What is a backlash? Reversible and irreversible reactions

Video tutorial 2: Chemical equilibrium shift

Lecture: Reversible and irreversible chemical reactions. Chemical balance. Shift in chemical equilibrium under the influence of various factors

From the previous lesson, you learned what the rate of a chemical reaction is and what factors influence it. In this lesson we will look at how these reactions occur. This depends on the behavior of the starting substances participating in the reaction - the reagents. If they are completely converted into final substances - products, then the reaction is irreversible. Well, if the final products are converted back into the original substances, then the reaction is reversible. Taking this into account, let us formulate the definitions:

Reversible reaction- this is a certain reaction that occurs under the same conditions in forward and reverse directions.

Remember, in chemistry lessons you were shown a clear example of a reversible reaction for the production of carbonic acid:

CO 2 + H 2 O<->H2CO3

Irreversible reaction- this is a certain chemical reaction that goes to completion in one specific direction.

An example is the phosphorus combustion reaction: 4P + 5O 2 → 2P 2 O 5

Some evidence of the irreversibility of a reaction is the formation of a precipitate or the release of gas.

When the rates of forward and reverse reactions are equal, it occurs chemical equilibrium.

That is, in reversible reactions, equilibrium mixtures of reactants and products are formed. Let us see with an example how a chemical equilibrium is formed. Let's take the reaction of hydrogen iodide formation:

H 2 (g) + I 2 (g)<->2HI(g)

We can heat a mixture of gaseous hydrogen and iodine or ready-made hydrogen iodine, the result in both cases will be the same: the formation of an equilibrium mixture of three substances H 2, I 2, HI.

At the very beginning of the reaction, before the formation of hydrogen iodide, a direct reaction occurs at a rate of ( v etc ). Let us express it by the kinetic equation v pr = k 1, where k 1 is the rate constant of the forward reaction. The product HI is gradually formed, which, under the same conditions, begins to decompose into H 2 and I 2. The equation for this process is as follows: v arr = k 2 2, where v rev – reverse reaction rate, k 2 – reverse reaction rate constant. At the moment when HI is sufficient for leveling v at v chemical equilibrium occurs. The amount of substances in equilibrium, in our case these are H 2, I 2 and HI, does not change over time, but only if there are no external influences. From the above it follows that chemical equilibrium is dynamic. In our reaction, hydrogen iodide is either formed or consumed.

Remember, changing the reaction conditions allows you to move the equilibrium in the desired direction. If we increase the concentration of iodine or hydrogen, it will increase v Thus, there will be a shift to the right, more hydrogen iodide will be formed. If we increase the concentration of hydrogen iodide, it will increase v arr, and the shift will be to the left. We can get more/less reagents and products.

Thus, chemical equilibrium tends to resist external influences. The addition of H 2 or I 2 ultimately leads to an increase in their consumption and an increase in HI. And vice versa. This process in science is called Le–Chatelier principle. It reads:

If a system that is in stable equilibrium is influenced from the outside (by changing temperature, or pressure, or concentration), then a shift will occur in the direction of a process that weakens this influence.

Remember, a catalyst cannot shift the equilibrium. He can only speed up its onset.

Change in concentration . Above, we looked at how this factor shifts the equilibrium either in the forward or in the opposite direction. If the concentration of reactants is increased, the equilibrium shifts to the side where this substance is consumed. If you reduce the concentration, it shifts to the side where this substance is formed. Remember, the reaction is reversible, and the reactants can be substances on both the right and left sides, depending on which reaction we are considering (direct or reverse).

Influencet . Its increase provokes a shift in equilibrium towards the endothermic reaction (- Q), and a decrease towards the exothermic reaction (+ Q). The reaction equations indicate the thermal effect of the forward reaction. The thermal effect of the reverse reaction is the opposite. This rule is only suitable for reactions with a thermal effect. If it is not there, then t is not capable of shifting the equilibrium, but its increase will accelerate the process of the emergence of equilibrium.

Effect of pressure . This factor can be used in reactions involving gaseous substances. If the moles of gas are zero, there will be no changes. As pressure increases, the equilibrium shifts towards smaller volumes. As the pressure decreases, the equilibrium will shift towards larger volumes. Volumes - look at the coefficients of gaseous substances in the reaction equation.

Among the numerous classifications of types of reactions, for example those that are determined by the thermal effect (exothermic and endothermic), by changes in the oxidation states of substances (redox), by the number of components participating in them (decomposition, compounds) and so on, reactions occurring in two mutual directions, otherwise called reversible . An alternative to reversible reactions are reactions irreversible, during which the final product (precipitate, gaseous substance, water) is formed. Among these reactions are the following:

Exchange reactions between salt solutions, during which either an insoluble precipitate is formed - CaCO 3:

Ca(OH) 2 + K 2 CO 3 → CaCO 3↓ + 2KON (1)

or a gaseous substance - CO 2:

3 K 2 CO 3 + 2H 3 RO 4 →2K 3 RO 4 + 3 CO 2+ 3H 2 O (2)

or a slightly dissociable substance is obtained - H 2 O:

2NaOH + H 2 SO 4 → Na 2 SO 4 + 2 H 2O(3)

If we consider a reversible reaction, then it proceeds not only in the forward direction (in reactions 1,2,3 from left to right), but also in the opposite direction. An example of such a reaction is the synthesis of ammonia from gaseous substances - hydrogen and nitrogen:

3H 2 + N 2 ↔ 2NH 3 (4)

Thus, a chemical reaction is called reversible if it proceeds not only in the forward direction (→), but also in the reverse direction (←) and is indicated by the symbol (↔).

The main feature of this type of reaction is that reaction products are formed from the starting substances, but at the same time, the starting reagents are formed from the same products. If we consider reaction (4), then in a relative unit of time, simultaneously with the formation of two moles of ammonia, their decomposition will occur with the formation of three moles of hydrogen and one mole of nitrogen. Let us denote the rate of direct reaction (4) by the symbol V 1, then the expression for this rate will take the form:

V 1 = kˑ [Н 2 ] 3 ˑ , (5)

where the value “k” is defined as the rate constant of a given reaction, the values ​​[H 2 ] 3 and correspond to the concentrations of the starting substances raised to powers corresponding to the coefficients in the reaction equation. In accordance with the principle of reversibility, the rate of the reverse reaction will take the expression:

V 2 = kˑ 2 (6)

At the initial moment of time, the rate of the forward reaction takes on the greatest value. But gradually the concentrations of the starting reagents decrease and the reaction rate slows down. At the same time, the rate of the reverse reaction begins to increase. When the rates of forward and reverse reactions become the same (V 1 = V 2), state of equilibrium , at which there is no longer a change in the concentrations of both the initial and the resulting reagents.

It should be noted that some irreversible reactions should not be taken literally. Let us give an example of the most frequently cited reaction of a metal with an acid, in particular, zinc with hydrochloric acid:

Zn + 2HCl = ZnCl 2 + H 2 (7)

In fact, zinc, when dissolved in acid, forms a salt: zinc chloride and hydrogen gas, but after some time the rate of the direct reaction slows down as the concentration of salt in the solution increases. When the reaction practically stops, a certain amount of hydrochloric acid will be present in the solution along with zinc chloride, so reaction (7) should be given in the following form:

2Zn + 2HCl = 2ZnНCl + H2 (8)

Or in the case of the formation of an insoluble precipitate obtained by merging solutions of Na 2 SO 4 and BaCl 2:

Na 2 SO 4 + BaCl 2 = BaSO 4 ↓ + 2NaCl (9)

the precipitated salt BaSO 4, albeit to a small extent, will dissociate into ions:

BaSO 4 ↔ Ba 2+ + SO 4 2- (10)

Therefore, the concepts of irreversible and irreversible reactions are relative. But nevertheless, both in nature and in the practical activities of people, these reactions are of great importance. For example, combustion processes of hydrocarbons or more complex organic substances, such as alcohol:

CH 4 + O 2 = CO 2 + H 2 O (11)

2C 2 H 5 OH + 5O 2 = 4CO 2 + 6H 2 O (12)

are completely irreversible processes. It would be considered a happy dream of humanity if reactions (11) and (12) were reversible! Then it would be possible to synthesize gas and gasoline and alcohol again from CO 2 and H 2 O! On the other hand, reversible reactions such as (4) or oxidation of sulfur dioxide:

SO 2 + O 2 ↔ SO 3 (13)

are basic in the production of ammonium salts, nitric acid, sulfuric acid, and other inorganic and organic compounds. But these reactions are reversible! And in order to obtain the final products: NH 3 or SO 3, it is necessary to use such technological methods as: changing the concentrations of reagents, changing pressure, increasing or decreasing the temperature. But this will already be the subject of the next topic: “Shift in chemical equilibrium.”

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Reversible reactions- chemical reactions, under given conditions, occurring simultaneously in two opposite directions (forward and reverse), the initial substances are not completely converted into products. for example: 3H 2 + N 2 ⇆ 2NH 3

The direction of reversible reactions depends on the concentrations of the substances participating in the reaction. Upon completion of the reversible reaction, i.e., upon reaching chemical equilibrium, the system contains both starting materials and reaction products.

A simple (one-stage) reversible reaction consists of two elementary reactions occurring simultaneously, which differ from each other only in the direction of the chemical transformation. The direction of the final reaction accessible to direct observation is determined by which of these mutually inverse reactions has a higher speed. For example, a simple reaction

N 2 O 4 ⇆ 2NO 2

consists of elementary reactions

N 2 O 4 ⇆ 2NO 2 and 2NO 2 ⇆ N 2 O 4

For the reversibility of a complex (multistage) reaction, it is necessary that all its constituent stages be reversible.

For reversible reactions The equation is usually written as follows: A + B AB.

Two oppositely directed arrows indicate that, under the same conditions, both forward and reverse reactions occur simultaneously

Irreversible These are chemical processes whose products are not able to react with each other to form the starting substances. From the point of view Thermodynamics - the initial things are completely transformed into products. Examples of irreversible reactions include the decomposition of berthollet salt upon heating 2КlО3 > 2Кl + 3О2,

Irreversible reactions are those reactions that occur:

1) the resulting products leave the reaction sphere - they precipitate and are released as a gas, for example BaCl 2 + H 2 SO 4 = BaSO 4 ↓ + 2HCl Na 2 CO 3 + 2HCl = 2NaCl + CO 2 ↓ + H 2 O

2) a slightly dissociated compound is formed, for example water: HCl + NaOH = H 2 O + NaCl

3) the reaction is accompanied by a large release of energy, for example the combustion of magnesium

Mg + 1 / 2 O 2 = MgO, ∆H = -602.5 kJ / mol

Chemical equilibrium is a state of a reaction system in which the rates of forward and reverse reactions are equal.

Equilibrium concentration of substances are the concentrations of substances in a reaction mixture that are in a state of chemical equilibrium. The equilibrium concentration is indicated by the chemical formula of the substance enclosed in square brackets.

For example, the following entry indicates that the equilibrium concentration of hydrogen in the equilibrium system is 1 mol/L.

Chemical equilibrium differs from the familiar concept of “equilibrium”. Chemical equilibrium is dynamic. In a system in a state of chemical equilibrium, both forward and reverse reactions occur, but their rates are equal, and therefore the concentrations of the substances involved do not change. Chemical equilibrium is characterized by an equilibrium constant equal to the ratio of the rate constants of the forward and reverse reactions.

The rate constants of the forward and reverse reactions are the rates of a given reaction at concentrations of the starting substances for each of them in equal units. Also, the equilibrium constant is equal to the ratio of the equilibrium concentrations of the products of the direct reaction in powers of stoichiometric coefficients to the product of the equilibrium concentrations of the reactants.

Н2+I2 = 2НI

If , then there are more starting materials in the system. If , then there are more reaction products in the system. If the equilibrium constant is significantly greater than 1, the reaction is called irreversible.

The position of chemical equilibrium depends on the following reaction parameters: temperature, pressure and concentration of substances. The influence that these factors have on a chemical reaction is subject to a pattern that was generally stated in 1884 by the French physical chemist Le Chatelier and confirmed in the same year by the Dutch physical chemist Van't Hoff. The modern formulation of Le Chatelier's principle is as follows : if the system is in a state of equilibrium, then any impact that is expressed in a change in one of the factors that determines the equilibrium causes a change in it that tends to weaken this impact.

In Le Chatelier's principle, we are talking about a shift in the state of dynamic chemical equilibrium; this principle is also called the principle of moving equilibrium, or the principle of shifting equilibrium.

Let's consider the use of this principle for various cases:

Effect of temperature. When the temperature changes, the shift in chemical equilibrium is determined by the sign of the thermal effect of the chemical reaction. In the case of an endothermic reaction, i.e. a reaction that occurs with the absorption of heat, an increase in temperature promotes its occurrence, since the temperature decreases during the reaction. As a result, the equilibrium shifts to the right, the concentrations of products increase, and their yield increases. If the temperature decreases, then the opposite picture is observed: the equilibrium shifts to the left (towards the reverse reaction, which occurs with the release of heat), the concentration and yield of products decrease.

For an exothermic reaction, on the contrary, an increase in temperature leads to a shift of the equilibrium to the left, and a decrease in temperature leads to a shift of the equilibrium to the right.

Changes in the concentration of products and reagents are due to the fact that when the temperature changes, the equilibrium constant of the reaction changes. An increase in the equilibrium constant leads to an increase in the yield of products, a decrease leads to a decrease.

For example, an increase in temperature in the case of an endothermic process of decomposition of calcium carbonate CaCO 3 (t) Û CaO (t)+ CO 2 (g) − Q causes a shift of equilibrium to the right, and in the case of an exothermic reaction of the decomposition of nitrogen monoxide into simple substances
2NO Û N 2 + O 2 +Q An increase in temperature shifts the equilibrium to the left, i.e., it promotes the formation of NO.

Effect of pressure. Pressure has a noticeable effect on the state of chemical equilibrium only in cases where at least one of the participants in the chemical reaction is a gas. An increase in pressure in such systems is accompanied by a decrease in volume and an increase in the concentration of all gaseous participants in the reaction.

If during a forward reaction the amount of gaseous substances increases, then the increase in pressure leads to a shift of equilibrium to the left (the amount of gases decreases during the reverse reaction). If during a reaction the amount of gaseous substances decreases, as the pressure increases, the equilibrium shifts to the right. If the quantities of gaseous reactants and products are equal, a change in pressure does not lead to a shift in chemical equilibrium.

It should be noted that changes in pressure do not affect the equilibrium constant.

Effect of concentration. According to Le Chatelier's principle, an increase in the concentration of one of the reaction participants should lead to its consumption. Thus, if a reagent is added to the system at V = const, the equilibrium will shift to the right, and if the reaction product - to the left. Removing a substance from the system (decreasing its concentration) has the opposite effect.

All of the above applies to both liquid and gaseous solutions (gas mixtures)

>> Chemistry: Reversible and irreversible reactions

CO2+ H2O = H2CO3

Let the resulting acid solution stand on a stand. After some time, we will see that the solution has turned purple again, as the acid has decomposed into its original substances.

This process can be carried out much faster if the solution is a third of carbonic acid. Consequently, the reaction to produce carbonic acid occurs both in the forward and in the reverse direction, that is, it is reversible. The reversibility of a reaction is indicated by two oppositely directed arrows:

Among the reversible reactions that underlie the production of the most important chemical products, let us cite as an example the reaction of synthesis (compound) of sulfur (VI) oxide from sulfur (IV) oxide and oxygen.

1. Reversible and irreversible reactions.

2. Berthollet's rule.

Write down the equations for the combustion reactions discussed in the text of the paragraph, noting that as a result of these reactions, oxides of the elements from which the original substances are built are formed.

Characterize the last three reactions carried out at the end of the paragraph according to plan: a) the nature and number of reagents and products; b) state of aggregation; c) direction: d) presence of a catalyst; e) release or absorption of heat

What inaccuracy was made in the writing of the equation for the reaction of limestone firing proposed in the text of the paragraph?

How true is it to say that compound reactions will generally be exothermic reactions? Justify your point of view using the facts given in the text of the textbook.

Chemical reactions proceeding in one direction are called irreversible.

Most chemical processes are reversible. This means that under the same conditions both forward and reverse reactions occur (especially if we are talking about closed systems).

For example:

a) reaction

in an open system irreversible;

b) the same reaction

in a closed system reversible.

Chemical equilibrium

Let us consider in more detail the processes occurring during reversible reactions, for example, for a conditional reaction:

Based on the law of mass action rate of forward reaction:

Since the concentrations of substances A and B decrease over time, the rate of the direct reaction also decreases.

The appearance of reaction products means the possibility of a reverse reaction, and over time the concentrations of substances C and D increase, which means that the reverse reaction speed.

Sooner or later a state will be reached in which the rates of forward and reverse reactions become equal = .

The state of the system in which the rate of the forward reaction is equal to the rate of the reverse reaction is called chemical equilibrium.

In this case, the concentrations of reactants and reaction products remain unchanged. They are called equilibrium concentrations. At the macro level, it seems that overall nothing is changing. But in fact, both the forward and reverse processes continue to occur, but at the same speed. Therefore, such equilibrium in the system is called mobile and dynamic.

Let us denote the equilibrium concentrations of substances [A], [B], [C], [D]. Then since = , k 1 [A] α [B] β = k 2 [C] γ [D] δ , where

where α, β, γ, δ are exponents, equal to the coefficients in the reversible reaction; K equal - chemical equilibrium constant.

The resulting expression quantitatively describes state of equilibrium and is a mathematical expression of the law of mass action for equilibrium systems.

At a constant temperature, the equilibrium constant is constant value for a given reversible reaction. It shows the relationship between the concentrations of reaction products (numerator) and starting substances (denominator), which is established at equilibrium.

Equilibrium constants are calculated from experimental data, determining the equilibrium concentrations of starting substances and reaction products at a certain temperature.

The value of the equilibrium constant characterizes the yield of reaction products and the completeness of its progress. If we get K » 1, this means that at equilibrium [C] γ [D] δ "[A] α [B] β , i.e., the concentrations of reaction products prevail over the concentrations of the starting substances, and the yield of reaction products is high.

At K equal to « 1, the yield of reaction products is correspondingly low. For example, for the hydrolysis reaction of acetic acid ethyl ester

equilibrium constant:

at 20 °C it has a value of 0.28 (that is, less than 1).

This means that a significant portion of the ester was not hydrolyzed.

In the case of heterogeneous reactions, the expression of the equilibrium constant includes the concentrations of only those substances that are in the gas or liquid phase. For example, for the reaction

The equilibrium constant is expressed as follows:

The value of the equilibrium constant depends on the nature of the reactants and temperature.

The constant does not depend on the presence of a catalyst, since it changes the activation energy of both the forward and reverse reactions by the same amount. The catalyst can only accelerate the onset of equilibrium without affecting the value of the equilibrium constant.

The state of equilibrium is maintained indefinitely under constant external conditions: temperature, concentration of starting substances, pressure (if gases participate in the reaction or are formed).

By changing these conditions, it is possible to transfer the system from one equilibrium state to another that meets the new conditions. This transition is called displacement or shift in equilibrium.

Let's consider different ways to shift the equilibrium using the example of the reaction between nitrogen and hydrogen to form ammonia:

Effect of changing the concentration of substances

When nitrogen N2 and hydrogen H2 are added to the reaction mixture, the concentration of these gases increases, which means the rate of forward reaction increases. The equilibrium shifts to the right, towards the reaction product, that is, towards ammonia NH 3.

N 2 +3H 2 → 2NH 3

The same conclusion can be drawn by analyzing the expression for the equilibrium constant. As the concentration of nitrogen and hydrogen increases, the denominator increases, and since K is equal. - the value is constant, the numerator must increase. Thus, the amount of the reaction product NH 3 in the reaction mixture will increase.

An increase in the concentration of the ammonia reaction product NH 3 will lead to a shift of equilibrium to the left, towards the formation of the starting substances. This conclusion can be drawn based on similar reasoning.

Effect of Pressure Change

A change in pressure affects only those systems where at least one of the substances is in a gaseous state. As pressure increases, the volume of gases decreases, which means their concentration increases.

Let's assume that the pressure in a closed system is increased, for example, by 2 times. This means that the concentrations of all gaseous substances (N 2, H 2, NH 3) in the reaction under consideration will increase by 2 times. In this case, the numerator in the expression for K equal will increase by 4 times, and the denominator by 16 times, i.e., the equilibrium will be disrupted. To restore it, the concentration of ammonia must increase and the concentrations of nitrogen and hydrogen must decrease. The balance will shift to the right. A change in pressure has virtually no effect on the volume of liquid and solid bodies, i.e. it does not change their concentration. Hence, the state of chemical equilibrium of reactions that do not involve gases does not depend on pressure.

Effect of temperature change

As the temperature increases, the rates of all reactions (exo- and endothermic) increase. Moreover, an increase in temperature has a greater effect on the rate of those reactions that have a higher activation energy, which means endothermic.

Thus, the rate of the reverse reaction (endothermic) increases more than the rate of the forward reaction. The equilibrium will shift towards the process accompanied by the absorption of energy.

The direction of the equilibrium shift can be predicted using Le Chatelier's principle:

If an external influence is exerted on a system that is in equilibrium (concentration, pressure, temperature changes), then the equilibrium shifts to the side that weakens this influence.

Thus:

As the concentration of reactants increases, the chemical equilibrium of the system shifts towards the formation of reaction products;

As the concentration of reaction products increases, the chemical equilibrium of the system shifts towards the formation of the starting substances;

As pressure increases, the chemical equilibrium of the system shifts towards the reaction in which the volume of gaseous substances formed is smaller;

As the temperature increases, the chemical equilibrium of the system shifts towards the endothermic reaction;

As the temperature decreases, it moves towards an exothermic process.

Le Chatelier's principle is applicable not only to chemical reactions, but also to many other processes: evaporation, condensation, melting, crystallization, etc. In the production of the most important chemical products, Le Chatelier's principle and calculations arising from the law of mass action make it possible to find such conditions to carry out chemical processes that provide maximum yield of the desired substance.

Reference material for taking the test:

Mendeleev table

Solubility table