Chemical equilibrium: the constant of chemical equilibrium and the ways of its expression

In 1885 the French physicist and chemist Le Chatelier was withdrawn, and in 1887 the German physicist Brown substantiated the law of chemical equilibrium and the constant of chemical equilibrium, and also studied their dependence on the influence of various external factors.

The essence of chemical equilibrium

Equilibrium is a dynamic state, meaning that things always move. The products are decomposed into reagents, and the reagents are combined into products. Things are moving, but the concentrations remain unchanged. The reaction is recorded with a double arrow instead of an equal sign to show that it is reversible.

Classical patterns

Even in the last century, chemists have discovered certain patterns that provide for the probability of changing the direction of the reaction in the same container. Knowledge of how chemical reactions occur are incredibly important, both for laboratory research and for industrial production. At the same time, it is of great importance to control all these phenomena. It is common for a person to interfere in many natural processes, especially reversible ones, in order to use them for their own benefit. From the knowledge of chemical reactions will be more useful, if you perfectly master the levers of their management.

The law of acting masses in chemistry is used by chemists to correctly calculate the rates of reaction. He gives a clear idea that no chemical process will be completed if it passes through a closed system. The molecules of the substances formed are in constant and erratic motion, and a reverse reaction may soon occur, at which the molecules of the starting material will be restored.

In industry, open systems are most often used. Vessels, apparatus and other containers, where chemical reactions pass, 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 type furnaces, blast furnaces function with a constant supply of air, and ammonia is synthesized with the continuous removal of ammonia itself.

Reversible and irreversible chemical reactions

Based on the name, it is possible to give appropriate definitions: irreversible reactions are considered to be carried out to the end, not changing their direction and proceeding along a given trajectory, regardless of pressure drops and temperature fluctuations. Their distinctive feature is that some products can leave the sphere of reaction. 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 thermal energy is released during the process, for example: 4P + 5O ₂ = 2P ₂ O ₅ + Q.

Virtually all reactions that occur in nature are reversible. Regardless of such external conditions as pressure and temperature, practically all processes can proceed simultaneously in different directions. As the law of the acting masses in chemistry states, the amount of absorbed heat will be equal to the amount extracted, which means that if one reaction was exothermic, then the second (inverse) would be endothermic.

Chemical equilibrium: the constant of chemical equilibrium

Reactions are the "verbs" of chemistry - the activity that chemists are studying. Many reactions pass to their completion, and then stop, which means that the reagents are completely converted into products, not being able to return to their original state. In some cases, the reaction is indeed irreversible, for example, when combustion alters both the physical and chemical properties of the 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 by means of certain laws that are applied in industries that seek to reduce the costs of production of specific chemicals. To understand the processes that preserve or potentially threaten people's health, the notion of chemical equilibrium is also useful. 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 reagents and products in solution.

Calculation of the equilibrium constant

This value is dimensionless, that is, not having a certain number of units. Although the calculation is usually recorded for two reagents and two products, it works for any number of participants in the reaction. Calculation and interpretation of the equilibrium constant depend on whether the chemical reaction is related to a homogeneous or heterogeneous equilibrium. This means that all the reactive components can be pure liquids or gases. For reactions that reach 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, there is only one value for the equilibrium constant, which changes only if the temperature at which the reaction occurs varies in one direction or another. You can make some predictions about the chemical reaction, based on whether the equilibrium constant is large or small. If the value is very large, then 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 of the equilibrium constant for the forward and reverse reactions are almost the same, the number of reagents and products will also be almost the same. This type of reaction is considered reversible.

Let us consider a specific reversible reaction

Let's 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 rate of the direct reaction, for v ₂ - the rate of the reverse reaction, k - the equilibrium constant. Using the law of mass action, we get the following expression:

V ₁ = k ₁ * c (H ₂ ) * c (I ₂ ),

V2 = k2 * c2 (HI).

When mixing the molecules of iodine (I ₂ ) and hydrogen (H ₂ ), their interaction begins. At the initial stage, the concentration of these elements is maximum, but by the end of the reaction the maximum concentration of the new compound - hydrogen iodide (HI) will be. Accordingly, the reaction rates will be different. At the very beginning, they will be maximum. Over time, there comes a time when these values are equal, it is a state called chemical equilibrium.

The expression for the chemical equilibrium constant is usually denoted by square brackets: [H ₂ ], [I ₂ ], [HI]. Since in the equilibrium state the velocities are equal, then:

K ₁ [H ₂ ] [I ₂ ] = k ₂ [H ₁ ] 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 system that is in equilibrium has a certain effect (change the conditions of chemical equilibrium by changing the temperature or pressure, for example), the balance will shift to partially counteract the effect of the change. In addition to chemistry, this principle is also applicable in several different forms to the fields of pharmacology and economics.

The constant of chemical equilibrium and the ways of its expression

Equilibrium expression can be expressed in terms of 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 affect the chemical equilibrium? If a pure liquid or solid substance participates in it, it is considered that it has K = 1, and accordingly ceases to be taken into account, except for highly concentrated solutions. For example, pure water has activity 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 can affect the equilibrium include the addition of a reagent or product (a change in concentration affects the balance). Adding a 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 in both directions has a constant 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 shows that equilibrium is in fact a very dynamic process, since the reaction moves in both directions at an equal pace.

Dynamic equilibrium is an example of a stable state function. For the system in a stable state, the behavior observed at the present time continues in the future. Therefore, once the reaction reaches equilibrium, the ratio of product and reagent concentrations remains the same, although the reaction continues.

How easy is it to tell about the complex?

Such concepts as chemical equilibrium and the constant of chemical equilibrium are quite complex for understanding. Let us take an example from life. Have you ever been stuck on a bridge between two cities and paid attention to the fact that the traffic in the other direction is smooth and measured while you are hopelessly stuck in a traffic jam? This is not good.

What if the machines were moving at the same speed and speed from 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 entire process is in a dynamic equilibrium.