical equilibrium constant
Reactions are the โverbsโ of chemistry โ an activity that chemists study. Many reactions proceed to their completion and then stop, which means that the reagents are completely converted into products, not having the opportunity to return to their original state. In some cases, the reaction is truly irreversible, for example, when burning alters both the physical and chemical properties of a substance. However, there are many other circumstances in which the reverse reaction is not only possible, but also continuous, since the products of the first reaction become reagents in the second.
A dynamic state in which the concentrations of reagents and products remain constant is called equilibrium. It is possible to predict the behavior of substances with the help of certain laws that apply in industries that seek to reduce the production costs of specific chemicals. The concept of chemical equilibrium is also useful for understanding processes that preserve or potentially threaten human health. The chemical equilibrium constant is the value of the reaction factor, which depends on the ionic strength and temperature, and does not depend on the concentrations of the reactants and products in the solution.
Calculation of the equilibrium constant
This value is dimensionless, that is, not having a certain number of units. Although the calculation is usually written for two reagents and two products, it works for any number of participants in the reaction. The calculation and interpretation of the equilibrium constant depends on whether the chemical reaction is associated with a homogeneous or heterogeneous equilibrium. This means that all reactive components can be pure liquids or gases. For reactions that achieve heterogeneous equilibrium, there is usually not one phase, but at least two. For example, liquids and gases or solids and liquids.
The value of the equilibrium constant
For any given temperature for the equilibrium constant, there is only one value that changes only if the temperature at which the reaction occurs changes in one direction or another. Some predictions can be made regarding a chemical reaction based on whether the equilibrium constant is large or small. If the value is very large, then the equilibrium favors the reaction to the right and more products are obtained than there were reagents. The reaction in this case can be called "complete" or "quantitative."
If the value of the equilibrium constant is small, then it favors the reaction to the left, where the amount of reagents was greater than the products formed. If this value tends to zero, we can assume that the reaction does not occur. If the values โโof the equilibrium constant for the direct and reverse reactions are almost the same, then the number of reagents and products will also be almost the same. This type of reaction is considered reversible.
Consider a specific reversible reaction
Take two chemical elements such as iodine and hydrogen, which when mixed give a new substance - iodine hydrogen.
H 2 + I 2 = 2HI
For v 1 we take the speed of the direct reaction, for v 2 - the speed of the reverse reaction, k - the equilibrium constant. Using the law of mass action, we obtain the following expression:
v 1 = k 1 * c (H 2 ) * c (I 2 ),
v 2 = k 2 * c 2 (HI).
When molecules of iodine (I 2 ) and hydrogen (H 2 ) are mixed, their interaction begins. At the initial stage, the concentration of these elements is maximum, but by the end of the reaction, the concentration of the new compound, hydrogen iodide (HI), will be maximum. Accordingly, the reaction rates will also be different. At the very beginning, they will be maximum. Over time, there comes a time when these values โโwill be equal, and it is a state called chemical equilibrium.
The expression of the chemical equilibrium constant is usually indicated using square brackets: [H 2 ], [I 2 ], [HI]. Since the equilibrium speeds are equal, then:
k 1 [H 2] [I 2] = k 2 [HI] 2,
so we get the equation of the chemical equilibrium constant:
k 1 / k 2 = [HI] 2 / [H 2 ] [I 2 ] = K.
The Le Chatelier-Brown Principle
There is the following regularity: if a certain effect is exerted on a system that is in equilibrium (changing the conditions of chemical equilibrium by changing temperature or pressure, for example), then the balance will shift to partially counteract the effect of the change. In addition to chemistry, this principle also applies in slightly different forms to the fields of pharmacology and economics.
Chemical equilibrium constant and methods of its expression
The equilibrium expression can be expressed in terms of the concentration of products and reagents. Only chemicals in the aqueous and gaseous phases are included in the equilibrium formula, since the concentrations of liquids and solids do not change. What factors influence chemical equilibrium? If a pure liquid or solid is involved in it, it is considered that it has K = 1, and accordingly it ceases to be taken into account, with the exception of highly concentrated solutions. For example, pure water has an activity of 1.
Another example is solid carbon, which can be formed by the reaction of two molecules of carbon monoxide to form carbon dioxide and carbon. Factors that may affect equilibrium include the addition of a reagent or product (a change in concentration affects the balance). Adding reagent can lead to equilibrium on the right in the chemical equation, where more forms of the product appear. Adding a product can lead to equilibrium on the left, as more forms of reagents become.
Equilibrium occurs when a reaction taking place in both directions has an unchanged ratio of products and reagents. In general, the chemical equilibrium is static, since the quantitative ratio of products and reagents is constant. However, a closer look reveals that equilibrium is actually a very dynamic process, as the reaction moves in both directions at the same pace.
Dynamic equilibrium is an example of a steady state function. For a system in steady state, the currently observed behavior continues in the future. Therefore, as soon as the reaction reaches equilibrium, the concentration ratio of the product and the reagent will remain the same, although the reaction continues.
How to just talk about the complex?
Concepts such as chemical equilibrium and chemical equilibrium constant are quite difficult to understand. Take an example from life. Have you ever been stuck on a bridge between two cities and paid attention to the fact that traffic in the other direction is smooth and measured, while you are hopelessly stuck in traffic jam? This is not good.
What if the cars measured and at the same speed moved on both sides? Would the number of cars in both cities remain constant? When the speed of entry and exit to both cities is the same, and the number of cars in each city is stable over time, this means that the whole process is in dynamic equilibrium.