reversibility of chemical reactions. Reversible and irreversible reactions

Chemical reactions are reversible and irreversible.

those. if some reaction A + B = C + D is irreversible, this means that the reverse reaction C + D = A + B does not occur.

i.e., for example, if a certain reaction A + B = C + D is reversible, this means that both the reaction A + B → C + D (direct) and the reaction C + D → A + B (reverse) proceed simultaneously ).

In fact, because both direct and reverse reactions proceed, reagents (starting substances) in the case of reversible reactions can be called both substances on the left side of the equation and substances on the right side of the equation. The same goes for products.

For any reversible reaction, it is possible that the rates of the forward and reverse reactions are equal. Such a state is called state of equilibrium.

In a state of equilibrium, the concentrations of both all reactants and all products are unchanged. The concentrations of products and reactants at equilibrium are called equilibrium concentrations.

Shift in chemical equilibrium under the influence of various factors

Due to such external influences on the system as a change in temperature, pressure or concentration of starting substances or products, the equilibrium of the system may be disturbed. However, after the cessation of this external influence, the system will pass to a new state of equilibrium after some time. Such a transition of a system from one equilibrium state to another equilibrium state is called shift (shift) of chemical equilibrium .

In order to be able to determine how the chemical equilibrium shifts with a particular type of exposure, it is convenient to use the Le Chatelier principle:

If any external influence is exerted on a system in a state of equilibrium, then the direction of the shift in chemical equilibrium will coincide with the direction of the reaction that weakens the effect of the impact.

The influence of temperature on the state of equilibrium

When the temperature changes, the equilibrium of any chemical reaction shifts. This is due to the fact that any reaction has a thermal effect. In this case, the thermal effects of the forward and reverse reactions are always directly opposite. Those. if the forward reaction is exothermic and proceeds with a thermal effect equal to +Q, then the reverse reaction is always endothermic and has a thermal effect equal to -Q.

Thus, in accordance with Le Chatelier's principle, if we increase the temperature of some system that is in a state of equilibrium, then the equilibrium will shift towards the reaction, during which the temperature decreases, i.e. towards an endothermic reaction. And similarly, if we lower the temperature of the system in a state of equilibrium, the equilibrium will shift towards the reaction, as a result of which the temperature will increase, i.e. towards an exothermic reaction.

For example, consider the following reversible reaction and indicate where its equilibrium will shift as the temperature decreases:

As you can see from the equation above, the forward reaction is exothermic, i.e. as a result of its flow, heat is released. Therefore, the reverse reaction will be endothermic, that is, it proceeds with the absorption of heat. According to the condition, the temperature is lowered, therefore, the equilibrium will shift to the right, i.e. towards a direct reaction.

Effect of concentration on chemical equilibrium

An increase in the concentration of reagents in accordance with the Le Chatelier principle should lead to a shift in equilibrium towards the reaction in which the reagents are consumed, i.e. towards a direct reaction.

Conversely, if the concentration of the reactants is lowered, then the equilibrium will shift towards the reaction that results in the formation of the reactants, i.e. side of the reverse reaction (←).

A change in the concentration of reaction products also affects in a similar way. If you increase the concentration of products, the equilibrium will shift towards the reaction, as a result of which the products are consumed, i.e. towards the reverse reaction (←). If, on the contrary, the concentration of products is lowered, then the equilibrium will shift towards the direct reaction (→), in order for the concentration of products to increase.

Effect of pressure on chemical equilibrium

Unlike temperature and concentration, a change in pressure does not affect the equilibrium state of every reaction. In order for a change in pressure to lead to a shift in chemical equilibrium, the sums of the coefficients in front of gaseous substances on the left and right sides of the equation must be different.

Those. from two reactions:

a change in pressure can affect the state of equilibrium only in the case of the second reaction. Since the sum of the coefficients in front of the formulas of gaseous substances in the case of the first equation on the left and right is the same (equal to 2), and in the case of the second equation it is different (4 on the left and 2 on the right).

From this, in particular, it follows that if there are no gaseous substances among both the reactants and the products, then a change in pressure will not affect the current state of equilibrium in any way. For example, pressure will not affect the equilibrium state of the reaction:

If the amount of gaseous substances is different on the left and on the right, then an increase in pressure will lead to a shift in equilibrium towards the reaction, during which the volume of gases decreases, and a decrease in pressure in the direction of the reaction, as a result of which the volume of gases increases.

Effect of a catalyst on chemical equilibrium

Since a catalyst equally accelerates both the forward and reverse reactions, its presence or absence does not affect to a state of equilibrium.

The only thing that a catalyst can affect is the rate of transition of the system from a non-equilibrium state to an equilibrium state.

The impact of all the above factors on chemical equilibrium is summarized below in a cheat sheet, which at first you can peek at when performing balance tasks. However, she will not be able to use it in the exam, therefore, after analyzing several examples with her help, she should be taught and trained to solve tasks for balance, no longer peeping into her:

Designations: T - temperature, p - pressure, With – concentration, – increase, ↓ – decrease

One of the most important characteristics of a chemical reaction is the depth (degree) of conversion, which shows how much the starting substances are converted into reaction products. The larger it is, the more economically the process can be carried out. The depth of conversion, among other factors, depends on the reversibility of the reaction.

reversible reactions , Unlike irreversible, do not proceed to the end: none of the reactants is completely consumed. At the same time, the reaction products interact with the formation of starting materials.

Consider examples:

1) equal volumes of gaseous iodine and hydrogen are introduced into a closed vessel at a certain temperature. If the collisions of the molecules of these substances occur with the desired orientation and sufficient energy, then the chemical bonds can be rearranged with the formation of an intermediate compound (an activated complex, see section 1.3.1). Further rearrangement of bonds can lead to the decomposition of the intermediate compound into two molecules of hydrogen iodide. Reaction equation:

H 2 + I 2 ® 2HI

But the molecules of hydrogen iodide will also randomly collide with molecules of hydrogen, iodine and among themselves. When HI molecules collide, nothing will prevent the formation of an intermediate compound, which can then decompose into iodine and hydrogen. This process is expressed by the equation:

2HI ® H 2 + I 2

Thus, two reactions will proceed simultaneously in this system - the formation of hydrogen iodide and its decomposition. They can be expressed by one general equation

H 2 + I 2 "2HI

The reversibility of the process is shown by the sign “.

The reaction directed in this case towards the formation of hydrogen iodide is called direct, and the opposite is called reverse.

2) if we mix two moles of sulfur dioxide with one mole of oxygen, create conditions in the system that are favorable for the reaction to proceed, and after the time has elapsed, analyze the gas mixture, the results will show that the system will contain both SO 3 - the reaction product, and the initial substances - SO 2 and O 2. If sulfur oxide (+6) is placed under the same conditions as the initial substance, then it will be possible to find that part of it will decompose into oxygen and sulfur oxide (+4), and the final ratio between the quantities of all three substances will be the same as when starting from a mixture of sulfur dioxide and oxygen.

Thus, the interaction of sulfur dioxide with oxygen is also one of the examples of a reversible chemical reaction and is expressed by the equation

2SO 2 + O 2 "2SO 3

3) the interaction of iron with hydrochloric acid proceeds according to the equation:

Fe + 2HCL ® FeCL 2 + H 2

With enough hydrochloric acid, the reaction will end when

all the iron is used up. In addition, if you try to carry out this reaction in the opposite direction - to pass hydrogen through a solution of iron chloride, then metallic iron and hydrochloric acid will not work - this reaction cannot go in the opposite direction. Thus, the interaction of iron with hydrochloric acid is an irreversible reaction.

However, it should be borne in mind that theoretically any irreversible process can be represented as reversible under certain conditions, i.e. In principle, all reactions can be considered reversible. But very often one of the reactions clearly prevails. This happens in those cases when the products of interaction are removed from the reaction sphere: a precipitate precipitates, a gas is released, during ion-exchange reactions practically non-dissociating products are formed; or when, due to a clear excess of starting substances, the opposite process is practically suppressed. Thus, the natural or artificial exclusion of the possibility of a reverse reaction allows you to bring the process almost to the end.

Examples of such reactions are the interaction of sodium chloride with silver nitrate in solution

NaCL + AgNO 3 ® AgCl¯ + NaNO 3 ,

copper bromide with ammonia

CuBr 2 + 4NH 3 ® Br 2,

neutralization of hydrochloric acid with sodium hydroxide solution

HCl + NaOH ® NaCl + H 2 O.

These are all examples only practically irreversible processes, since silver chloride is somewhat soluble, and the complex cation 2+ is not absolutely stable, and water dissociates, although to an extremely small extent.

DEFINITION

Chemical reaction called the transformation of substances in which there is a change in their composition and (or) structure.

The reaction is possible with a favorable ratio of energy and entropy factors. If these factors balance each other, the state of the system does not change. In such cases, the systems are said to be in equilibrium.
Chemical reactions that proceed in one direction are called irreversible. Most chemical reactions are reversible. This means that under the same conditions, both forward and reverse reactions occur (especially when it comes to closed systems).

The state of a 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 (equilibrium concentrations).

Equilibrium constant

Consider the reaction for obtaining ammonia:

N 2 (g) + 3H 2 (g) ↔ 2 NH 3 (g)

Let us write down the expressions for calculating the rates of direct (1) and reverse (2) reactions:

1 = k 1 [ H 2 ] 3

2 = k 2 2

The rates of the forward and reverse reactions are equal, so we can write:

k 1 3 = k 2 2

k 1 / k 2 = 2 / 3

The ratio of two constants is a constant. The equilibrium constant is the ratio of the rate constants of the forward and reverse reactions.

K = 2 / 3

In general terms, the equilibrium constant is:

mA + nB ↔ pC +qD

K = [C] p [D] q / [A] m [B] n

The equilibrium constant is the ratio of the products of the concentrations of the reaction products raised to the powers equal to their stoichiometric coefficients to the product of the concentrations of the starting substances raised to the powers equal to their stoichiometric coefficients.

If K is expressed in terms of equilibrium concentrations, then K s is most often denoted. It is also possible to calculate K for gases in terms of their partial pressures. In this case, K is denoted as K p. There is a relationship between K s and K p:

K p \u003d K c × (RT) Δn,

where Δn is the change in the number of all moles of gases during the transition from reactants to products, R is the universal gas constant.

K is independent of concentration, pressure, volume, and the presence of a catalyst, and depends on temperature and the nature of the reactants. If K is much less than 1, then there are more starting substances in the mixture, and in the case of much more than 1, there are more products in the mixture.

Heterogeneous equilibrium

Consider the reaction

CaCO 3 (tv) ↔ CaO (tv) + CO 2 (g)

The expression for the equilibrium constant does not include the concentrations of the components of the solid phase, therefore

Chemical equilibrium occurs in the presence of all components of the system, but the equilibrium constant does not depend on the concentrations of substances in the solid phase. Chemical equilibrium is a dynamic process. K gives information about the course of the reaction, and ΔG - about its direction. They are related to each other:

ΔG 0 = -R × T × lnK

ΔG 0 = -2.303 × R × T × lgK

Shift in chemical equilibrium. Le Chatelier's principle

From the point of view of technological processes, reversible chemical reactions are not beneficial, since it is necessary to have knowledge of how to increase the yield of the reaction product, i.e. it is necessary to learn how to shift the chemical equilibrium towards the products of the reaction.

Consider a reaction in which it is necessary to increase the yield of ammonia:

N 2 (g) + 3H 2 (g) ↔ 2NH 3 (g), ΔН< 0

In order to shift the equilibrium in the direction of a direct or reverse reaction, it is necessary to use Le Chatelier's principle: if a system in equilibrium is affected by some factor from outside (increase or decrease in temperature, pressure, volume, concentration of substances), then the system counteracts this effect.

For example, if the temperature is increased in an equilibrium system, then out of 2 possible reactions, one will go that will be endothermic; if you increase the pressure, then the equilibrium will shift towards the reaction with a large number of moles of substances; if the volume in the system is reduced, then the equilibrium shift will be directed to an increase in pressure; if the concentration of one of the starting substances is increased, then out of 2 possible reactions, one will go that will lead to a decrease in the equilibrium concentration of the product.

So, in relation to the considered reaction, in order to increase the yield of ammonia, it is necessary to increase the concentration of the starting substances; lower the temperature, since the direct reaction is exothermic, increase the pressure or decrease the volume.

Examples of problem solving

EXAMPLE 1

What is a reversible reaction? This is a chemical process that proceeds in two mutually opposite directions. Consider the main characteristics of such transformations, as well as their distinctive parameters.

What is the meaning of balance

Reversible chemical reactions do not lead to specific products. For example, when sulfur oxide (4) is oxidized simultaneously with the production of sulfur oxide (6), the original components are again formed.

Irreversible processes involve the complete transformation of the interacting substances, such a reaction is accompanied by the production of one or more reaction products.

Decomposition reactions are examples of irreversible interactions. For example, when potassium permanganate is heated, metal manganate, manganese oxide (4), is formed, and gaseous oxygen is also released.

A reversible reaction does not imply the formation of precipitation, the release of gases. This is precisely its main difference from irreversible interaction.

Chemical equilibrium is such a state of an interacting system in which the reversible occurrence of one or more chemical reactions is possible provided that the rates of the processes are equal.

If the system is in dynamic equilibrium, there is no change in temperature, concentration of reagents, other parameters in a given period of time.

Equilibrium shift conditions

The equilibrium of a reversible reaction can be explained using Le Chatelier's rule. Its essence lies in the fact that when an external influence is exerted on a system that is initially in dynamic equilibrium, a change in the reaction is observed in the direction opposite to the influence. Any reversible reaction with the help of this principle can be shifted in the right direction in the event of a change in temperature, pressure, and also the concentration of interacting substances.

The Le Chatelier principle "works" only for gaseous reagents, solid and liquid substances are not taken into account. There is an inverse relationship between pressure and volume, defined by the Mendeleev-Clapeyron equation. If the volume of the initial gaseous components is greater than the reaction products, then in order to change the equilibrium to the right, it is important to increase the pressure of the mixture.

For example, during the transformation of carbon monoxide (2) into carbon dioxide, 2 mol of carbon monoxide and 1 mol of oxygen enter into the reaction. This produces 2 moles of carbon monoxide (4).

If, according to the condition of the problem, this reversible reaction should be shifted to the right, it is necessary to increase the pressure.

The concentration of reactants also has a significant effect on the course of the process. According to the Le Chatelier principle, in the case of an increase in the concentration of the initial components, the equilibrium of the process shifts towards the product of their interaction.

In this case, the decrease (withdrawal from the reaction mixture) of the resulting product contributes to the flow of the direct process.

In addition to pressure, concentration, a change in temperature also has a significant effect on the course of a reverse or forward reaction. When the initial mixture is heated, the equilibrium is shifted towards the endothermic process.

Examples of reversible reactions

Consider on a specific process ways to shift the equilibrium towards the formation of reaction products.

2CO + O 2 -2CO 2

This reaction is a homogeneous process, since all substances are in the same (gaseous) state.

There are 3 volumes of components on the left side of the equation, after the interaction this indicator decreased, 2 volumes are formed. For the direct process to proceed, it is necessary to increase the pressure of the reaction mixture.

Given that the reaction is exothermic, the temperature is lowered to produce carbon dioxide.

The equilibrium of the process will shift towards the formation of a reaction product with an increase in the concentration of one of the initial substances: oxygen or carbon monoxide.

Conclusion

Reversible and irreversible reactions play an important role in human life. The metabolic processes occurring in our body are associated with a systematic shift in the chemical balance. In chemical production, optimal conditions are used to direct the reaction in the right direction.

Among the numerous classifications of types of reactions, for example, those that are determined by the thermal effect (exothermic and endothermic), by the change in the oxidation states of substances (redox), by the number of components involved in them (decompositions, compounds), and so on, reactions occurring in two mutual directions, otherwise called reversible . An alternative to reversible reactions are the reactions irreversible, during which the final product (precipitate, gaseous substance, water) is formed. These reactions include 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↓ + 2KOH (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 poorly dissociated substance is obtained - H 2 O:

2NaOH + H 2 SO 4 → Na 2 SO 4 + 2 H2O(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 (→) but also in the reverse direction (←) and is indicated by the symbol (↔).

The main feature of this type of reaction is that the reaction products are formed from the starting materials, but at the same time, the starting reagents are formed from the same products, inversely. If we consider reaction (4), then in a relative unit of time, simultaneously with the formation of two moles of ammonia, they will decompose with the formation of three moles of hydrogen and one mole of nitrogen. Let us denote the rate of the 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 of "k" is defined as the rate constant of a given reaction, the values ​​of [H 2 ] 3 and correspond to the concentrations of the starting substances raised to the power 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 direct reaction takes on the maximum value. But gradually the concentrations of the initial 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 the forward and reverse reactions become the same (V 1 \u003d V 2), it comes state of equilibrium , at which there is no change in the concentrations of both the initial and the formed 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 the interaction of a metal with an acid, in particular, zinc with hydrochloric acid:

Zn + 2HCl \u003d 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 + H 2 (8)

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

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

the precipitated BaSO 4 salt, 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 is relative. Nevertheless, both in nature and in the practical activities of people, these reactions are of great importance. For example, the combustion processes of hydrocarbons or more complex organic substances, such as alcohol:

CH 4 + O 2 \u003d CO 2 + H 2 O (11)

2C 2 H 5 OH + 5O 2 \u003d 4CO 2 + 6H 2 O (12)

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

SO 2 + O 2 ↔ SO 3 (13)

are the main ones in the production of ammonium salts, nitric acid, sulfuric acid, etc., both inorganic and organic compounds. But these reactions are reversible! And in order to obtain 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 temperature. But this will already be the subject of the next topic: "Displacement of chemical equilibrium."

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